Category Weather & Climate

            Most outdoor sports events can be affected by adverse weather conditions in one way or another. “Rain stopped play” is a phrase familiar to followers of cricket in England, where the often unpredictable summer weather regularly interrupts a game. Tennis is similarly affected when heavy rain makes play impossible on open-air grass courts. Some sports can be played in almost all weathers (only severe snow and freezing temperatures will stop a soccer or rugby match), but the conditions can influence tactics and the outcome of the game.

            The effects of weather on sport are varied, with some events unable to take place while others are changed considerably. The performance of participants can be reduced or improved, and some sporting world records are invalid if set under certain weather conditions. While outdoor sports are most affected, those played indoors can still be impacted by adverse or advantageous weather conditions.

            Temperature has a significant impact on the performance of athletes. High temperature can cause various heat illnesses such as heat cramps and heat stroke, while very low temperatures may lead to hypothermia.

             Some major sporting events cannot be held when the temperature is too high. When AFC selected Qatar as the host of the 2011 AFC Asian Cup they opted to run the event in the January window rather than July or August because they considered it to be “too hot in the Gulf region”. However, the also set to be held in Qatar, is scheduled for the late fall and early winter.

            Some sports are cancelled because of precipitation. Some are deemed too dangerous to play when the ground is damp because of the danger of injury to a player through slipping.

            When the rain is excessive an event might be canceled because of a waterlogged pitch. Winter sports can sometimes be canceled due to the amount of snow on the ground, be it too little or too much.

            Wind can blow the equipment in a sporting event, changing the direction or travel of a ball. In golf the wind levels may influence the way a shot is taken.A headwind can slow runners, while they may gain wind assistance from a tailwind.Some sports rely on the presence of wind, especially surface water sports.

            Some sports cannot be played if there is insufficient visibility as it can make them dangerous or can be disadvantageous to a competitor. Cricket test matches often finish when the umpire decides that the light level is too low and the timing of this can sometimes be controversial. The difficulties of playing in bad light conditions is also disputed. Some events are called off when there is heavy fog.

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            Farmers need to pay special attention to the weather in order to tend their crops or feed their animals. Knowledge of a severe frost or rain will influence the time that they sow seeds or harvest crops. Accurate weather forecasts also help farmers to decide when to treat crops with chemicals. For example, should it rain shortly after pesticides are applied, they will be washed away and have little effect — a waste of time and money for the farmer. Forecasts for farmers provide as much information as possible about the weather for the next week or so.

            Most people know that the weather has a significant impact on the agriculture industry. Indeed, crops need the basics of moisture, warmth, and sun to thrive. But what’s less obvious is how the details of weather information can drive a grower’s business decisions, helping them to plan efficiently, minimize costs and maximize yields—and profits—as a result.

             While farmers must make many day-to-day decisions related to weather conditions, there are four primary areas of farming that are fundamentally affected impacted by weather:

            Crop Growth/Irrigation: Crop growth, or crop yield, requires appropriate amounts of moisture, light, and temperature. Detailed and accurate historical, real-time and forecast weather information can help farmers better understand and track the growth status/stage to make informed decisions. Having access to this data can guide farmers in making significant and potentially costly decisions, such as whether, when and how much to irrigate.

            Fertilizer Timing and Delivery: One of the many decisions that farmers have to make is determining the proper time to apply fertilizer, as well as the application rate and fertilizer form to use. A misapplied application caused by weather can wipe away the entire field’s profits. Weather forecasts can be used to ensure that fertilizer is applied in the right conditions—when it’s dry enough so that it doesn’t wash away (which would create a waste of resources and money) but moist enough so that it gets worked into the soil.

            Pest and Disease Control: Certain weather conditions encourage the development and growth of pests and diseases, which can destroy crops. Forecast guidance incorporated into pest and disease modeling can help determine whether—and when—it’s appropriate to apply pest or disease controls. Wind forecasts also play a role in this decision, as crop dusters, aircraft that spray fungicidal or insecticidal chemicals on plants from above, must be utilized when wind conditions are not apt to cause sprayed chemicals to miss their targets.

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            People who work at sea depend heavily on detailed, specialized weather forecasts because their lives can be at risk when stormy conditions bring high winds and steep seas. Fishermen may decide where to fish according to weather conditions, while sport sailors pay close attention to wind details to plan their racing tactics. All mariners listen to radio stations and coastguard broadcasts for advance warn-ings of weather conditions, which focus on the speed and direction of the wind, visibility and barometer readings.

            We receive weather information every day in a variety of ways – through television, radio, on our smartphones and through conversations around the water cooler. But how do you get weather forecasts on the high seas where WiFi is rarely an option? Across our oceans, NOAA’s Ocean Prediction Center (OPC) is delivering critical weather forecasts to keep you safe – at sea.

           Did you know 11 million Americans travel on cruise ships each year, and that our nation’s maritime shipping industry – the way we primarily receive goods from other countries – is a 1.5 trillion dollar economic activity annually for the U.S? This means accurate and reliable weather forecasts at sea are an incredibly important part of our nation’s economy.

            Every day, expert weather forecasters at OPC deliver more than 150 different products – like forecasting maps and guidance – for weather events across the Atlantic and Pacific Ocean basins, including the waters around Alaska. This forecast guidance makes its way into the hands of commercial shipping vessels allowing ship captains to dodge hurricanes, cruise ships to route around nor’easters and recreational boaters to return home safely.

            Hazards at sea can vary greatly and OPC forecasts for all of them. High winds, large waves, thunderstorms, sea ice, freezing spray, and volcanic ash all present significant safety issues to mariners traveling with precious cargo- the lives of their passengers and crew.

            Even with these challenges, OPC delivers life-saving forecasts around the clock each day, while also working hard to modernize the ways they provide forecasts to a geographically diverse community. And OPC has some great partners. OPC forecasters collaborate closely with the U.S. Coast Guard, the U.S. Navy and the international maritime community to continually advance services and ensure critical forecasts reach those who rely on them to make safety decisions at sea.

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            Some experts believe that when glaciers melted 7000 years ago, this caused the Mediterranean to overflow into the Black Sea, then a small freshwater lake. This may form the basis of Middle Eastern tales, such as the one recorded in the Old Testament, of a hugely destructive flood.

           A flood of Biblical proportions just like in the story of Noah’s Ark may have actually happened, according to the oceanographer who found the Titanic.

            Acclaimed underwater archaeologist Robert Ballard claims his team of researchers have uncovered evidence that suggests The Great Flood described in the Bible was actually based on real events.

            Mr Ballard told how he investigated a controversial theory proposed by two scientists from Columbia University that there was a massive flood in the Black Sea region.

           In an interview with ABC News, he said around 12,000 years ago much of the world was covered in ice and the Black Sea had been a freshwater lake surrounded by farmland.

            But when the glaciers began to melt during a warming period in the cycle of the Earth’s temperature around 5600BC water rushed toward the world’s oceans, Mr Ballard said. This, he claimed, caused floods all around the world and water cascaded through Turkey’s Straits of Bosporus towards the Black Sea.

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            Weather forecasts are used by everybody, but some people pay closer attention to them than others. Severe weather conditions can endanger lives on the roads, at sea and in the air, so transport and safety organizations are regularly updated on the weather situation. Many businesses, from farming and fishing to hotels and restaurants can be affected by the weather, so a forecast can help with business planning.

            This chapter examines recent and expected developments in the scientific capability to make seasonal-to-interannual climate forecasts and discusses the types of forecasts that are likely to be socially useful. As background for readers unfamiliar with climate forecasting, we begin by discussing the distinction between weather and climate and how climate forecasts are made.

            We are all familiar with the progression of the weather. Every few days, the temperature changes, rain comes and goes, or a severe storm hits. The characteristic time scale for changes in weather in the mid-latitudes is a few days or less. In the tropics, especially over the ocean, the weather tends to be much steadier, with sunny weather and steady trade winds punctuated by an hour of daily downpour (usually in the late afternoon) or by a squall every few days.

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          The river Nile was the source of life and prosperity in Egypt. The Ancient Egyptians relied on the annual floods of the Nile to irrigate their crops, but studies have shown that the way in which the river floods varies considerably. Working together, historians and climatologists have found links between years of low flooding and periods of instability in Egyptian society. Records show that the famines that followed low floods led to disease and civil unrest — possibly causing the collapse of the Old Kingdom.

          The flooding of the Nile is the result of the yearly monsoon between May and August causing enormous precipitations on the Ethiopian Highlands whose summits reach heights of up to 4550 m (14,928 ft). Most of this rainwater is taken by the Blue Nile and by the Atbarah River into the Nile, while a less important amount flows through the Sobat and the White Nile into the Nile. During this short period, those rivers contribute up to ninety percent of the water of the Nile and most of the sedimentation carried by it, but after the rainy season, dwindle to minor rivers.

          These facts were unknown to the ancient Egyptians who could only observe the rise and fall of the Nile waters. The flooding as such was foreseeable, though its exact dates and levels could only be forecast on a short term basis by transmitting the gauge readings at Aswan to the lower parts of the kingdom where the data had to be converted to the local circumstances. What was not foreseeable, of course, was the extent of flooding and its total discharge.

          The Egyptian year was divided into the three seasons of Akhet (Inundation), Peret (Growth), and Shemu (Harvest). Akhet covered the Egyptian flood cycle. This cycle was so consistent that the Egyptians timed its onset using the heliacal rising of Sirius, the key event used to set their calendar.

          The first indications of the rise of the river could be seen at the first of the cataracts of the Nile (at Aswan) as early as the beginning of June, and a steady increase went on until the middle of July, when the increase of water became very great. The Nile continued to rise until the beginning of September, when the level remained stationary for a period of about three weeks, sometimes a little less. In October it often rose again, and reached its highest level. From this period it began to subside, and usually sank steadily until the month of June when it reached its lowest level, again. Flooding reached Aswan about a week earlier than Cairo, and Luxor 5 – 6 days earlier than Cairo. Typical heights of flood were 45 feet (13.7 metres) at Aswan, 38 feet (11.6 metres) at Luxor (and Thebes) and 25 feet (7.6 metres) at Cairo.

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          1200 years ago, the Mayan civilization thrived in what are now southern Mexico, Belize and Guatemala. The Mayans were brilliant astronomers and mathematicians, and their society was very stable and established. However, at some point during the 9th century, their civilization suffered a sudden and devastating collapse. Archaeologists have struggled to find an explanation for the Mayans’ fate, but recent studies suggest that a massive drought was responsible. Analysis of mud samples from the bottom of Lake Chichancanab in the Yucatan area of Mexico has found that the region’s climate in the 9th century was the driest that it had been for 7000 years.

           The Maya civilisation, which dominated southern Mexico for hundreds of years, appears to have been brought to its knees at least in part by a series of severe, decades-long droughts, scientists say. Conditions were so bad, says Nicholas Evans, a geochemist at the University of Cambridge, UK, that rainfall decreased by 50% on average. During the worst periods, he says, it decreased by up to 70%. The drought was further exacerbated by a 2-to-7% drop in relative humidity, his team found.

          The climate shift coincided with an era called the Terminal Classic Period, between 800 and 1000 CE, when the Maya civilisation was in decline and permanently abandoned many of its cities. The idea that drought may have contributed to this collapse isn’t new. “[It] has been debated for at least 100 years,” says Christopher Baisan, a dendrochronologist, or tree-ring scientist, at the US University of Arizona’s Laboratory of Tree-Ring Research, who was not involved in the new study.

          But just how severely the climate had changed was not clear. All that was really known was that it was drier than at the height of Maya influence. Evans’ team took core samples of sediments in a lake in the central Yucatan peninsula. “These sediments contain muds,” Evans says, “but importantly, they also contain a mineral known as gypsum.”

          Gypsum is a crystal that precipitates out of water when the mineral content grows too large — something that can occur during a drought. It is predominately composed of calcium and sulfate, but it also includes trapped water molecules.

          By examining hydrogen and oxygen isotopes in these molecules and comparing them to water in lake today, Evans says, scientists can chart changes in the lake. From these, he says, it’s possible to deduce variations in rainfall patterns.

          The result isn’t perfect. To begin with, gypsum only forms during periods of drought, when minerals become concentrated enough to precipitate to the bottom. Also, the isotope levels of the trapped water reflect multi-year averages of climate conditions in and around the lake, not an instantaneous measure.

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          Three years after his retreat from Russia, Napoleon faced the allied forces of Britain and Prussia at Waterloo. Again, the weather was to play its part. Very heavy rain in the region made the ground muddy, which delayed Napoleon’s attack. The delay meant that the allies, under the leadership of the Duke of Wellington, were able to send in additional troops and supplies, which ultimately helped them to victory.

          Two months before Napoleon’s historic defeat at Waterloo, a volcanic eruption in Indonesia caused heavy rains in Europe that soon succeeded in bringing him down.

          The defeat of French emperor Napoleon Bonaparte at the Battle of Waterloo in 1815 is widely believed to be due to the inclement weather in England. But a new study suggests that Napoleon’s misfortune with the rain and mud was caused by a massive volcanic eruption in Indonesia two months prior to the battle.

          On the night before Napoleon’s final battle, heavy rains flooded the Waterloo region of Belgium and as a result, the French Emperor elected to delay his troops. Napoleon was worried that the soggy ground would slow down his army.

          While that might have been viewed as a wise choice on Napoleon’s part, the extra time allowed the Prussian Army to join the British-led Allied army and help defeat the French. 25,000 of Napoleon’s men were killed and wounded, and once he returned to Paris, Napoleon abdicated his rule and lived the rest of his life in exile on the remote island of Saint Helena.

          And none of that may have happened if not for one of the largest volcanic eruptions in history. The eruption of Mount Tambora could be heard from up to 1,600 miles away with ash falling as far as 800 miles away from the volcano itself. For two days after the explosion, the 350-mile region that surrounds the mountain was left in pitch darkness.

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          Napoleon Bonaparte was one of the finest military leaders in history. His clever tactics brought a series of victories that allowed him to rule over large parts of Europe over 200 years ago. However, it was the weather that was to prove instrumental in his downfall. He invaded Russia in the summer of 1812 and captured Moscow, following the Russians deeper into the country. By November, a lack of supplies forced Napoleon and his army to retreat, and the extremely harsh winter killed many thousands of troops as they returned to France.

          In the year 1812, the infamous Napoleon assembled the largest army Europe had ever seen, more than 600 000 men strong. His plan was to march into Russia, and his last concern was the approaching winter chills. Napoleon confidently captured Moscow; his soldiers pillaged the city, stealing jewels, furs, and war prizes. However, it was too soon to be celebrating – since Napoleon had failed to consider how very cold Russia can be. As Napoleon’s army marched away with their prizes, temperatures dropped to minus 40 degrees Celsius. Many soldiers died of frostbite and starvation, and in one 24-hour period 50 000 horses died from the cold – leaving men to struggle on foot through the icy environment. Even with their stolen furs to wrap themselves up in – of the 600 000 men who marched into Russia, only 150 000 limped home. This was the beginning of the end for Napoleon’s empire, and heralded the emergence of Russia as a power in Europe.

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          Throughout history, the weather has had a major influence on the outcome of certain events. Adverse weather conditions have helped decide the outcome of battles and military campaigns, while over longer periods of time, climate change is thought to have brought about the end of some civilizations and the beginning of others.

          While searching for some topic of interest to bumble on about in this blog, I remembered an article I read ages ago that left an impression. Maybe the weather is something that most of us at CSAG think about on a daily basis (I hope), but it is interesting to hear how the weather has helped shaped history – and thus the societal world we live in.  As will be discussed shortly, the weather can be a huge deciding point in what happens when, and it is interesting to hear about events that may or may not have happened because of weather conditions (and I’m not talking about a picnic at Kirstenbosch event).

          On the 6th August 1945 it was a fine summer day in Hiroshima. At 7:09am a weather reconnaissance plane passed overhead and radioed back: “Cloud cover less than three-tenths. Advice: bomb primary.” Thus, the sky was clear enough to drop the first nuclear weapon used in war. The lack of cloud cover sealed Hiroshima’s fate, and spared the back-up target. Even more dramatic was the effect of cloud cover on Kokura. On the 8th August 1945, the second nuclear weapon was loaded into a B-29, however the skies were overcast over the primary target, Kokura. Instead, the bomb was released over the backup target: Nagasaki.

          In the 13th century, Kublai Khan, leader of the Mongol Empire, set his sights on the conquest of Japan, but was defeated by not one, but two monsoons. Shinto priests, who believed the storms were the result of prayer, called them kamikaze or “divine wind.”

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          The damage caused to crops by large hailstones has prompted many attempts to prevent hail forming. Techniques similar to those used in cloud seeding have been tried, aiming to turn hailstones into rain, but this does not seem to work. In the early 20th century, people tried using “anti-hail guns”. These would fire huge amounts of debris into the clouds in an attempt to break up the hailstones. They were tried many times, unsuccessfully, in the vineyards of France.

          A Hail cannon is a shock wave generator claimed to disrupt the formation of hailstones in the atmosphere.

          These devices frequently engender conflict between farmers and neighbors when used, because they are repeatedly fired every 1 to 10 seconds while a storm is approaching and until it has passed through the area, yet there is no scientific evidence for their effectiveness.

          In the French wine-growing regions, church-bells were traditionally rung in the face of oncoming storms and later replaced by firing rockets or cannons.

          A mixture of acetylene and oxygen is ignited in the lower chamber of the machine. As the resulting blast passes through the neck and into the cone, it develops into a shock wave. This shock wave then travels at the speed of sound through the cloud formations above, a disturbance which manufacturers claim disrupts the growth phase of hailstones.

          Manufacturers claim that what would otherwise have fallen as hailstones then falls as slush or rain. It is said to be critical that the machine is running during the approach of the storm in order to affect the developing hailstones, although all manufacturers unanimously agree that the area of effect of their device is only 100 to 200 square meters directly above.

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          The next generation of lightning conductor could be a type of laser gun. A laser beam fired from the ground into a storm cloud could charge the air molecules along the way, creating a path for the lightning bolt to follow. Once the lightning is set on a direct path, its charge can be neutralized. It is thought that such a device could be used to steer lightning away from exposed structures such as power lines.

          Thousands of lightning bolts strike the Earth’s surface roughly every couple of seconds, but despite their ubiquity, this phenomena is somewhat poorly understood. Lightning is also unpredictable. While humans have been placing lightning rods for centuries to increase the probability of striking in a certain fixed point, its path cannot be controlled. That may be true in nature, but in the confinement of a lab of the INRS Energie Materiaux Telecommunications research centre (Varennes, QC, Canada), scientists have defied this common knowledge and used lasers to coax lighting to follow a predefined path.

          Lighting is one of the most powerful forces found in nature (if one single lightning strike was harnessed, the energy would power an entire home for a whole week), but at its core we can say that lightning is nothing but a discharge of static electricity. What we know from static electricity is that these discharges are caused by separation of charges into positive and negative ions.  Over time more of one charge builds until its natural attraction to the opposite charge causes it to migrate in an electrical discharge. In the case of lightning, the charge is built up in water.

          So, when you discharge static electricity between two tiny electrodes that’s basically a mini lightning strike – a couple of million volts short of the real deal discharged in thunder clouds. Electric arcs are used for all kinds of applications, from things as simple as ignition in a vehicle, to pollution control, to micromachining. Now, if you could also control the path of electric arc, then a slew of other potential applications could open up.

          One first baby step was made by the team at Advanced Laser Light Source facility, INRS. Their experiment was based on the self-healing properties of certain laser beams. When a laser beam is obstructed by an object, it can sometimes reconstruct its intensity once past the object. Using various laser shapes, like Airy beams and Bessel beams, the researchers guided electrical discharges and effectively controlled the path of mini lightning bolts, as described in Science advances.

          “Our fascination with lightning and electric arcs aside, this scientific discovery holds out significant potential and opens up new fields of research,” said Yves Begin, vice dean of research and academic affairs  at INRS. “This spectacular proof of concept, which was conducted over a distance of a few centimetres, required the high-power lasers, state-of-the-art facilities, and extraordinary research environment that our professors helped to create at INRS. Being able to work in such cutting-edge labs enables our students and postdoctoral fellows to embark on the path of scientific discovery even while still in school.”

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          Scientists believe that it may be possible to “kill” a tornado. Space satellites could be used to fire beams of microwave energy towards the base of a thunderstorm. The theory is that this would heat up the cool downdraft of air that helps create the tornado, effectively knocking it out. This sounds very much like science fiction, and many scientists claim that it could never work.

          The most intense tornadoes emerge from what are called supercell thunderstorms. For such a storm to form, you first “need the ingredients for a regular thunderstorm,” says Brooks. Those ingredients include warm moisture near the surface and relatively cold, dry air above. “The warm air will be buoyant, and like a hot-air balloon it will rise,” says Brooks.

          A supercell requires more: winds that increase in strength and change direction with height. “Then the updraft tends to rotate, and that makes a supercell,” explains Brooks. The supercell churns high in the air and, in about 30 percent of cases; it leads to the formation of a tornado below it. This happens when air descending from the supercell causes rotation near the ground.

          Even then, “we still don’t know why some thunderstorms create tornadoes while others don’t,” tornado-chaser Tim Samaras said in early 2013. Samaras was a scientist and National Geographic grantee who was killed by a twister on May 31, 2013, in El Reno, Oklahoma.

          Brooks says scientists believe strong changes in winds in the first kilometer of the atmosphere and high relative humidity are important for the formation of tornadoes. He adds that there also needs to be a downdraft in just the right part of the storm.

          Tornado formation also requires a “Goldilocks” situation, in which air must be cold but not too cold. It should be a few degrees more frigid than surrounding air, Brooks says.

          He adds, “We don’t understand how tornadoes die: Eventually the air gets too cold and it chokes off the inflow of new air into the storm, but we don’t know the details.”

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          Cloud seeding is a scientific process that makes clouds produce rain and snow. It works by sending tiny particles of silver iodide, or other substances such as dry ice or liquid propane, into rain-bearing clouds, usually by aircraft. These substances stimulate the production of rain by providing something for water droplets to freeze on to — scientists call them ice nuclei. Once enough of the droplets take hold, they become heavy enough to fall to the ground. Cloud seeding cannot produce clouds — it can only make existing clouds produce rain.

          Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud. The usual intent is to increase precipitation (rain or snow), but hail and fog suppression are also widely practised in airports where harsh weather conditions are experienced. Cloud seeding also occurs due to ice nucleators in nature, most of which are bacterial in origin.

          The most common chemicals used for cloud seeding include silver iodide, potassium iodide and dry ice (solid carbon dioxide). Liquid propane, which expands into a gas, has also been used. This can produce ice crystals at higher temperatures than silver iodide. After promising research, the use of hygroscopic materials, such as table salt, is becoming more popular.

          In mid-altitude clouds, the usual seeding strategy has been based on the fact that the equilibrium vapor pressure is lower over ice than over water. The formation of ice particles in supercooled clouds allows those particles to grow at the expense of liquid droplets. If sufficient growth takes place, the particles become heavy enough to fall as precipitation from clouds that otherwise would produce no precipitation. This process is known as “static” seeding.

          Seeding of warm-season or tropical cumulonimbus (convective) clouds seeks to exploit the latent heat released by freezing. This strategy of “dynamic” seeding assumes that the additional latent heat adds buoyancy, strengthens updrafts, ensures more low-level convergence, and ultimately causes rapid growth of properly selected clouds.

          Cloud seeding chemicals may be dispersed by aircraft or by dispersion devices located on the ground (generators or canisters fired from anti-aircraft guns or rockets). For release by aircraft, silver iodide flares are ignited and dispersed as an aircraft flies through the inflow of a cloud. When released by devices on the ground, the fine particles are carried downwind and upward by air currents after release.

          An electronic mechanism was tested in 2010, when infrared laser pulses were directed to the air above Berlin by researchers from the University of Geneva. The experimenters posited that the pulses would encourage atmospheric sulfur dioxide and nitrogen dioxide to form particles that would then act as seeds.

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          The Appearance of blossom on trees is traditionally said to mark the beginning of spring and the end of winter. Trees do only come into bloom in mild weather, but as anyone who lives in a temperate part of the world knows, cold weather will often return after the blossom appears! The dates on which blossom appears have been recorded in some weather records for many years, which helps to show what the weather was like in the past.

          The cherry blossoms will be blooming again soon around the Tidal Basin in Washington, D.C., as they have every year since Japan shipped its arboreal gift of more than 3,000 cherry trees to the United States in 1912. And for this year, anyway, climate change won’t have much effect on the timing.

          The projected peak days for 2019 are right around the historic average of the past several decades, according to the park service. The agency determined the projected dates after analyzing a variety of data, including winter temperatures and the forecast for March, according to NPS acting superintendent Jeffrey Rein bold. He said that the development of the blossoms will depend on variable weather conditions.

          Experts at the park service and elsewhere say that local conditions of daylight and heat are the main factors that determine the blooming time in temperate ecosystems. However, they say that although bloom times can vary from year to year because of those local conditions, the long-term trends clearly show the impact of climate change on the trees in Washington, D.C., and elsewhere.

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          Pine Cones make one of the best natural weather indicators. The scales of a pine cone open out when the weather is dry and close up when the air is humid — a good sign that rain is coming. The natural state of the cone is closed — the scales are shrivelling up when it is dry. When the air is moist, the cone becomes flexible again and returns to its regular shape.

          There are two main types of superstitions tying plants and animals to weather forecasting: Those that imply that the flora and fauna “know” what the coming season (typically winter), will bring, and those that rely on the physics, chemistry and biology of living things responding to changing conditions. The former generally don’t hold up — plants and animals react to their past or present environments, they don’t predict the future — but there’s definitely hope for the latter.

          For example, some say that a profusion of pine cones in fall means a cold winter to follow. This one’s a bust: Actually, pine trees can take three years to fully grow cones, and varying their cone production from year to year helps them throw off predators. However, you can use pine cones to predict weather in another way: watching as they open or close.

          Pine cones are the procreative parts of pine trees. Male versions produce pollen, and pollenated female forms yield seeds. Under dry conditions, the outer parts of the cones’ scales dry more than the inner parts, causing the cone to open. This is good news for the tree, since dry, calm weather provide a better environment for seed dispersal. In wet weather, the scales absorb moisture and swell shut, shielding the seeds until better days roll around.

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          A piece of seaweed is an excellent indicator of humidity. Any moisture in the seaweed evaporates when the air is dry, making it brittle and hard to the touch. When humidity levels increase, the seaweed absorbs moisture again, making it expand and become soft. A high level of moisture in the air is a sure sign that rain will follow soon after. Pieces of seaweed are often seen hanging outside the houses in seaside towns.

          The tradition is to hang dried seaweed outside with a nail. If the seaweed stays dry the weather will be sunny and dry. If the seaweed is wet and flexible, as if it had just been from the ocean, then rain is coming.

          The origin of this tradition is unknown, but the kernel of truth is valid. Seaweeds are able to dehydrate and re-hydrate over and over. If there is enough moisture in the air the dried seaweed will re-hydrate. It turns out that moisture in the air can be a decent indicator of rain.

          Over the weekend, people in the USA sought weather advice from a rodent. Groundhog Day (Feb. 2nd) is a superstitious tradition where if a groundhog emerging from its burrow on this day sees its shadow due to clear weather, it will retreat to its den and winter will persist for six more weeks, and if it does not see its shadow because of cloudiness, spring will arrive early.  There are many different superstitions across cultures to predict weather; some have a kernel of truth while others are outright ridiculous.

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          It is often said that when cows lie down in a field, rain is on the way. This piece of folklore is based on the idea that the cows can sense dampness in the air, so they lie down to make sure they have a dry space to stay. As much as this saying is well known, it is also rarely accurate. Cows will lie down when they are tired, not just when they think it might rain, so they are probably not the best weather forecasters!

          Animal behavior has long been a favorite weather indicator, and a whole herd of superstitions crowd around cows. It’s said, for example, that a cow fed its own hairs will forget its previous home, or that a cow with a piece of its tail lopped off will never run away [source: Farmer’s Almanac].

          Cows have a long history as weather predictors, too. One superstition claims that a cow lies down when rain is coming. Given that cows lie down for a variety of reasons, including cud chewing, it’s tempting to dismiss this claim as “udderly” ridiculous, but further rumination suggests that it might have a leg to stand on after all. The reason? A possible, albeit tenuous, link between crouching cows and wet weather: body heat.

          It turns out that cows tend to stand more often when their bodies overheat, so an upright Guernsey could arguably mean hotter weather while a seated shorthorn implies cooling weather or a storm a’ brewin’. Still, we wouldn’t bet the farm on it, as this maxim is likely a case of over-milking a coincidence.

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          The saying “red sky at night, shepherd’s delight; red sky in morning, shepherd’s warning” probably originated in Europe. There, prevailing winds bring weather systems from the west, so a clear red sky at sunset is thought to indicate the arrival of good weather. A similar sight in the eastern skies at dawn could suggest that the fine weather is passing away.

          The complete saying states: Red sky in morning, sailors warning; red sky at night, sailors delight. This saying only applies to mid-latitude locations (winds are easterly in the tropics / in the high latitudes the sun rises and sets at a large deviation from the east-west trajectory). Storm systems in the middle latitudes generally move west to east. A red sky in the morning implies the rising sun in the east is shining on clouds to the west and conditions are clear to the east. Clouds moving from the west (especially upper level cirrus) indicate an approaching storm system. A red sky at night implies the sun (setting in the west) is shining on clouds to the east and conditions are clear to the west (because the sun can be seen setting). If you can see the sunset, the sky will be redder. Clouds to the east indicate an existing storm system in the middle latitudes. Upper level clouds (especially cirrus) are noted for giving the sky a reddish hue during dawn or dusk. As a mid-latitude cyclone approaches, it is the upper level cirrus that are seen first, followed by lower clouds. The approach of upper level cirrus from the west often indicates an approaching storm system. The sky will not be as red at night if a storm system is approaching because the sun is setting behind the clouds approaching from the west. A red sky at night implies “the storm system moving through has ended!”; The clouds have broken and the sun is shining on and reddening the exiting clouds. The sun will continue to shine on clouds for a period of time after the sun has dipped below the horizon (especially cirrus). Keep in mind this saying was developed before satellite, radar and modern meteorological knowledge. Much of the knowledge of an approaching storm system back then was cloud and wind patterns. Of course, this saying (weather folklore) has some profound problems such as:

(1) The sky can be “reddish” near the sun at dawn and dusk (with or without clouds)
(2) Storm systems do not always move straight west to east

(3) Cirrus can occur without a storm system approaching or leaving. Clouds can cover one side of the sky or the other without being directly associated with a storm system.

(4) The meteorological sailor may delight at an approaching storm system even while taking precautions at the same time

(5) Rarely do the clouds from an approaching or exiting storm system only cover 1/2 of the eastern or western sky. The saying represents an ideal case.

SUMMARY: If you can see the sunrise but the west part of the sky is dark: look out for approaching bad weather. If you can see the sunset: the weather conditions will be nice.

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          People have been forecasting the weather for thousands of years, based on changes seen in the world around them. Many such observations are little more than folklore and superstition, but it is true that certain plants and animals can detect variations in the air that people cannot, providing us with a natural sign of a change in the weather.

          When it comes to predicting the weather by using nature signs, old civilizations were masters in doing it. If you’re on a wilderness trip without checking the weather forecast before, there are plenty of ways to read the language of nature and tell what the weather will be like in the next hours and even days.

          It’s of extreme importance to know what climate you are going to face on a wild adventure because you need to stay safe and take precautions if there’s a storm coming closer. Predicting weather is a crucial skill that every hiking and backpacking enthusiast should learn.

         Reading the height and the shape of the clouds is something you must learn if you want to have weather accuracy. High and white clouds mean a sunny and bright day. Black and low clouds are a sign of a rainstorm. A grey veil formed by clouds means that you need to run to a shelter.

        A red sky at dusk and dawn is one of the most beautiful views that we all enjoy, especially near the beach. If the sky is red at dusk, it means that the next day will be sunny. On the contrary, a red sky at dawn means that the low pressure air is bringing moisture and rain.

        Ancient people closely observed the behavior of animals to predict weather changes. Watch animals to see how their behavior changes with the weather change. For example, insect-eating birds like swallows fly low right before the rain, and ants tend to build their anthills with higher, steeper sides. Also, if you notice that the bees and butterflies disappear from the flowers they usually visit; it means the storm is coming.

       The flower’s scent is strongest just before the rain because aromas spread more when there is moisture in the air. You may also notice the sweet, sharp smell of ozone just before a summer storm as it is carried to lower altitudes by the storm’s downdraft. So, if you notice an unusually strong smell, it often means rain is on the way.

       If you see milky-white rings around the Sun or Moon, it’s a sign of extreme humidity and moisture coming closer to the Earth’s surface.

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            At Sea, weather conditions are monitored by ships, which take measurements of pressure and sea and air temperatures. Ships may be used to launch weather balloons. Free-floating buoys are also used to collect weather data. They drift with ocean currents, transmitting sea-level weather details to satellites. They are much less expensive to maintain than specialist weather ships.

            Marine weather forecasting is the process by which mariners and meteorological organizations attempt to forecast future weather conditions over the Earth’s oceans. Mariners have had rules of thumb regarding the navigation around tropical cyclones for many years, dividing a storm into halves and sailing through the normally weaker and more navigable half of their circulation. Marine weather forecasts by various weather organizations can be traced back to the sinking of the Royal Charter in 1859 and the RMS Titanic in 1912.

            The wind is the driving force of weather at sea, as wind generates local wind waves, long ocean swells, and its flow around the subtropical ridge helps maintain warm water currents such as the Gulf Stream. The importance of weather over the ocean during World War II led to delayed or secret weather reports, in order to maintain a competitive advantage. Weather ships were established by various nations during World War II for forecasting purposes, and were maintained through 1985 to help with transoceanic plane navigation.

            Voluntary observations from ships, weather buoys, weather satellites, and numerical weather prediction have been used to diagnose and help forecast weather over the Earth’s ocean areas. Since the 1960s, numerical weather prediction’s role over the Earth’s seas has taken a greater role in the forecast process. Weather elements such as sea state, surface winds, tide levels, and sea surface temperature are tackled by organizations tasked with forecasting weather over open oceans and seas. Currently, the Japan Meteorological Agency, the United States National Weather Service, and the United Kingdom Met Office create marine weather forecasts for the Northern Hemisphere.

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            Satellites serve two purposes in weather forecasting. Communications satellites are used to send weather data around the world, while dedicated weather satellites monitor the movement of weather systems and the patterns of cloud cover: There are two types of weather satellite. Geostationary satellites are fixed in one position, observing a certain area from their orbit high above the Equator. Polar-orbiting satellites circle the Earth from pole to pole. The planet’s rotation means that each orbit takes in a different part of the Earth.

            In order to predict the weather accurately for the hours and days ahead weather forecasters must analyze the information they receive from number of sources – including local weather observes, weather balloons, weather stations, and satellites.

           Speaking of satellites – NASA has a train of satellites called the Afternoons Constellation, nicknamed the “A”-Train, which are orbiting the Earth and are collecting all sorts of data. This includes data that will help predict weather and climate change.

            For example NASA has the Cloudsat satellite for studding the aspect of clouds, and Caplipso for studding the climate change, and how aerosols and particles affect the Earth’s atmosphere.

            Many businesses and people rely on accurate weather conditions for different purposes – farmers need to know the best time to plant they crops; airplane takeoffs, landings, and flight paths are scheduled according to local weather conditions. Weather forecast alert people for severe storms that can be danger to personal property. Most people want to know what the weather will be like as they go to and from work, school, or plan outdoor activities.

           The atmosphere is constantly changing and even though the scientist receive weather data from variety of sources – stations, satellites, observers, and balloons – it is still impossible to predict the weather correctly 100% of the time.

            One of the ways of improving weather prediction and climate is to develop new technologies that helps scientists understand how the atmosphere works.

            One such tech is the NASA’s Cloudsat satellite. This instrument provides a vertical cloud profiling from space improving weather and climate forecast.

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            Computers are used to collect weather information and also to help meteorologists predict the weather. Special software uses the data to develop a “model” of the expected weather.

            Throughout history, numerous techniques and experiments were performed by meteorologists to predict the weather with greater efficacy over time. Due to substantial advances in technology, it is now possible to forecast the weather days and even months in advance—which was not truly possible before the mid-20th century. The use of computer models became widespread mainly throughout the 1960s, as the first weather satellites were launched. The types of computer models that are used in forecasts depend mostly on the type of climate and weather conditions.

            Climate models are primarily used to forecast substantial changes in the earth’s climate. Climate is the average weather conditions in an area for a prolonged period of time. Therefore, climate models use a combination of statistical and current data to provide a reasonable forecast. The CFS is one of the primary climate models used for forecasting planetary scale weather conditions such as: El Nino, Madden Julian Oscillations (MJO), and monsoons.

            Statistical models are primarily used to help meteorologist provide accurate analog forecasts. Statistical models use data from previous storms and weather conditions to help meteorologists get a better idea of how to track current weather systems. Statistical models are commonly used to track tropical and mid latitude cyclones. If the dynamical model consensus is not reasonable, meteorologists often use statistical models to provide better forecasts.

            Although atmospheric computer models are effective tools for weather forecasting, they are not impeccably accurate. Computer models are usually less efficient during the preliminary runs. For instance during the first stages of tropical cyclogenesis (tropical cyclone formation), computer models are usually not initialized enough to provide a reasonable forecast. Long range forecasts (beyond a week) are usually less accurate, because there are many atmospheric factors that can come into play beyond that time. Dynamic models are most accurate for three- to five-day forecasts.

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            Weather data in remote areas is collected by automated weather stations. Equipped with a wide range of instruments and computers, the stations record and transmit information via satellite every hour. Individual observers with a small number of simple instruments also play an important part in all levels of weather forecasting.

            A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, atmospheric pressure, humidity, wind speed, wind direction, and precipitation amounts. Wind measurements are taken with as few other obstructions as possible, while temperature and humidity measurements are kept free from direct solar radiation, or insolation. Manual observations are taken at least once daily, while automated measurements are taken at least once an hour. Weather conditions out at sea are taken by ships and buoys, which measure slightly different meteorological quantities such as sea surface temperature (SST), wave height, and wave period. Drifting weather buoys outnumber their moored versions by a significant amount.

Typical weather stations have the following instruments:

• Thermometer for measuring air and sea surface temperature
• Barometer for measuring atmospheric pressure
• Hygrometer for measuring humidity
• Anemometer for measuring wind speed
• Pyranometer for measuring solar radiation
• Rain gauge for measuring liquid precipitation over a set period of time.
• Wind sock for measuring general wind speed and wind direction
• Wind vane, also called a weather vane or a weathercock: it shows whence the wind is blowing.

In addition, at certain automated airport weather stations, additional instruments may be employed, including:

• Present Weather/Precipitation Identification Sensor for identifying falling precipitation
• Disdrometer for measuring drop size distribution
• Transmissometer for measuring visibility
• Ceilometer for measuring cloud ceiling

More sophisticated stations may also measure the ultraviolet index, leaf wetness, soil moisture, soil temperature, water temperature in ponds, lakes, creeks, or rivers, and occasionally other data.

Exposure

           Except for those instruments requiring direct exposure to the elements (anemometer, rain gauge), the instruments should be sheltered in a vented box, usually a Stevenson screen, to keep direct sunlight off the thermometer and wind off the hygrometer. The instrumentation may be specialized to allow for periodic recording otherwise significant manual labour is required for record keeping. Automatic transmission of data, in a format such as METAR, is also desirable as many weather station’s data is required for weather forecasting.

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            Research Aircraft are used to obtain detailed information about the atmosphere. They carry very sophisticated radar and laser equipment that records a three-dimensional picture of clouds at various levels in the atmosphere. Some planes are dedicated to monitoring hurricanes, often flying into the centre of the storm itself. The information collected by aircraft is much more detailed than that collected by weather balloons.

            A recent example of unexpected utility is the meteorological application of transponders developed for air-traffic management. Accurate wind information for the upper atmosphere is a key requirement for weather prediction. Currently, most wind data comes from weather balloons, wind profilers, Doppler radars and satellites. Mode-S EHS, a novel source of wind data from aircraft flight levels, is helping us to make more accurate weather forecasts.

            Under European regulations, all large aircraft must carry Mode-S EHS- enhanced surveillance navigation apparatus. Aircraft equipped with EHS transponders are interrogated every four seconds by ground-based radar and, in response, send information on position, flight level, magnetic heading, air speed and ground speed. Air-traffic control monitors this data to ensure efficient and safe operations.

            Suppose a plane is heading eastwards at 200m per second. Its position is known accurately by the satellite-based global positioning system or GPS. Four seconds later, it should be 800m east of its initial position. But suppose there is a wind of 50m per second from the southeast. This will slow the plane and cause it to drift to the north.

            The GPS location shows precisely where the plane has gone in four seconds and determines the ground speed. Since ground speed (G) is the vector sum of air speed (A) and wind speed (W), a simple vector calculation gives us the wind. What a delightfully simple application of vectors; what a shame that vectors have been dropped from Leaving Cert maths.

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            Weather balloons are used to take measurements of humidity, pressure and temperature at altitudes of up to 20km (12 miles). The readings are taken by instruments called radiosondes carried beneath the balloon. These transmit the information to processing stations on the ground. Wind strength and direction is monitored by tracking the movement of the balloon.

            A weather or sounding balloon is a balloon (specifically a type of high-altitude balloon) that carries instruments aloft to send back information on atmospheric pressure, temperature, humidity and wind speed by means of a small, expendable measuring device called a radiosonde. To obtain wind data, they can be tracked by radar, radio direction finding, or navigation systems (such as the satellite-based Global Positioning System, GPS). Balloons meant to stay at a constant altitude for long periods of time are known as transosondes. Weather balloons that do not carry an instrument pack are used to determine upper-level winds and the height of cloud layers. For such balloons, a theodolite or total station is used to track the balloon’s azimuth and elevation, which are then converted to estimated wind speed and direction and/or cloud height, as applicable.

            One of the first persons to use weather balloons was Léon Teisserenc de Bort, the French meteorologist. Starting in 1896 he launched hundreds of weather balloons from his observatory in Trappes, France. These experiments led to his discovery of the tropopause and stratosphere. Transosondes, weather balloons with instrumentation meant to stay at a constant altitude for long periods of time to help diagnose radioactive debris from atomic fallout, were experimented with in 1958.

            Weather balloons are launched around the world for observations used to diagnose current conditions as well as by human forecasters and computer models for weather forecasting. About 800 locations around the globe do routine releases, twice daily, usually at 0000 UTC and 1200 UTC. Some facilities will also do occasional supplementary “special” releases when meteorologists determine there is a need for additional data between the 12-hour routine launches in which time much can change in the atmosphere. Military and civilian government meteorological agencies such as the National Weather Service in the US typically launch balloons, and by international agreements almost all the data are shared with all nations.

            Specialized uses also exist, such as for aviation interests, pollution monitoring, photography or videography and research. Examples include pilot balloons (Pibal). Field research programs often use mobile launchers from land vehicles as well as ships and aircraft (usually dropsondes in this case). In recent years weather balloons have also been used for scattering human ashes at high-altitude. The weather balloon was also used to create the fictional entity ‘Rover’ during production of the 1960s TV series The Prisoner in Portmeirion, Gwynedd, North Wales, UK in September 1966. This was retained in further scenes shot at MGM Borehamwood UK during 1966-67.

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            Meteorologists forecast the weather based on information gathered from a huge variety of sources. To get the clearest picture about the weather, both people and technology are employed around the world to continuously take weather measurements. Instruments on land, at sea, in the air and in space feed the information into a global network, where it is accessed and analyzed by the world’s weather experts.

            Meteorologists use a variety of tools to help them gather information about weather and climate. Some more familiar ones are thermometers which measure air temperature, anemometers which gauge wind speeds, and barometers which provide information on air pressure. These instruments allow meteorologists to gather data about what is happening near Earth’s surface. Collecting data from other sources—and other parts of the atmosphere—helps to create a more descriptive picture of weather.

            Meteorological phenomena are observable weather events that are explained by the science of meteorology. Meteorological phenomena are described and quantified by the variables of Earth’s atmosphere: temperature, air pressure, water vapour, mass flow, and the variations and interactions of those variables, and how they change over time. Different spatial scales are used to describe and predict weather on local, regional, and global levels.

           Meteorology, climatology, atmospheric physics, and atmospheric chemistry are sub-disciplines of the atmospheric sciences. Meteorology and hydrology compose the interdisciplinary field of hydrometeorology. The interactions between Earth’s atmosphere and its oceans are part of a coupled ocean-atmosphere system. Meteorology has application in many diverse fields such as the military, energy production, transport, agriculture, and construction.

            The ability to predict rains and floods based on annual cycles was evidently used by humans at least from the time of agricultural settlement if not earlier. Early approaches to predicting weather were based on astrology and were practiced by priests. Cuneiform inscriptions on Babylonian tablets included associations between thunder and rain. The Chaldeans differentiated the 22° and 46° halos.

            Ancient Indian Upanishads contain mentions of clouds and seasons. The Samaveda mentions sacrifices to be performed when certain phenomena were noticed. Var?hamihira’s classical work Brihatsamhita, written about 500 AD, provides evidence of weather observation.

            In 350 BC, Aristotle wrote Meteorology. Aristotle is considered the founder of meteorology. One of the most impressive achievements described in the Meteorology is the description of what is now known as the hydrologic cycle.

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            Meteorologists draw up special weather maps called synoptic charts to show a forecast. The long curved lines — isobars — show areas of equal pressure. Black circles mark the centre of low- and high-pressure areas. Lines of red semicircles indicate a warm front, and a cold front is shown by a line of blue triangles. A combination of triangles and semicircles indicates an occluded front. Ideally, all the observations shown on a synoptic chart should be made at the same time (“synoptic” means “seen together”), but this is rarely possible, so slight variations must be taken into account when interpreting a chart. The synoptic chart illustrated below shows a weather system over north-west Europe.

            The word ‘synoptic’ simply means a summary of the current situation. In weather terms, this means the pressure pattern, fronts, wind direction and speed and how they will change and evolve over the coming few days.Temperature, pressure and winds are all in balance and the atmosphere is constantly changing to preserve this balance. This is why the UK sees such changeable weather.

         The circular lines you see on the chart are isobars, which join areas of the same barometric pressure. The pressure pattern is important because we can use it to tell us where the wind is coming from and how strong it is. It also shows areas of high and low pressure.

            Air moves from high to low pressure along a gradient (similar to squash that is left in a glass of water becoming evenly distributed as it becomes less concentrated). If the difference between areas of high and low pressure is greater then we have a large gradient and the air will move faster to try and balance out this difference. This is shown on a synoptic chart with isobars that are very close together and we feel strong winds as a result.

            In terms of the wind direction, air moves around high pressure in a clockwise direction and low pressure in an anticlockwise direction, so isobars also tell us the direction and speed of the wind.

Cold fronts and warm fronts

            Also on a synoptic chart are the lines, triangles and semi-circles representing ‘fronts’. With the atmosphere trying to balance temperature, pressure and wind there are different sorts of air, known as air masses, circulating around the Earth. The differences are mostly between how warm, cold, dry and moist the air is, and fronts simply mark the boundary between these different types of air.

            A warm front is shown with a red line and red semi-circles and a cold front with a blue line and blue triangles. The way in which the semi-circles or triangles point shows the direction in which the front is moving. The position of a front depends on a number of meteorological factors, such as changes in wind direction or temperature, which we get from our network of weather observation sites. A few things to remember are that warm air follows a warm front and cold air follows a cold front. We also tend to see increased amounts of cloud and rainfall along the front itself.

            Sometimes the red or blue line of a front will be broken by crosses. This indicates that the front is weakening and the difference in the warmth or dryness of the air is becoming less marked.

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            Weather Satellites produce images by interpreting different levels of heat and light. When an area is lit by sunlight, different features — clouds, land, sea, ice, and so on — reflect different amounts of light, which are recorded by the satellite as varying shades of grey. When an area is in darkness, heat emissions are recorded by infrared equipment to produce a similar picture. The information is trans-mitted to a base station, where it is converted into images. Television forecasts often put a series of satellite images together to produce a “movie” of a moving weather system.

          IR or infrared satellite imagery is sort of a temperature map. The weather satellite detects heat energy in the infrared spectrum (infrared energy is invisible to the human eye). The satellite image displays objects(whether clouds, water or land surfaces) based on the temperature of the object. Warm temperatures appear in dark shades. Cold temperatures appear in light shades. A temperature scale(in degrees Celsius) is depicted to the left of the image.

           The chief advantage of IR imagery is that it’s not dependent on sunlight. Visible imagery(like the photos you take with a normal camera) relies on sufficient sunlight reflecting off a surface to be viewable. It’s useless at night, but IR imagery relies on emitted heat energy(detectable day or night if you have the right equipment).

            You can infer relative altitudes of clouds from their temperature. Since temperature, in general, decreases with increasing height, high altitude clouds will appear whiter than low altitude clouds.

            A visible satellite image is created by looking only at the visible portion of the light spectrum and is thus only really useful during daylight hours. The Infrared (IR) image comes from the satellite detecting heat energy in the infrared sepectrum and thus does not depend on visible light. For this reason we switch between the visible and IR images at 1500 GMT (8:00am PDT) and 0200 GMT (7:00pm PDT).

            This is a three letter identifier for each station. Example: ‘SFO’ is San Fransisco. Use the station search engine to find the name of the plotted stations (select the “Call Sign” option on the search form before attempting a search).

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            Detailed, short-range weather forecasts can usually be made for the next 24 hours. However, meteorologists today have access to information that enables them to make a fairly accurate, long-range forecast for up to a week ahead.

           One of the country’s leading commercial forecasters, AccuWeather, said earlier this month that it could predict the weather conditions and temperature three months ahead of time. It’s a bold claim that has been refuted by some meteorologists who say such a 90-day forecast will only be as good as historical averages and not much use to someone planning a hike or outdoor wedding this summer.

            That’s because the Earth’s atmosphere is a chaotic system that doesn’t follow an easily predictable path, according to Keith Seitter, executive director of the American Meteorological Society in Boston.

           “If anybody kept track about how (AccuWeather) did, they would find it’s a pretty horrible forecast,” Seitter said.The best weather forecasters can do now is seven to 10 days. After that, accuracy drops off quickly.

            The good news is that forecasting has gotten better over the years. Improvements in computer technology, data collection and weather models have improved this forecasting number about one day each decade.

            One of the biggest advancements has come in boosting computer power. The National Oceanic and Atmospheric Administration, the parent agency of the National Weather Service, operates supercomputers in Reston, Va., (“Luna”) and Orlando, Fla., (“Surge”) to come up with weather forecasts.

            After a $44 million upgrade in January, each one has the capacity of 2.89 petaflops, or 2.89 quadrillion calculations per second, according to Richard Michaud, director of NOAA’s office of central processing. That’s up from 778 teraflops (1 petaflop equals 1,000 teraflops) of computing power last year.

            Better predict the amount, timing and type of precipitation in both winter storms and thunderstorms Create “water forecasts” and more accurately predict drought and floods Connect the air, ocean and waves to track eight hurricanes at once These supercomputers are the brains behind the weather forecasts you see on TV each night or your smartphone when you wake up.

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          Meteorologists and weather forecasters are employed by national and regional weather centres, as well as by organizations such as the military and by airports. They make forecasts based on their knowledge of weather patterns and information received from local, national and global sources .The forecasts are delivered to the public through television, radio, newspapers and the Internet.

          Meteorologists use a process called numerical weather prediction to create forecasts by inputting current conditions — which they call the “nowcast” — into computer models. The more current and accurate information available to these models, the better the forecast will be. Ground radar, weather balloons, aircraft, satellites, ocean buoys and more can provide three-dimensional observations that a model can use. This allows meteorologists to simulate what the atmosphere is currently doing and predict what will happen in the next few days or, for some models, hours.

          Weather models divide a region, say a single state or even the whole globe, into a set of boxes, or cells. The size of these cells — the resolution of the model — affects its forecasting accuracy. Large boxes mean poor resolution, or the inability to tell what’s happening over small areas, but a broad picture of large-scale weather trends over long timelines. This big-picture forecast is helpful when you want to know how a big storm will move across the U.S. over the course of a week.

          Smaller boxes mean higher resolution, which can forecast smaller storms. These models are more expensive in terms of computing power, and only run to the one- or two-day mark to tell people whether it might storm in their local area. Although all models are based on the same physics, each translates those physics into computer code differently, says Judt. Some models might prioritize certain kinds of data — such as wind speed, temperature and humidity — over others to generate predictions, or simulate physical processes slightly differently than another model. That’s why two models might spit out slightly different results, even with exactly the same starting observations.

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          Television weather forecasts are the most easily understood and widely seen source of weather information for the general public. The weather forecasters may be trained meteorologists who work at a weather centre, or television presenters may read out forecasts provided for them. A detailed forecast is presented as a sequence of weather maps generated on a computer. They usually show temperatures and wind speed and direction, and give some indication of the expected weather conditions for different parts of the country. Local television stations will present a more detailed forecast for their region.

          Television is the main medium through which forecasts are viewed. Television stations use a variety of media to portray forecast information. These may be icons, for example showing a sunshine symbol, or contours, for example showing an area which may be affected by rain.

          Broadcasts are available to view on standard television sets. New technology is changing viewing habits and increasingly weather forecasts are viewable through other methods such as via a desktop computer or downloaded onto a mobile mp4 player.

          Forecast data are often displayed on maps of the area of interest. Before assessing the weather forecast for the area they are interested in, the viewer must be able to ascertain where on the map they are located. As revealed in the survey carried out by Thornes (1992) this is something which the public are not generally able to do with confidence. It does seem that an attempt to pinpoint one’s location to within a general area can be made, but more detailed identification of the location is more difficult.

          During the past few years television weather forecast graphics have evolved, from a ‘hand’ drawing on a weather chart, to fully integrated 3D graphics capable of showing fly?throughs of weather, anywhere in the world. When television weather forecasts were first broadcast, the forecaster would often draw expected conditions directly onto a map using a pen. In the 1970s the BBC introduced magnetic symbols which ‘stuck’ to a base map as the forecaster described the changed weather. These symbols are now perhaps the most well-known of weather symbols.

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          There are many theories about the origin of mysterious patterns that appear in fields of corn around the world – commonly known as crop circles. While some people believe they are the work of alien spacecraft, many of them are known to be man-made. Unusual weather, such as small tornadoes or electrical storms, is thought to be the cause of some of the patterns.

          George Meaden, a meteorologist and physicist, explained crop circles in terms of atmospheric physics, as the effect of a plasma-vortex. Meaden likened the phenomenon to ball-lightning, but larger and longer-lasting. He expanded his theories in later research to include ionisation and electromagnetism. Physicist Stephen Hawking appeared to give credence to this view, writing in 1991 that corn circles are either hoaxes or formed by vortex movement of air.

           Meaden’s ideas were taken up by Ralph Noyes, a senior Ministry of Defence official and an expert in UFO phenomena, who wrote as follows:

          If Meaden is right, our atmosphere is sometimes able to produce a short-lived but vigorously swirling disturbance with strong electrical properties. A layman grasps after the analogy of something between ball-lightning and a mini-tornado. Depending on conditions, this transient energy-form can manifest as a globe of light, often with associated sounds. It may be able to interfere with the ignition system of automobiles and perhaps to affect close witnesses. Descending to earth, it can make a cropfield circle. Acting more vigorously it may well cause more violent circular damage at ground level. In short, a good meteorologist whose sole concern has been to investigate cropfield circles has ended by describing much of the UFO phenomenon!

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            El Nino is Spanish for “Boy Child” — a reference to Jesus. It was named in the 17th century by Spanish-speaking fishermen who lived in Peru, South America. It was given this name because the unusual weather associated with El Nino began around Christmas.

            El Niño Spanish is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including the area off the Pacific coast of South America. The ENSO is the cycle of warm and cold sea surface temperature (SST) of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. El Niño phases are known to occur close to four years, however, records demonstrate that the cycles have lasted between two and seven years. During the development of El Niño, rainfall develops between September–November. The cool phase of ENSO is La Niña, with SSTs in the eastern Pacific below average, and air pressure high in the eastern Pacific and low in the western Pacific. The ENSO cycle, including both El Niño and La Niña, causes global changes in temperature and rainfall.

            Developing countries that depend on their own agriculture and fishing, particularly those bordering the Pacific Ocean, are usually most affected. In American Spanish, the capitalized term El Niño means “the boy”. In this phase of the Oscillation, the pool of warm water in the Pacific near South America is often at its warmest about Christmas. The original name of the phase, El Niño de Navidad, arose centuries ago, when Peruvian fishermen named the weather phenomenon after the newborn Christ. La Niña, chosen as the “opposite” of El Niño, is American Spanish for “the girl”.

            Originally, the term El Niño applied to an annual weak warm ocean current that ran southwards along the coast of Peru and Ecuador at about Christmas time. However, over time the term has evolved and now refers to the warm and negative phase of the El Niño–Southern Oscillation and is the warming of the ocean surface or above-average sea surface temperatures in either the central and eastern tropical Pacific Ocean. This warming causes a shift in the atmospheric circulation with rainfall becoming reduced over Indonesia and Australia, while rainfall and tropical cyclone formation increases over the tropical Pacific Ocean. The low-level surface trade winds, which normally blow from east to west along the equator, either weaken or start blowing from the other direction.

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          Fog is cloud that forms close to the ground. It appears when the wind is light, the air is damp and the sky is relatively clear. It often forms when moisture in the air close to the ground condenses and spreads upwards — this is called radiation fog. It is most common at the beginning or end of the day, when the ground cools down quickly.

          Fog is a natural weather conditions that can cause visibility to become zero. It can cause accidents on normally safe roads and is such a serious weather condition that schools delay the start of the day until the sun burns it off. So how does fog form? First it is important to understand that fog is basically a cloud on the ground. This means like clouds it is a collection of tiny water droplets formed when evaporated water is cooled. The way it is cooled determines how fog is formed.

          The first way that fog is formed is by infrared cooling. Infrared cooling happens due to the change of seasons from summer to fall and winter. During the summer the ground absorbs solar radiation. As air passes over it is made warm and moist. When the seasons change this mass of warm moist air collides with the cooler that is now prevalent. This cause is the water vapor in the air mass to condense quickly and fog is formed. This fog is often called radiation fog due to the way it forms. This kind is the most common type of fog. It also happens when an unseasonable day of warm weather combined with high humidity is followed by dropping temperatures.

          The next way that fog forms is through advection. Advection is wind driven fog formation. In this case warm air is pushed by winds across a cool surface where it condenses into fog. There are also other kinds of fog like hail fog or freezing fog. Each of these conditions is where condensed water droplets are cooled to the point of freezing. There is also fog formed over bodies of water. One type is sea smoke. This is a type of fog that forms when cool air passes over a warm body of water or moist land.

          In general we see that fog is formed whenever there is a temperature difference between the ground and the air. When the humidity is high enough and there is enough water vapor or moisture fog is sure to form. However the kind of fog and how long is last and its effects will depends on the different conditions mentioned. One interesting kind of fog actually helps to make snow melt faster.

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          One of the simplest ways to measure humidity is to use a hair hygrometer. This uses a piece of human hair, which stretches or contracts according to the amount of water in the air. In a weather house — a type of hair hygrometer — a hair attached to a turntable stretches and contracts, making the man appear in humid conditions and the woman appear when it is drier.

          These devices use a human or animal hair under some tension. The hair is hygroscopic (tending toward retaining moisture); its length changes with humidity, and the length change may be magnified by a mechanism and indicated on a dial or scale. In the late 17th century, such devices were called by some scientists hygroscopes; that word is no longer in current use, but hygroscopic and hygroscopy, which derive from it, still are. The traditional folk art device known as a weather house works on this principle. Whale bone and other materials may be used in place of hair.

          In 1783, Swiss physicist and geologist Horace Bénédict de Saussure built the first hair-tension hygrometer using human hair.

          It consists of a human hair eight to ten inches long, b c, fastened at one extremity to a screw, a, and at the other passing over a pulley, c, being strained tight by a silk thread and weight, d.

          The pulley is connected to an index which moves over a graduated scale (e). The instrument can be made more sensitive by removing oils from the hair, such as by first soaking the hair in diethyl ether.

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          To Measure humidity accurately, meteorologists look at relative humidity. This is the amount of water in the air, relative to the maximum amount of water that it can hold at that temperature. To measure relative humidity, a wet and a dry thermometer are used. The wet bulb is covered with wet muslin. The water in the muslin evaporates, making the temperature around the wet bulb cooler than that around the dry bulb. The amount of water that evaporates increases along with the dryness of the air — the greater the difference in temperature, the lower the humidity. A smaller difference means higher humidity. The thermometers are housed in a Stevenson screen, to shade them from the Sun.

           Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the dew point.

              Climate control refers to the control of temperature and relative humidity in buildings, vehicles and other enclosed spaces for the purpose of providing for human comfort, health and safety, and of meeting environmental requirements of machines, sensitive materials (for example, historic) and technical processes.

              A hygrometer is a device used for measuring the humidity of air.

             The humidity of an air and water vapor mixture is determined through the use of psychrometric charts if both the dry bulb temperature (T) and the wet bulb temperature (Tw) of the mixture are known. These quantities are readily estimated by using a sling psychrometer.

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           Desert regions have very low levels of humidity — often less than 10%. The low levels of water vapour in the air and, indeed, the general scarcity of water makes conditions for life very difficult. Agriculture is practically impossible in such areas and is only really successful in places where levels of humidity tend to be moderate.

           Deserts cover 20 percent of the earth’s surface yet are the driest regions in the world. Their lack of humidity is particularly striking because hot areas can hold so much moisture. Rainforests, for instance, combine warm air and high precipitation to produce some of the highest areas of humidity in the world. Deserts, on the other hand, are very dry, so they’re antithetical to most life.

          Deserts are prone to having long periods of little to no rain before receiving short bursts of precipitation, but the amount of humidity that does enter the air is rare. The desert air is so dry that the rate of evaporation regularly exceeds the rainfall rate, and the rainfall may even evaporate before it hits the ground.

          Desert conditions are also exacerbated by the fact that desert organisms respond to the low humidity by preserving as much water as they can without losing it to evaporation. Many desert plants have evolved a waxy structure called a cuticle that can keep water inside. Small leaves and white hairs that reflect heat may also be strategies for dealing with desert conditions.

         Humidity also affects human health conditions in general (some more and some less of course). For example, high humidity levels in the hot temperatures will make the feeling of heat much worse, since the sweat which regulates our body temperature cannot evaporate as quickly. The best relative humidity, that feels the most comfortable is somewhere around 45%. But nor extremely high neither extremely low humidity is good for human health, even though both high and low humidity climate conditions have pros and cons. 

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          Humidity is highest in tropical areas, where the climate is warm. A continuous cycle of water movement exists, where water evaporates from the sea into the air and falls again in heavy rainfall. The conditions are ideal for plants and other forms of life — the plants themselves add more moisture to the atmosphere.

          It’s often been suggested (or, at least, hoped) that no matter how much the climate warms, that humans might be able to adapt. But there’s a hard upper limit on that: If the “wet-bulb temperature” — measured by wrapping the bulb of a thermometer in a wet cloth and taking the temperature of the air — exceeds 95 degrees Fahrenheit (or, 35ºC), humans are pretty much toast. It might seem like an odd way to measure the upper limit of human survivability, but that’s the simplest way to measure how much a human body could theoretically cool itself, assuming a perfectly healthy body.

          “You rapidly approach a situation where it’s thermodynamically impossible to keep your body cool,” Radley Horton, an associate research professor at Columbia University’s Earth Institute and the Lamont-Doherty Earth Observatory, a co-author of the study, told VICE News.

          As humidity increases, so does the wet-bulb temperature. Because we never hit a wet-bulb temperature of 95 degrees Fahrenheit in today’s climate, it’s hard to say what the societal effects would be. But wet-bulb temperatures between 84 degrees and 88 degrees Fahrenheit (29–31ºC) have been responsible for tens of thousands of deaths around the world. A wet-bulb temperature of 86 degrees Fahrenheit (30ºC) was recorded during a heat wave in 2015 in the southeastern coastal Indian state of Andhra Pradesh that killed at least 2,500 people.

          The risks are starkest for places that already see high heat and humidity, like the Persian Gulf and the Tropics. And some of the regions most at risk of these spikes are the most densely populated — places like Northeast India, East China, West Africa, and the Southeastern U.S. Many of those places don’t currently have good access to medical care, and the population flight from these regions could make it even harder to help the most vulnerable.

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          Dew is condensed water vapour, which forms when air comes into contact with a cold surface. It will form on a clear, still night, but it is especially noticeable after a night of fog, when there is a lot of water vapour in the air close to the ground. Dew will appear as water droplets on any cold surface.

          As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets.

          When temperatures are low enough, dew takes the form of ice; this form is called frost.

          Because dew is related to the temperature of surfaces, in late summer it forms most easily on surfaces that are not warmed by conducted heat from deep ground, such as grass, leaves, railings, car roofs, and bridges.

          Dew should not be confused with guttation, which is the process by which plants release excess water from the tips of their leaves.

          Water vapour will condense into droplets depending on the temperature. The temperature at which droplets form is called the dew point. When surface temperature drops, eventually reaching the dew point, atmospheric water vapor condenses to form small droplets on the surface. This process distinguishes dew from those hydrometeors (meteorological occurrences of water), which form directly in air that has cooled to its dew point (typically around condensation nuclei), such as fog or clouds. The thermodynamic principles of formation, however, are the same. Dew is usually formed at night.

          Adequate cooling of the surface typically takes place when it loses more energy by infrared radiation than it receives as solar radiation from the sun, which is especially the case on clear nights. Poor thermal conductivity restricts the replacement of such losses from deeper ground layers, which are typically warmer at night. Preferred objects of dew formation are thus poor conducting or well isolated from the ground, and non-metallic, while shiny metal coated surfaces are poor infrared radiators. Preferred weather conditions include the absence of clouds and little water vapor in the higher atmosphere to minimize greenhouse effects and sufficient humidity of the air near the ground. Typical dew nights are classically considered calm, because the wind transports (nocturnally) warmer air from higher levels to the cold surface. However, if the atmosphere is the major source of moisture (this type is called dewfall), a certain amount of ventilation is needed to replace the vapor that is already condensed. The highest optimum wind speeds could be found on arid islands. If the wet soil beneath is the major source of vapor, however (this type of dew formation is called distillation), wind always seems adverse.

          The processes of dew formation do not restrict its occurrence to the night and the outdoors. They are also working when eyeglasses get steamy in a warm, wet room or in industrial processes. However, the term condensation is preferred in these cases.

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          Frost can sometimes create beautiful patterns on the inside of windows. The delicate shapes, called fern frost because of their resemblance to fern plant, form when water vapour condenses into tiny droplets on the window pane. Ice crystals form, making water freeze on to the sharp points of the ice crystals, creating a chain reaction that creates the patterns.

          Window frost (also called fern frost or ice flowers) forms when a glass pane is exposed to very cold air on the outside and warmer, moderately moist air on the inside. If the pane is not a good insulator (for example, if it is a single pane window), water vapour condenses on the glass forming frost patterns. With very low temperatures outside, frost can appear on the bottom of the window even with double pane energy efficient windows because the air convection between two panes of glass ensures that the bottom part of the glazing unit is colder than the top part. On unheated motor vehicles the frost will usually form on the outside surface of the glass first. The glass surface influences the shape of crystals, so imperfections, scratches, or dust can modify the way ice nucleates. The patterns in window frost form a fractal with a fractal dimension greater than one but less than two. This is a consequence of the nucleation process being constrained to unfold in two dimensions, unlike a snowflake which is shaped by a similar process but forms in three dimensions and has a fractal dimension greater than two.

          If the indoor air is very humid, rather than moderately so, water will first condense in small droplets and then freeze into clear ice.

          Fern frost can form on windowpanes when the air outside is very cold and the air inside is moist. The outside air temperature on the winter’s day when this photo was snapped was 15 degrees F (-9 C) and the inside air temperature was 66 F (19 C). Crystal formation is affected by surface features of the glass, like dust and dirt particles, which serve as nucleation points for crystalline growth. Towels had been left to dry near the old and weathered bathroom window shown here  provided just the right level of moisture conducive to frost formation. If there had been considerably more moisture in the air, for instance after a steamy shower, or if the windowpane had not been extremely cold, the water vapor would have merely formed ice or water droplets on the glass. The very low temperature of the glass allowed the moisture to go directly from a gaseous state to a solid state— in the form of frost.

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        The air absorbs water from oceans, rivers, lakes and also from trees and plants. Humidity describes the amount of water vapour that the air contains. The warmer the weather, the more moisture the air can hold. The air can reach a point of saturation, where it is no longer able to absorb any more water — this is 100% humidity. In such conditions, water vapour condenses to form mist, clouds and rain.

        Humidity is the concentration of water vapour present in air. Water vapour, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The amount of water vapour needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C.

        Three primary measurements of humidity are widely employed: absolute, relative and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metre or grams per kilogram. Relative humidity, expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass.

        Humidity plays an important role for surface life. For animal life dependent on perspiration (sweating) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces. This effect can be calculated using a heat index table, also known as a humidex.

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          It is possible for a type of icicle to  form upside down. This happens in small, shallow pools of water — ornamental bird baths, for example. When the water freezes, it expands and forms a dome of ice in the centre. A crack in the dome will allow water out, which then freezes. As this happens over time, an “ice spike” will form.

          An ice spike is an ice formation, often in the shape of an inverted icicle that projects upwards from the surface of a body of frozen water. Ice spikes created by natural processes on the surface of small bodies of frozen water have been reported for many decades, although their occurrence is quite rare. A mechanism for their formation, now known as the Bally–Dorsey model, was proposed in the early 20th century but this was not tested in the laboratory for many years. In recent years a number of photographs of natural ice spikes have appeared on the Internet as well as methods of producing them artificially by freezing distilled water in domestic refrigerators or freezers. This has allowed a small number of scientists to test the hypothesis in a laboratory setting and, although the experiments appear to confirm the validity of the Bally–Dorsey model, they have raised further questions about how natural ice spikes form, and more work remains to be done before the phenomenon is fully understood. Natural ice spikes can grow into shapes other than a classic spike shape, and have been variously reported as ice candles, ice towers or ice vases as there is no standard nomenclature for these other forms. One particularly unusual form takes the shape of an inverted pyramid.

          Although natural ice spikes are usually measured in inches or centimeters, a report that appeared in the Harbor Creek Historical Society Newsletter by Canadian Gene Heuser, who hiked across frozen Lake Erie in 1963, spoke of “small pinholes in the ice through which the water below was periodically forced under pressure to spout up into the air and freeze” producing five-foot-high (1.5 m) “frozen spurts that looked to him like telephone poles standing straight up all over the lake”.

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          Rime frost forms on leaves, branches and other solid objects when an icy wind freezes water droplets over them. It forms a solid white crust on the windward side of objects and can cause damage to buildings and other structures if it is allowed to build up. Rime is usually found in very cold, exposed areas.

          Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. There are also intermediate formations.

• Soft rime is less dense than hard rime and is milky and crystalline, like sugar. Soft rime appears similar to hoar frost.
• Hard rime is somewhat less milky, especially if it is not heavy.
• Clear ice is transparent and homogeneous and resembles ice-cube ice in appearance. Its amorphous, dense structure helps it cling tenaciously to any surface on which it forms.

          Both rime types are less dense than clear ice and cling less tenaciously, therefore damage due to rime is generally minor compared to clear ice. Glaze ice is similar in appearance to clear ice but it is the result of a completely different process, occurring during freezing rain or drizzle.

          These three types occur also when ice forms on the surface of an aircraft, particularly on the leading edges and control surfaces, when it flies through a cloud made of supercooled water liquid droplets. Rime ice is the least dense, milky ice is intermediate and clear ice is the most dense. All forms of ice can spoil lift and may have a catastrophic effect on an airborne aircraft. Ice is hazardous to flight as it disrupts airflow, increases weight, and adds drag. Ice forming on propellors and/or engine inlets can cause severe vibration and/or damage if ingested.

Rime is also a weather form.

          Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. This fog freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between ?2 and ?8 °C (28.4 and 17.6 °F).

          Soft rime is a white ice deposition that forms when the water droplets in light freezing fog or mist freeze to the outer surfaces of objects, with calm or light wind. The fog freezes usually to the windward side of tree branches, wires, or any other solid objects.

          Soft rime is similar in appearance to hoar frost; but whereas rime is formed by vapour first condensing to liquid droplets (of fog, mist or cloud) and then attaching to a surface, hoar frost is formed by direct deposition from water vapour to solid ice. A heavy coating of hoar frost, called white frost, is very similar in appearance to soft rime, but the formation process is different: it happens when there is no fog, but very high levels of air relative humidity (above 90%) and temperatures below ?8 °C (17.6 °F).

          Soft rime formations have the appearance of white ice needles and scales; they are fragile and can be easily shaken off objects. Factors that favour soft rime are small drop size, slow accretion of liquid water, high degree of supercooling, and fast dissipation of latent heat of fusion. The opposite conditions favour ice with higher densities, such as hard rime or clear ice.

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          Hoar frost is the most common type of frost. It covers every exposed surface with layers of crunchy ice crystals. In certain conditions, the ground may be covered with a very thick layer of white hoar frost, which looks like snow.

          Hoar frost refers to white ice crystals deposited on the ground or loosely attached to exposed objects, such as wires or leaves. They form on cold, clear nights when conditions are such that heat radiates out to the open air faster than it can be replaced from nearby sources, such as wind or warm objects. Under suitable circumstances, objects cool to below the frost point of the surrounding air, well below the freezing point of water. Such freezing may be promoted by effects such as flood frost or frost pocket. These occur when ground-level radiation losses cool air until it flows downhill and accumulates in pockets of very cold air in valleys and hollows. Hoar frost may freeze in such low-lying cold air even when the air temperature a few feet above ground is well above freezing.

          The word hoar comes from an Old English adjective that means “showing signs of old age”. In this context, it refers to the frost that makes trees and bushes look like white hair.

Hoar frost may have different names depending on where it forms:

• Air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, and wires.
• Surface hoar refers to fern-like ice crystals directly deposited on snow, ice or already frozen surfaces.
• Crevasse hoar consists of crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions.
• Depth hoar refers to faceted crystals that have slowly grown large within cavities beneath the surface of banks of dry snow. Depth hoar crystals grow continuously at the expense of neighbouring smaller crystals, so typically are visibly stepped and have faceted hollows.

          When surface hoar covers sloping snowbanks, the layer of frost crystals may create an avalanche risk; when heavy layers of new snow cover the frosty surface, furry crystals standing out from the old snow hold off the falling flakes, forming a layer of voids that prevent the new snow layers from bonding strongly to the old snow beneath. Ideal conditions for hoarfrost to form on snow are cold clear nights, with very light, cold air currents conveying humidity at the right rate for growth of frost crystals. Wind that is too strong or warm destroys the furry crystals, and thereby may permit a stronger bond between the old and new snow layers. However, if the winds are strong enough and cold enough to lay the crystals flat and dry, carpeting the snow with cold, loose crystals without removing or destroying them or letting them warm up and become sticky, then the frost interface between the snow layers may still present an avalanche danger, because the texture of the frost crystals differs from the snow texture and the dry crystals will not stick to fresh snow. Such conditions still prevent a strong bond between the snow layers.

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          ICICLES usually form when the water from thawed ice and snow freezes again. This happens during a bright winter’s day when sunny areas are warm, but shaded areas remain below freezing, or when a cold night follows a warmer day. When the melt-water drips over the edge of a surface, the drops will freeze to form an icicle.

          Icicles can form during bright, sunny, but subfreezing weather, when ice or snow melted by sunlight or some other heat source (such as a poorly insulated building), refreezes as it drips off under exposed conditions. Over time continued water runoff will cause the icicle to grow. Another set of conditions is during ice storms, when rain falling in air slightly below freezing slowly accumulates as numerous small icicles hanging from twigs, leaves, wires, etc. Thirdly, icicles can form wherever water seeps out of or drips off vertical surfaces such as road cuts or cliffs. Under some conditions these can slowly form the “frozen waterfalls” favored by ice climbers.

          Icicles form on surfaces which might have a smooth and straight, or irregular shape, which in turn influences the shape of an icicle. Another influence is melting water, which might flow toward the icicle in a straight line or which might flow from several directions. Impurities in the water can lead to ripples on the surface of the icicles.

          Icicles elongate by the growth of ice as a tube into the pendant drop. The wall of this ice tube is about 0.1 mm (0.0039 in) and the width 5 mm (0.20 in). As a result of this growth process, the interior of a growing icicle is liquid water. The growth of an icicle both in length and in width can be calculated and is a complicated function of air temperature, wind speed, and the water flux into the icicle. The growth rate in length typically varies with time, and can in ideal conditions be more than 1 cm (0.39 in) per minute.

          Given the right conditions, icicles may also form in caves (in which case they are also known as ice stalactites). They can also form within salty water (brine) sinking from sea ice. These so-called brinicles can actually kill sea urchins and starfish, which was observed by BBC film crews near Mount Erebus, Antarctica.

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          Rivers and lakes can become frozen over if the temperatures get very low. This will usually depend on the depth of the water — the shallower it is, the more likely it is to freeze over. A layer of ice forms on the surface that is strongest and thickest nearest to the banks. Weaker spots will exist further out, making it very dangerous to walk or skate on the ice – anyone falling in could easily become trapped in the freezing water.

          Ice found on natural bodies of water raises the risk of danger due to natural variables. There is no way to judge the strength and safety of ice by looking at it or by the temperature of the day. Fluctuating temperatures, exposure to sunlight and other factors affect the consistency of the ice. It can be a several inches thick in one spot and only an inch thick 10 feet away. Be especially wary of ice covered with snow. Snow can hide cracks and weaknesses in open ice. Parents should educate their children about the danger of going out onto a frozen body of water, including never following a friend or a pet into these potential danger zones.

Basic tips

• Parents should always supervise children skating or playing on or near ice. Educate them on the risks of playing on ice, and outfit them with lifejackets. Never leave children alone on or near ice covered bodies of water.
• Adults should prepare before going on ice. Wait to walk out onto ice until there is a minimum of four inches of clear, solid ice measured from multiple locations. Start measurements in an area where the water is shallow. If the thickness in the shallow area is less than three inches, do not walk on the ice.
• Take someone with you, wear a life jacket, and bring safety equipment, including a cell phone, in case of an emergency.
• Always keep your pets on a leash near frozen bodies of water. If a pet falls through the ice, do not attempt to rescue your pet, call 9-1-1 or go for help.
• Stay clear of white ice. White ice contains air and snow within it, therefore, making it weaker.
• Stay off river ice. Currents can quickly change the thickness of ice, making it more fragile.

When Someone Falls Through Ice

• If you come across someone who has fallen through the ice, don’t attempt a rescue yourself. Call 9-1-1 or immediately or go for help. Local public safety officers have proper training and equipment to handle ice emergencies.
• If the ice did not support the victim’s weight, it will not support you. Avoid going onto the ice to attempt a rescue, but extend a ladder, rope, jumper cables, or tree branch to the victim along with something that will keep them afloat.
• Once the person is rescued from the cold water, help the victim into dry clothes as soon as possible. If dry clothes aren’t an option, leave the wet clothes on for insulation to trap body heat.
• Transport the victim to get medical attention if necessary.

If You Fall Through Ice

• Remain calm, and try not to panic. The body will undergo cold water shock when suddenly immersed in cold water, and you will experience an increase in heart rate and blood pressure.
• Face the direction you came from and spread your arms out on the unbroken ice. Kick your feet and try to pull yourself onto the ice.
• Once out of the cold water, remain lying on the ice (do not attempt to stand) to keep your weight distributed and avoid breaking through the ice. Roll away from the hole and crawl back to solid land. This keeps your weight distributed.
• Treat yourself for hypothermia and seek medical attention.

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          If you were to cut through a hailstone, you would see that it is made up of several layers of ice and frost. Each layer is produced by one journey up and down the height of the cloud. The greater the amount of turbulence inside the cloud, the greater the number of layers and the bigger the hailstones.

          If you’ve ever seen a tree that has been cut down, you’re probably familiar with the many rings that you will find inside. If you slice through a hailstone, you would find that the cross-section has rings similar to the inside of a tree.

          This layered appearance inside the hailstone is due to the coatings of ice that were added over time while the hailstone was suspended aloft in the thunderstorm. The series of photos above, some more vivid that others, illustrate the layers that can be found in hailstones.

          So, why are some hailstones so much larger than others?  The answer lies in how many ice layers are formed on a hailstone.

          Thunderstorms have an inflow of warm, moist air that rises into the storm. This rising unstable air is referred to as the updraft.

          The updrafts suspend some initial small ice kernels aloft, where water droplets collide with the small hail and make it grow larger with each additional ice coating.

          While the majority of thunderstorms contain at least some hail, many times it does not reach the ground because the hailstones are small and melt in the above-freezing layer in the lower atmosphere. These storms likely have a weak updraft.

          For very large hail to reach the ground, a strong and long-lasting updraft is needed to allow the hailstone to stay aloft and add more layers of ice. Sometimes the updraft in the storm can exceed 100 mph!

          Eventually, the hailstone will reach a point where it becomes too heavy and the updraft can no longer support it aloft. This hail then falls to the ground below in various sizes and shapes.

          Every year we see thunderstorms with strong updrafts that produce hail the size of baseballs and softballs in the nation’s heartland. The largest hailstone ever measured in the United States fell from the sky.

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          Frost most often forms over the land when the air remains clear after a cold winter’s day. The ground loses its heat quicker than the air above it, and if the temperature falls below freezing, any moisture in the air will freeze, covering almost any surface with frost. Different conditions can produce different types of frost.

          Morning condensation (dew) is very common in some regions and can easily be forecasted. The favorable weather elements for dew include clear skies, light wind, decent soil moisture, and low night-time dew point depressions.

          Dew forms when the temperature becomes equal to the dew point. This often happens first at ground level for two reasons. First, longwave emission causes the earth’s surface to cool at night. Condensation requires the temperature to decrease to the dew point. Second, the soil is often the moisture source for the dew. Warm and moist soils will help with the formation of dew as the soil cools overnight.

          The cooling of warm and moist soil during the night will cause condensation especially on clear nights. Clear skies allow for the maximum release of longwave radiation to space. Cloudy skies will reflect and absorb while re-emitting longwave radiation back to the surface and that prevents as much cooling from occurring. Light wind prevents the mixing of air right at the surface with drier air aloft. Heavier dew will tend to occur when the wind is light as opposed to when the wind is strong. Especially when soils are moist, the moisture concentration will be higher near the earth’s surface than higher above the earth’s surface. As the air with higher moisture concentration cools, this air will produce condensation first.

          Soil moisture is EXTREMELY critical to producing dew (especially heavy dew). Dry regions that have not received rain in over a week or two are much less likely to have morning dew (especially a heavy dew). Once the soil gets a good soaking from a rain, it takes several days for the soil to lose the moisture through evaporation. If nights are clear after a good rain, dew can be expected every morning for the next few days (especially in regions with abundant vegetation, clear skies and light wind). The dew point depression is important because it determines how much the air will need to cool to reach saturation. With a large dew point depression (greater than 25 units of F), quite a bit of night-time cooling will need to take place in order to produce dew. A low dew point depression with the other factors favorable for dew is more likely to produce heavy dew.

          Dew is important to forecast since it impacts people. Dew can produce a thick film of water all over the car in the morning (can be especially annoying for people that don’t have a garage). Time has to be spent wiping the water off the windows in order to see on-coming traffic. Dew is also important to agriculture. Dew recharges the soil moisture and limits evaporation from the soil during the time the dew is forming. Dew can make the mowing of the lawn more difficult. It is much easier to mow the lawn in the late afternoon when the dew has evaporated than it is in the morning. Wet grass clumps together and sticks to everything. Also, you are more prone to getting a dirty shoe when walking on dew covered grass as compared to dry grass.

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          Hailstones are essentially frozen raindrops. They are made inside very tall cumulonimbus clouds that have great differences in temperature between the top and bottom. Freezing temperatures at the top and warmer temperatures at the bottom of the cloud create very strong upward and downward currents of air. Ice crystals and super-cooled water droplets are thrown around by these currents and collide with each other. As they do so, they are coated with more and more layers of ice. The layers of ice buildup until the hailstones are heavy enough to fall to the ground.

          Hailstones are formed when raindrops are carried upward by thunderstorm updrafts into extremely cold areas of the atmosphere and freeze. Hailstones then grow by colliding with liquid water drops that freeze onto the hailstone’s surface. If the water freezes instantaneously when colliding with the hailstone, cloudy ice will form as air bubbles will be trapped in the newly formed ice. However, if the water freezes slowly, the air bubbles can escape and the new ice will be clear. The hail falls when the thunderstorm’s updraft can no longer support the weight of the hailstone, which can occur if the stone becomes large enough or the updraft weakens.

          Hailstones can have layers of clear and cloudy ice if the hailstone encounters different temperature and liquid water content conditions in the thunderstorm. The conditions experienced by the hailstone can change as it passes horizontally across or near an updraft. The layers, however, do not occur simply due to the hailstone going through up and down cycles inside a thunderstorm. The winds inside a thunderstorm aren’t simply up and down; horizontal winds exist from either a rotating updraft, like in supercell thunderstorms, or from the surrounding environment’s horizontal winds. Hailstones also do not grow from being lofted to the top of the thunderstorm. At very high altitudes, the air is cold enough (below -40°F) that all liquid water will have frozen into ice, and hailstones need liquid water to grow to an appreciable size.

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          Snow is often described as being “dry” or “wet”. The snowflakes that make wet snow are relatively large and form when the temperature is at freezing point or thereabouts. Wet snow packs together easily when it falls and is the best snow for making snowmen and snowballs. Dry snow is formed at lower temperatures, and the snowflakes are smaller than those that make wet snow. It is referred to as “powder” by skiers and snow-boarders and is the best form of snow for such winter sports enthusiasts. It is lighter and much easier to clear away.

          The temperature profile in the vertical is a critical factor in determining the wetness of snow. Wet snow generally has a snow depth to melted liquid depth ratio of less than 10. Dry snow generally has a snow depth to melted liquid depth ratio of greater than 20. Values between 10 and 20 are a hybrid of the two. Wet snow is great for making snow balls and it is a sticky snow. Dry snow is powdery, easily blown around by the wind and is not sticky.

          What determines the wetness or dryness of the snow is the amount of liquid content within the falling snow. When the temperature aloft goes just above freezing (see diagram below) then some of the snow will melt and it will fall as a wet snow. In the dry snow profile the temperature is well before freezing and thus the snow will have a low liquid content.

          Other factors that influence the wetness of snow are day vs. night, ground temperature, and if the sun is out. Even in cloudy conditions, some sunlight passes through clouds. Some of this light absorbs into the falling snowflakes and thus this can make a snow wetter even at temperatures well below freezing. This does not happen at night thus all being equal a night snow will be a little drier. If the ground surface is above freezing, some or all of the snow will melt after it reaches the ground. This will make a snow wetter. Sun shining on snow on the ground, even when temperatures are well before freezing, will cause snow to become wetter due to the absorption of solar energy into the snow. At night, when temperatures fall below freezing then snow on the ground will tend to have its liquid water content freeze up and the snow will become a hard crusty snow.

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          There are thought to be about 80 different varieties of snowflakes, which form into shapes ranging from needles and columns to stars, prisms, plates and hexagons. All snowflakes have a symmetrical, six-sided pattern, but no two snowflakes have been found with exactly the same shape. Scientists think the shape of a snowflake depends on the height and temperature at which it was formed.

          Do you enjoy watching snow fall on a cold winter day? We love to sip on hot cocoa while we watch snowflakes pile up outside. Later, we’ll head out to build snowmen, make snow angels, and start a snowball fight! Snowy days can be a lot of fun. Have you ever heard that no two snowflakes are exactly alike? Well, that isn’t exactly true. However, the chance of finding twin snowflakes is very, very, very low.

          Scientists say the chances of two snowflakes being exactly alike are about 1 in 1 million trillion. That’s a one followed by 18 zeros, so it’s very unlikely! Meteorologists think there are 1 trillion, trillion, trillion (a one with 36 zeros!) types of snowflakes.

          A snowflake has three basic ingredients: ice crystals, water vapor and dust. The ice crystals form as water vapor freezes on a tiny piece of dust. The dust particles come from many places. It could be from flower pollen, volcanic ash, or even meteors.

          Snow forms in very cold clouds. As water droplets attach themselves to the ice crystal, they freeze, creating an even larger ice crystal. In any crystal, molecules line up in a pattern. In ice crystals, water molecules line up and form a six-sided shape called a hexagon. This is why all snowflakes are six-sided!

          The temperature of the cloud determines the shape of an ice crystal. Likewise, the amount of moisture in the cloud determines the size of the ice crystal. More moisture will create a bigger crystal. When several ice crystals stick together, they form a snowflake.

          As snowflakes tumble through the air, swirling and spiraling, they each take a different path to the ground. Each snowflake falls and floats through clouds with different temperatures and moisture levels. This shapes each snowflake differently. Two snowflakes from the same cloud will have different sizes and shapes because of their different journeys to the ground.

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          Sleet is usually snow that has half-melted, or it can be formed from rain-drops that have partly evaporated and then cooled down as they fall to the ground. It often feels like very cold, wet rain when it falls on you.

          Sleet is rain or melted snow that freezes into ice pellets before hitting the ground. Sleet only happens under very specific weather conditions.

          There must be a layer of air near the ground whose temperature is below freezing, where water turns to ice. Above this layer of freezing air must be a layer of warmer air. As snow falls through the warm air, it melts or partially melts into raindrops. As the melted snow falls through the cold layer of air, it re-freezes. It forms ice pellets, or sleet, before hitting the ground.

          Sleet falls as tiny, hard pieces of ice. Sleet usually cannot do severe damage to crops or transportation systems the way heavy snow, freezing rain, or hail can. In fact, sleet is so light and tiny it usually bounces when it hits a hard surface.

          Sleet is not the same as freezing rain. Freezing rain also falls through a cold layer of air close to the ground. However, the rain does not freeze until it touches the surface of an object. When you see trees coated in jackets of ice, you are seeing the results of freezing rain. The rain was liquid when it landed on the tree branch, then immediately froze solid.

          Sleet also is not the same as hail. Hail, like sleet, is a collection of ice pellets. But hail forms in a cloud, while sleet forms as it falls. Hail freezes from the inside out, while sleet freezes from the outside in. Hail also tends to fall during thunderstorms in the spring and summer. Sleet usually falls in the winter.

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          Snow forms in clouds when the temperature is within the range -20°C to —40°C (-4°F to —40°F). Ice crystals in the clouds begin to melt and join together with super-cooled water droplets. They then freeze together and form into snowflakes which, provided the air temperature is low enough, fall from the clouds. The process of forming snowflakes is called accretion.

          Whether winter storms produce snow relies heavily on temperature, but not necessarily the temperature we feel here on the ground. Snow forms when the atmospheric temperature is at or below freezing (0 degrees Celsius or 32 degrees Fahrenheit) and there is a minimum amount of moisture in the air. If the ground temperature is at or below freezing, the snow will reach the ground. However, the snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this higher temperature layer; the melting creates evaporative cooling which cools the air immediately around the snowflake. This cooling retards melting. As a general rule, though, snow will not form if the ground temperature is at least 5 degrees Celsius (41 degrees Fahrenheit).

          While it can be too warm to snow, it cannot be too cold to snow. Snow can occur even at incredibly low temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur when there is relatively warm air near the ground—typically -9 degrees Celsius (15 degrees Fahrenheit) or warmer—since warmer air can hold more water vapor.

          Because snow formation requires moisture, very cold but very dry areas may rarely receive snow. Antarctica’s Dry Valleys, for instance, form the largest ice-free portion of the continent. The Dry Valleys are quite cold but have very low humidity, and strong winds help wick any remaining moisture from the air. As a result, this extremely cold region receives little snow.

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          Rainfall is usually measured in millimetres or inches. Rain water is collected in a metal drum about 50cm (20in) tall called a rain gauge. The rain gauge is placed on the ground, just high enough to avoid splashes. The rainwater is collected in a funnel t the top and passes into the drum.

          Rain gauges are thought to be the most ancient weather instruments, and they’re believed to have been used in India more than 2,000 years ago. A rain gauge is really just a cylinder that catches rain. If an inch collects in the cylinder, it means an inch of rain has fallen. It’s that simple. Most standard rain gauges have a wide funnel leading into the cylinder and are calibrated so that one-tenth of an inch of rain measures one inch when it collects inside. The funnel is 10 times the cross-sectional area of the tube. Rainfall as low as .01 inches can be measured with this instrument. Anything under .01 inches is considered a trace. This standard rain gauge is shown in the following figure.

          In the more modern era, a common rain gauge is called the tipping bucket type. A bucket doesn’t really tip—a pair of small receiving funnels alternate in the collection of the rain. When one fills up with water, it tips and spills out, and the other comes into place to do the collecting. These little funnels tip each time rainfall amounts to .01 inches. The tip triggers a signal that is transmitted and recorded.

          Of course, these rain gauges have a problem when the temperature drops below freezing, so the standard versions are heated for the occasion.

          When snow falls on these heated rain gauges, it melts, and a water equivalent is determined. The recorded precipitation is always expressed in terms of rainfall or melted snow. The snow depth doesn’t count—unless, of course, you have to shovel it! Sometimes a foot of snow amounts to just a half-inch of water, other times it amounts to three inches of water. It really depends on the water equivalent of the snow, which varies widely.

          On the average, 10 inches of snow is equivalent to one inch of rain, but that’s only an average. If a rain gauge measures one inch of water during a snowstorm, an observer can’t automatically assume that 10 inches of snow has fallen. The snow depth can only be determined the old-fashioned way—by measuring it.

          That depth is determined by taking an average of three or more representative spots. A ruler is stuck into the snow, and its depth is recorded. Because of blowing and drifting, the determination of three or more representative locations is not always easy. You would think that there would be a better way, but there really isn’t.

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          Desert regions experience very little rainfall, but flowers may still bloom after the rains come. Some flower seeds will survive in the desert soil for years, suddenly coming into bloom at the first sign of rain. These flowers will grow long enough to produce seeds, so that the cycle may continue.

          A desert bloom is a climatic phenomenon that occurs in various deserts around the world. The phenomenon consists of the blossoming of a wide variety of flowers during early-mid spring in years when rainfall is unusually high.

          The blossoming occurs when the unusual level of rainfall reach the seeds and bulbs that have been in a latent or dormant state and causes them to germinate and flower in early spring. It is accompanied by the proliferation of insects, birds and small species of lizards.

          In the Atacama Desert, a desert bloom occurs between the months of September and November in years when rainfall is unusually high. Normally, the Atacama Desert receives less than 12 mm (0.47 in) of rain a year.

          At its height, the phenomenon can be seen from just south of the city of Vallenar to just north of the city of Copiapo throughout the coastal valleys and Chilean Coast Range from September to November.

          Climatically, the event is related to the El Nino phenomenon, a band of anomalously warm ocean water temperatures that occasionally develops off the western coast of South America, which can lead to an increase in evaporation and therefore precipitation.

          The flowering desert is a popular tourist attraction with tourists visiting the phenomenon from various points around the southern Atacama, including Huasco, Vallenar, La Serena, Copiapo and Caldera.

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          An area that has experienced a very long period of hot, dry weather or drought may suffer a flood should heavy rain follow. This is because the soil will have become baked so hard and dry that the water produced by very heavy rainfall will not be able to drain away. This is sometimes called a flash flood.

          About one in five California residents now lives in a flood-prone area. The replacement value of buildings vulnerable to floods exceeds $575 billion. Climate change could increase these risks. Yet the amount we spend on flood management is currently just a fraction of state water spending.

          Now is the time to look at improving flood responses across the state, not when rivers are overflowing. If we’re serious about reducing flood damages, we will need to adopt an “all of the above” approach to managing flood risks. The palette of solutions includes reinforcing and improving structural protections such as levees and floodwalls, encouraging residents and businesses to buy flood insurance, utilizing green approaches like wetlands, and avoiding new development on the most flood-prone lands.

          The first line of defense—particularly for those in the so-called 100-year flood zone, which is prone to more frequent floods—has typically been levees. Our system of levees is in great need of improvement, and funds from a 2006 state bond have boosted investments to shore up these flood defenses. Yet we still have a large funding gap. We need to increase spending to protect our most vulnerable communities, while ensuring we get the biggest bang for our bucks with these investments. For example, setting back levees to allow rivers to have more room to flood can reduce flood damages, boost habitat benefits and even increase recharge to aquifers.

          While necessary in many places, structural systems are also the most costly and environmentally damaging elements of our flood-protection system. They’re also far from infallible, particularly in light of a changing climate. Flood insurance is an underutilized tool that complements structural protections, and can help people recover more quickly. Yet too few at-risk Californians carry flood insurance. State and local agencies must find new ways to promote greater adoption of insurance. One novel approach would give local or regional flood management agencies authority to buy insurance for the community. Pooling resources this way would increase coverage and cut costs. The legislature could encourage this by creating mechanisms to recover costs through assessments or fees.

          Finally, one of the best defenses against flooding is land-use planning and regulation that keeps people out of harm’s way. Many communities already discourage development in the most at-risk zones, but more needs to be done. The state doubled the protection standard for urban areas in the Central Valley in 2007. The state should consider using the higher standard major urban areas elsewhere in the state, which also face significant risk.

          There is only one certainty about California’s variable climate: the drought that is hitting the state today will, at some point, give way to floods. Strengthening flood management could have big pay-offs in protecting the public health and safety and the state’s economy.

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          A Drought occurs when there is less than 0.2mm (1/100in) of rainfall in an area over a period of about two weeks. Droughts are usually caused by an area of high pressure that remains in one place for a long period of time. This is called a blocking high. The blocking high will prevent the movement of low-pressure systems into an area, meaning that hot, dry weather will dominate that area, leading to a drought. Parts of Africa, Asia and Central America often experience periods of drought.

          A drought is a prolonged period with less-than-average amounts of rain or snow in a particular region. The severity of the drought depends on the amount of time that a region receives below-average precipitation.

          For example, a few weeks without rain could stress a farmer’s crops during the growing season. This is called a flash drought. But it could take a much longer dry period to see a full drought that would affect a region’s water supply.

          A drought is caused by drier than normal conditions that can eventually lead to water supply problems. Really hot temperatures can make a drought worse by causing moisture to evaporate from the soil. Just because a region is hot and dry doesn’t necessarily mean it is going through a drought. Droughts only occur when an area is abnormally dry. Here’s why:

          Rain and snow don’t fall evenly across Earth. Some regions are routinely wet and others are routinely dry. From season to season — and from year to year — the amount of rain or snow in a location can vary.

          However, over a period of many years, the average amount of precipitation in a region is fairly consistent. For example, in the deserts of the American Southwest, the average precipitation is less than 3 inches per year. But, the average yearly precipitation in Atlanta is about 50 inches.

          When a particular area gets less rain than usual, the soil gets much less moisture, too. The soil starts drying out and plants die. When this pattern continues for several weeks, months or years, the flow of streams and rivers decreases and water levels in lakes, reservoirs and wells fall. Eventually, the unusual dry weather causes water supply issues, and the dry period becomes a drought.

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          If less than 0.5mm (1/48in) of rain falls in an hour, it is described by meteorologists as light. When more than 4mm (1/6in) falls, the rain is described as heavy. The heaviest rainfall is experienced in the tropical and monsoon regions of the world. In other areas, periods of heavy rain rarely last longer than an hour.

          Concentrations of raindrops typically range from 100 to 1,000 per cubic m (3 to 30 per cubic foot); drizzle droplets usually are more numerous. Raindrops seldom have diameters larger than 4 mm, because as they increase in size they break up. The concentration generally decreases as diameters increase. Except when the rain is heavy, it does not reduce visibility as much as does drizzle. Meteorologists classify rain according to its rate of fall. The hourly rates relating to light, moderate, and heavy rain are, respectively, less than 2.5 mm, 2.8 to 7.6 mm, and more than 7.6 mm.

          Raindrops may form by the coalescence of small water droplets that collide or from the melting of snowflakes and other ice particles as they fall into warm air near the ground.

          Mount Waialeale, Hawaii, with a 20-year annual average of 11,700 mm (460 inches) from tropical easterlies, is the wettest known point on the Earth. The nearest competitor is Cherrapunji, Megh?laya, with an annual average of 11,430 mm from the moist tropical monsoon. Less than 250 mm and more than 1,500 mm per year represent approximate extremes of rainfall for all of the continents. Rainfall is slight in the central regions of the subtropical anticyclones, which are therefore the desert regions of the Earth. In parts of the desert no appreciable rain has ever been observed.

          Over most of Europe, South America, eastern North America, and central Africa, the annual rainfall exceeds 500 mm (20 inches), while over most of Asia, excluding India, Tibet, and China, the annual rainfall is less than 500 mm, being less than 250 mm in a long tongue extending from Arabia across to northeast Mongolia.

          The central regions of Australia, most of northern and a part of southwest Africa, portions of the intermontane area of the United States, and portions of the west-central coast and southern east coast of South America also have less than 250 mm of rain in the year. Portions of the western coast of Africa, between the Equator and 10° N, a strip of the western coast of India, parts of Assam, a coastal strip of Myanmar (Burma), windward mountain slopes in the temperate latitudes of North and South America, and many isolated tropical stations average more than 2,500 mm of rain in the year. Rainfall intensities greater than 30 mm in five minutes, 150 mm in one hour, or 500 mm per day are quite rare, but these intensities on occasion have been more than doubled for the respective durations.

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          Tropical areas experience a lot of rain because high temperatures cause a large amount of water to evaporate from the sea to make clouds. Coastal areas of the world tend to experience more rainfall than those inland. One side of a mountain range may be drier than the other, because the mountains block the winds that bring the rain. These and other factors account for the varying amounts of rainfall around the world.

          Changes in rainfall and other forms of precipitation will be one of the most critical factors determining the overall impact of climate change. Rainfall is much more difficult to predict than temperature but there are some statements that scientists can make with confidence about the future.

          A warmer atmosphere can hold more moisture, and globally water vapour increases by 7% for every degree centigrade of warming. How this will translate into changes in global precipitation is less clear cut but the total volume of precipitation is likely to increase by 1-2% per degree of warming.

          There’s evidence to show that regions that are already wet are likely to get wetter, but details on how much wetter and what impacts there will be on a local scale are more difficult to ascertain. The dry regions of the subtropics are likely to get drier and will shift towards the poles. For much of Europe, wetter winters are expected, but with drier summers over central and southern Europe.

          It is the changes in weather patterns that make predicting rainfall particularly difficult. While different climate models are in broad agreement about future warming on a global scale, when it comes to predicting how these changes will impact weather – and consequently rainfall – there is less agreement at a detailed level.

          It is likely that in a warmer climate heavy rainfall will increase and be produced by fewer more intense events. This could lead to longer dry spells and a higher risk of floods.

          So far, any impact that climate change may have had generally on regional rainfall cannot be distinguished from natural variations. However, for some specific cases a signal is starting to emerge. A recent study showed that man-made climate change substantially increased the odds of damaging floods occurring in England and Wales in autumn 2000. For the UK, current understanding suggests that increases in heavy rainfall during winter may start to become discernible more generally in the 2020s.

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          Umbrellas have been used for over 1000 years, and it is thought that they probably originated in China. Early umbrellas were made of paper and bamboo and waterproofed with varnish.

         We have seen evidence of umbrellas in the ancient art and artifacts of Egypt, Assyria, Greece, and China.

          These ancient umbrellas or parasols, were first designed to provide shade from the sun. The Chinese were the first to waterproof their umbrellas for use as rain protection. They waxed and lacquered their paper parasols in order to use them for rain.

          The word “umbrella” comes from the Latin root word “umbra”, meaning shade or shadow. Starting in the 16th century umbrella became popular to the western world, especially in the rainy weather of northern Europe. At first it was considered only an accessory suitable for women. Then the Persian traveler and writer, Jonas Hanway (1712-86), carried and used an umbrella publicly in England for thirty years, and he popularized umbrella use among men. English gentleman often referred to their umbrellas as a “Hanway.”

          The first all umbrella shop was called “James Smith and Sons“. The shop opened in 1830, and is still located at 53 New Oxford St., in London, England.

          The early European umbrellas were made of wood or whalebone and covered with alpaca or oiled canvas. The artisans made the curved handles for the umbrellas out of hard woods like ebony, and were well paid for their efforts.

          In 1852, Samuel Fox invented the steel ribbed umbrella design. Fox also founded the “English Steels Company“, and claimed to have invented the steel ribbed umbrella as a way of using up stocks of farthingale stays, steel stays used in women’s corsets. African-American, inventor, William C. Carter patented an umbrella stand on August the 8th, 1885. After that, compact collapsible umbrellas were the next major technical innovation in umbrella manufacture, over a century later.

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          Rain can form in two ways. In tropical areas, where temperatures are warm, any water droplets in the clouds join together to form raindrops that are heavy enough to fall from the clouds. Elsewhere, rain starts life as snow in the freezing temperatures of the high clouds. As the snow falls nearer the ground, it will turn to rain if the temperature is above freezing.

          While it may be tempting to say that rain comes from clouds, you can also say that rain is clouds, giving up on their dreams of being water vapor and falling back down to Earth, where they start their journey through the precipitation cycle again. If you want a better understanding of why rain comes down from clouds, start with that precipitation cycle, the mechanism through which water moves from the Earth to the atmosphere and back again.

          The amount of water available on Earth never changes. But its state (liquid or gas/vapor) does, and that’s all thanks to thermal energy from the sun. As liquid water is heated by the sun, it receives enough energy to break its molecules apart and transform into water vapor.

          The warmer the air, the more water vapor it can hold. That warm, moisture-saturated air rises, along with the water vapor it contains, and as it rises it cools. Once the air has cooled past the “dew point,” it condenses around “condensation nuclei,” which are usually teeny-tiny particles of dust, smoke or even salt that are suspended in the air. (If you’ve ever looked through a shaft of sunlight and seen dust particles dancing in the air, that’s a great visual.)

          The tiny water droplets that initially form are what you see as clouds – and if you pay close attention to clouds in the sky, you’ll see that they’re constantly shrinking and growing in response to the warring forces of evaporation and condensation.

          Water vapor that has condensed into tiny droplets and formed clouds is well on its way to becoming rain – but it’s not there yet. For now, the water droplets are so tiny that the air currents keep them aloft, just as swirling particles of dust can stay in the air. But as those droplets continue to rise, buoyed by rising bodies of warm air, they have two routes for making it back to Earth.

          The first is when water droplets collide and coalesce with other droplets, eventually becoming heavier than the uplift of the air around them, at which point they fall down through the cloud. Or, through something called the Bergeron-Findeisen-Wegener process, the ice process of precipitation or simply the Bergeron process, the droplets rise high enough to freeze into ice crystals, attracting more water vapor to themselves and growing quickly until they’re heavy enough to fall as snow or melt and fall as rain.

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          Aircraft flying at high altitudes will leave a white trail behind them when the air is very cold. This is caused by the exhaust gases expelled by the aircraft’s engines. The gases contain a large amount of water vapour, which condenses and freezes in the cold, high-altitude air, leaving behind cloud-like trails called contrails.

          The condensation trails left behind jet aircrafts are called contrails. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust. Cloud formation by a mixing process is similar to the cloud you see when you exhale and “see your breath”. The figure below represents how saturation vapor pressure varies as a function of temperature. The blue line is the saturation vapor pressure for ice as a function of temperature (in degrees Kelvin). Air parcels in the region labeled saturated will form a cloud. Imagine two parcels of air, A and B as located on the diagram. Both parcels are unsaturated. If B represents the engine exhaust, then as it mixes with the environment (parcel A) its temperature and corresponding vapor pressure will follow the dotted line. Where this dotted line intersects the blue line is where the parcel becomes saturated.

          If you are attentive to contrail formation and duration, you will notice that they can rapidly dissipate or spread horizontally into an extensive thin cirrus layer. How long a contrail remains intact, depends on the humidity structure and winds of the upper troposphere. If the atmosphere is near saturation, the contrail may exist for some time. On the other hand, if the atmosphere is dry then as the contrail mixes with the environment it dissipates. Contrails are a concern in climate studies as increased jet aircraft traffic may result in an increase in cloud cover. It has been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20%. An increase in cloud amount changes the region’s radiation balance. For example, solar energy reaching the surface may be reduced, resulting in surface cooling. They also reduce the terrestrial energy losses of the planet, resulting in a warming. Jet exhaust also plays a role in modifying the chemistry of the upper troposphere and lower stratosphere. NASA and the DOE are sponsoring a research program to study the impact contrails have on atmospheric chemistry, weather and climate. In this series of satellite images we will investigate the duration of contrails. The satellite images are from the GOES-8 visible channel. Each image is separated in time by approximately 15 minutes. The GOES-8 image has a spatial resolution of approximately 1 km. The satellite image is a view of upper mid-west including southern Wisconsin and northern Illinois. Madison is located on the image. Contrails were observed from the ground during this period. At this wavelength, the GOES-8 imager is measuring the amount of radiant energy reflected by the surface and the clouds. The whiter a given portion of the image, the larger the amount of reflected visible light. White portions of the image represent thick clouds and dark regions are water or heavily vegetated regions. Contrails show up on the image as white streaks, similar to how they appear from a surface view.

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          Meteorologists measures cloud cover in oktas — the number of oktas indicates how much of the sky is covered with cloud. On a scale of 0 to 8, 0 oktas means that there are no clouds; 8 oktas means the sky is completely covered.

          In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast). In addition, in the SYNOP code there is an extra cloud cover indicator ‘9’ indicating that the sky is totally obscured, usually due to dense fog or heavy snow.

          When used in weather charts, okta measurements are shown by means of graphic symbols (rather than numerals) contained within weather circles, to which are attached further symbols indicating other measured data such as wind speed and wind direction.

          Although relatively straightforward to measure (visually, for instance, by using a mirror), oktas only estimate cloud cover in terms of the area of the sky covered by clouds. They do not account for cloud type or thickness, and this limits their use for estimating cloud albedo or surface solar radiation receipt.

          Cloud oktas can also be measured using satellite imagery from geostationary satellites equipped with high-resolution image sensors such as Himawari-8. Similar to traditional approaches, satellite images do not account for cloud composition.

          Oktas are often referenced in aviation weather forecasts and low level forecasts: SKC = Sky clear (0 oktas); FEW = Few (1 to 2 oktas); SCT = Scattered (3 to 4 oktas); BKN = Broken (5 to 7 oktas); OVC = Overcast (8 oktas); NSC = nil significant cloud; CAVOK = ceiling and visibility okay.

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          The air contains millions of microscopic dust particles, which absorb water from rivers, lakes and seas. This happens when the water is heated. The heat turns the water into an invisible gas called vapour — a process called evaporation. When the warm, moist air cools down, it condenses (turns back into a liquid) on the surface of the dust particles. When the tiny droplets of water group together, a cloud forms. Clouds can be formed in several different ways, such as by warm air rising up through thermals, or when warm air is forced over hills and mountains. They can also be formed when two air masses meet and the cold air pushes under the warm air, forcing it up.

          Clouds are created when water vapor, an invisible gas, turns into liquid water droplets. These water droplets form on tiny particles, like dust, that are floating in the air.

          You hang up a wet towel and, when you come back, it’s dry. You set out a bowl of water for your dog and when you look again, the water level in the bowl has dropped even though Woofy has been nowhere near the bowl.

          Where did the missing water go? It evaporated. That means some of the liquid water in the towel or bowl changed into an invisible gas called water vapor and drifted away into the atmosphere. (Notice that “evaporated” contains the word “vapor.”) The same thing is constantly happening with oceans, lakes, rivers, swamps, swimming pools – and everywhere water is in contact with air.

          Liquid water changes into a gas when water molecules get extra energy from a heat source such as the Sun or from other water molecules running into them. These energetic molecules then escape from the liquid water in the form of gas. In the process of changing from liquid to gas, the molecules absorb heat, which they carry with them into the atmosphere. That cools the water they leave behind.

          The air can only hold a certain amount of water vapor, depending on the temperature and weight of the air – or atmospheric pressure – in a given area. The higher the temperature or atmospheric pressure, the more water vapor the air can hold. When a certain volume of air is holding all the water vapor it can hold, it is said to be “saturated.”

          What happens if a saturated volume of air cools or the atmospheric pressure drops? The air is no longer able to hold all that water vapor. The excess amount changes from a gas into a liquid or solid (ice). The process of water changing from a gas to a liquid is called “condensation,” and when gas changes directly into a solid, it is called “deposition.” These two processes are how clouds form.

          Condensation happens with the help of tiny particles floating around in the air, such as dust, salt crystals from sea spray, bacteria or even ash from volcanoes. Those particles provide surfaces on which water vapor can change into liquid droplets or ice crystals.

          A large accumulation of such droplets or ice crystals is a cloud. We usually think of clouds as being up in the sky, but when conditions are right, a cloud can form at ground level, too. Then it’s called “fog.” If you’ve ever walked through fog, you’ve walked through a cloud.

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          Cumulus Clouds form at different heights, although they are most often seen in the middle of the cloud layer. Fluffy in appearance, cumulus clouds are often grey on the bottom and a very bright white at the top. Sometimes known as cauliflower clouds, they are usually seen on dry, sunny days.

          Cumulus clouds are clouds which have flat bases and are often described as “puffy”, “cotton-like” or “fluffy” in appearance. Their name derives from the Latin cumulo-, meaning heap or pile.[ Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.

          Cumulus clouds are often precursors of other types of clouds, such as cumulonimbus, when influenced by weather factors such as instability, moisture, and temperature gradient. Normally, cumulus clouds produce little or no precipitation, but they can grow into the precipitation-bearing congestus or cumulonimbus clouds. Cumulus clouds can be formed from water vapour, supercooled water droplets, or ice crystals, depending upon the ambient temperature. They come in many distinct subforms, and generally cool the earth by reflecting the incoming solar radiation. Cumulus clouds are part of the larger category of free-convective cumuliform clouds, which include cumulonimbus clouds. The latter genus-type is sometimes categorized separately as cumulonimbiform due to its more complex structure that often includes a cirriform or anvil top. There are also cumuliform clouds of limited convection that comprise stratocumulus (low-etage), altocumulus (middle-etage) and cirrocumulus (high-etage). These last three genus-types are sometimes classified separately as stratocumuliform.

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          Cirrus clouds form at heights above 600m ( 20,000ft ). At this altitude, it is so cold that water inside the clouds is frozen into crystals of ice. They have a feathery, wispy appearance and are sometime called “mares’ tails”. A large number of cirrus clouds will occasionally form a complete layer of white cloud.

          Cirrus is a genus of atmospheric cloud generally characterized by thin, wispy strands, giving the type its name from the Latin word cirrus, meaning a ringlet or curling lock of hair. This cloud can form at any altitude between 16,500 ft (5.0 km; 3.13 mi) and 45,000 ft (14 km; 8.5 mi) above sea level. The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of “mares’ tails”.

          From the surface of Earth, cirrus typically appears white, or a light grey in color. It forms when water vapor undergoes deposition at altitudes above 5,500 m (18,000 ft) in temperate regions and above 6,400 m (21,000 ft) in tropical regions. It also forms from the outflow of tropical cyclones or the anvils of cumulonimbus clouds. Since cirrus clouds arrive in advance of the frontal system or tropical cyclone, it indicates that weather conditions may soon deteriorate. While it indicates the arrival of precipitation (rain), cirrus clouds only produce fall streaks (falling ice crystals that evaporate before landing on the ground).

          Jet stream-powered cirrus can grow long enough to stretch across continents while remaining only a few kilometers deep. When visible light interacts with the ice crystals in cirrus cloud, it produces optical phenomena such as sun dogs and halos. Cirrus is known to raise the temperature of the air beneath the main cloud layer by an average of 10 °C (18 °F). When the individual filaments become so extensive that they are virtually indistinguishable from one another, they form a sheet of high cloud called cirrostratus. Convection at high altitudes can produce another high-based genus called cirrocumulus, a pattern of small cloud tufts that contain droplets of supercoiled water. Some polar stratospheric clouds can resemble cirrus, while noctilucent clouds are typically structured in a way that is similar to cirrus.

          Cirrus clouds form on other planets, including Mars, Jupiter, Saturn, Uranus, and possibly Neptune. They have even been seen on Titan, one of Saturn’s moons. Some of these extraterrestrial cirrus clouds are composed of ammonia or methane ice rather than water ice. The term cirrus is also used for certain interstellar clouds composed of sub-micrometer-sized dust grains.

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          Stratus clouds form at the lowest levels of the cloud around 500m (1600ft). They form in Layers that can build up across the whole sky. Stratus clouds produce light rain and drizzle and, in hilly areas, will often produce wet fog and mist over the ground.

          Stratus does not usually produce precipitation, but when it does occur it is in the form of minute particles, such as drizzle, ice crystals, or snow grains. Stratus often occurs in the form of ragged patches, or cloud fragments (stratus fractus), in which case rapid transformation is a common characteristic. Stratus clouds have characteristically low vertical velocities, usually less than 1 m s-1. When the sun is seen through the cloud, its outline is clearly discernible, and it may be accompanied by corona phenomena. In the immediate area of the solar disk, stratus may appear very white. Away from the sun, and at times when the cloud is sufficiently thick to obscure it, stratus gives off a weak, uniform luminance. The particulate composition of stratus is quite uniform, usually of fairly widely dispersed water droplets and, at lower temperatures, of ice crystals (although this is much less common). Halo phenomena may occur with this latter composition. Dense stratus often contains particles of precipitation. The prior existence of any other cloud in the low or middle levels is seldom required for the formation of stratus. A common mode of stratus development is the transformation of fog, the lower part of which evaporates while the upper part may rise (St nebulomutatus). As can be expected by its close relationship to fog, stratus follows a diurnal cycle with a maximum (over land) in the night and early morning. Insolation tends to dissipate this cloud rapidly, and often brings about the transformation of stratus fragments to cumulus clouds. Fog arriving from the sea frequently becomes stratus over the adjacent land. Coastal regions also provide the low-level moisture and frequently the lapse-rate stability conducive to its formation, and therefore these areas have the greatest stratus status. Stratus also develops from stratocumuls when the undersurface of the latter descends or for any reason loses its relief or apparent subdivisions. Nimbostratus and cumulonimbus often produce stratus fractus, as precipitation from these clouds causes low-level condensation (St fractusnimbostratogenitus or St fractus cumulonimbogenitus). Stratus fractus in this form constitutes the accessory feature pannus of these mother-clouds. Stratocumulus and nimbostratus are the clouds most difficult to distinguish from stratus. Stratus is lower and lacks the uniform undulations or relief of stratocumulus. More difficulty is encountered when differentiating it from nimbostratus. Their modes of formation are different, nimbostratus usually having been formed from a preexisting mid- or low-level cloud; nimbostratus is more dense and has a wetter aspect, and its precipitation is of the ordinary varieties. As a final distinction, the wind accompanying nimbostratus is usually stronger than that with stratus.

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          Like many things in the natural world, thunder had a mythological and spiritual significance for some early peoples, who endowed their gods with the power of many natural forces. The Greeks attributed the might of storms to Zeus, the king of the gods. When angry, he would smite the world below with his thunderbolts. In early Scandinavian mythology, the god Thor had some of the same attributes, being the god of the sky and controller of storms, lightning, rain and thunder. Farmers prayed to Thor for good harvests and fine weather.

Early Theories

          Early man probably considered lightning to be the ultimate weapon or a weapon of their gods. The Navajo Native Americans believed the Thunderbird, a mythical bird, flapped its wings and created the sound of thunder and the source of lightning was reflected sunlight from its eyes. It was the Norse god Thor, the Greek god Zeus, and the Roman god Jupiter who wielded the mighty bolt of lightning to keep man in his place. There continues today a phrase about lightning coming from a supernatural or divine power. The phrase goes something like: “Let lightning strike me if I’m ______.” The word ‘bolt’, often used to describe lightning, has no meaning in meteorology and is an inappropriately used term.

          Some of the earliest theories about thunder originated during the Greek and Roman Empires and from the Viking (Scandinavian) people. Beliefs about thunder included that it occurred before lightning, it was a burning wind, it was caused by the collision of clouds, the sound was produced by resonance between high and low clouds, and by high clouds descending and colliding onto low clouds. By the mid-19th century, the accepted theory was the vacuum theory, whereby lightning produced a vacuum along its path (channel), and thunder was due to the subsequent motion of air rushing into the vacuum. The second half of the 19th century saw the steam explosion theory, created when water along the lightning channel was heated and exploded by lightning’s heat. Another theory was the chemical explosion theory that suggested gaseous materials were created by lightning and then exploded.

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          Clouds are named according to their shape, height and size. They are normally associated with rain, snow, sleet or hail, but not all clouds mean that bad weather is on the way. Dark, angry-looking clouds normally bring wet and windy weather, but a sky full of fluffy white clouds on a warm and sunny day usually means that the weather will stay that way.

          Clouds are given different names based on their shape and their height in the sky. Some clouds are puffy like cotton while others are grey and uniform. Some clouds are near the ground, while others are near the top of the troposphere. The diagram on the right shows where different types of clouds are located in the sky.

          Most clouds can be divided into groups (high/middle/low) based on the height of the cloud’s base above the Earth’s surface. Other clouds are grouped not by their height, but by their unique characteristics, such as forming alongside mountains (Lenticular clouds) or forming beneath existing clouds (Mammatus clouds).

          The table below provides information about cloud groups and any cloud types associated with them. Click on the cloud images in the table to learn more about each cloud type.

          The cloud heights provided in this table are for the mid-latitudes. Cloud heights are different at the tropics and in the polar regions. In addition, a few other cloud types are found in higher layers of the atmosphere. Polar stratospheric clouds are located in a layer of the atmosphere called the stratosphere. Polar mesospheric clouds, which are also called noctilucent clouds, are located in the atmospheric layer called the mesosphere.

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          Because a Lightning strike seeks out the quickest route to the ground, it is unwise to shelter close to an isolated tall point, such as a tree, should you find yourself out in the open during a thunderstorm. The inside of a car is one of the safest places to shelter because if the car is struck, the electricity is conducted to the ground over the surface of the car. Of course, a secure building is the safest place to be during a storm.

          The safest place to be is a place where the lightning will not strike you or where you will not come into contact with any of the stray power surge that happens.

          Some people think that under a tree is safe because the lightning will hit the tree and not them. The electrons that are travelling through that lightning strike (they start on the ground and go up) are spread out in the wet grass. if you are standing in that grass you might feel quite a jolt. about 10 years ago, several people doing exactly that, taking shelter when a storm drifted over a soccer field where they were playing. One man died and several were taken to hospital. The rain hadn’t even started yet.

          If you are in a car, you should know that rubber tires, especially when they are wet are not very good insulators. If the lightning strikes a car there is a good chance that bad things will happen. If you’re driving and you blow a tire, accident. If the lightning sparks the fuel, boom. If the lightning surges through the interior of the car, don’t touch anything near the car’s metal parts.

          A house may be a decent place, especially one that has a lightning rod attached to a high point in the roof. That will be the point from which the lightning will jump up into the clouds and the electrons will travel up the very thick ground wire that runs from the lightning rod to a very big iron spike driven deep into the ground. A house that has no protection may not be so good. My mom told me the story of one thunderstorm she experienced as a kid in the ‘30s in a pre-electric house (yes, water from the well and oil lamps for light). A ball of lightning came in through her kitchen window and drifted over to the stove where it caused a lot of sparks to fly as it connected with the stove and stove pipe, which was running up and out the roof, acting like a non-grounded lightning rod. She had heard of someone who got in the path of that kind of thing and was badly burned.

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          A Waterspout is literally a tornado over water. It may look like a waterfall rising from the water surface as condensed water vapour is pulled into the updraft. Waterspouts are very rarely as powerful as tornadoes, but wind speeds of over 400km/h (250mph) can make them a severe hazard to nearby boats.

          Tornadoes concentrate more destructive energy in a smaller area than any other weather phenomenon. The highest winds on Earth are found inside the strongest tornadoes – some surpassing 300 miles per hour. But because tornadoes are so violent, it is very difficult for scientists to comprehend their inner workings. Only recently have meteorologists come to a good understanding of how and why tornadoes form.

          Tornadoes most often form within powerful rotating thunderstorms called supercells. Vast amounts of energy are released when the water vapor in rising air condenses to form thunderstorm clouds. Some of this energy is converted into vigorous vertical winds that move both upward (updrafts) and downward (downdrafts). When conditions are right, the updrafts of a rotating supercell can narrow into a powerful vortex, forming a tornado. This happens because, like water in a bathtub drain, the upward moving air begins to spiral as it meets resistance from downdrafts. As this spiral narrows, the energy it contains is concentrated into an ever smaller area, which results in the uniquely powerful winds found inside tornadoes.

           The strength of tornadoes is ranked along the Fujita scale according to their destructive capacity. The scale ranges from F0 to F5, based on damage, rather than funnel size. The intensity of a tornado is independent of its actual size – a small funnel can be either weak or strong, and the same is true for a large funnel. However, by examining the damage a storm has caused, engineers and scientists can determine the actual wind speed, a key factor in the Fujita scale.

          Tornadoes are most likely to occur in an area referred to as ? Tornado Alley,? located in the central United States between the Rocky Mountains and the Appalachian Mountains. This is the perfect landscape for tornados to form because the land is relatively flat, warm, humid air rises from the Gulf of Mexico, and cold, drier air descends from Canada. Tornadoes do, however, form in other areas and have been reported in all 50 states. They can even form over warm bodies of water, where they are known as waterspouts.

          Although they were commonly misunderstood and classified as hurricanes until the 19th century, records of tornadoes can be traced all the way back to the 1600s. Prior to the development of warning systems, radio and television, tornadoes were extremely deadly events. The worst single tornado on record occurred on March 18, 1925 across the states of Missouri, Illinois, and Indiana causing widespread destruction and killing 695 people. Due to the advancement of warning systems, better building structures, and increased public knowledge and awareness, we do not experience nearly as many deaths or as much destruction due to tornado activity.

          All forms of Lightning are produced when electrical charge passes between positively and negatively charged areas. In forked lightning, an initial stroke (known as a leader) travels to the ground at a speed of around 100km per second (62 miles per second), creating a path of electrically charged air. A return stroke that travels immediately back along the path is what we see. Lightning also flashes between points within the cloud itself and between the cloud and the air, lighting up the sky. If the flash of lightning is hidden by cloud cover, it appears to make the cloud glow. This is called sheet lightning.

          Any lightning that you can see is a potential danger. Lightning is a life-threatening event and should never be taken lightly.

          Forked lightning is quite visible as in the image above. Lightning comes out of a cloud and sometimes it forks, sometimes not, and strikes an object on the ground (cloud to ground) or another cloud (cloud to cloud) or even another part of the same cloud (in-cloud.)

          Sheet lightning can light up a entire cloud in a brief but spectacular display. It’s almost as cool looking as a long-lasting multi-forked cloud to ground strike.

          Far away sheet lightning seems to produce no sound and is sometimes called heat lightning. If you were closer, you would hear the thunder.

          They are all dangerous. Sheet lightning is a bolt that you can’t see because it is hidden by the cloud that it lights up. Most often, it is cloud to cloud or in-cloud (from one part of a cloud to another part of the same cloud.)

          Lightning can strike “out of the blue” on a clear day. Sometimes called “clear air lightning,” it originates within a cloud that is near or beyond the horizon—just a few miles on relatively flat ground. It starts as a cloud to cloud or in-cloud bolt, then exits the cloud and follows a horizontal path for some distance until it turns toward the ground. And that is why sheet lightning is just as dangerous. You can’t predict where it will go.

          In a thunderstorm, the strong upward draft of air in the center of the cell carries ice crystals and super-cooled (-10 F) water droplets up toward the top of the cell. Small hail, which is heavier, is suspended or falls slowly in the moving air. As the water and ice crystals travel up though the hail, the hail gathers positive charges from the water and crystals which leaves them negatively charged. (explaining it simplistically here)

          This leaves the mid and lower sections of the cell negatively charged and the top, or anvil, positively charged. As water condenses at the very bottom of the cloud, it too becomes positively charged in warmer air and falling rain.

          As the thunderstorm moves, the electrical charges in the cloud induce a smaller, opposite charge on the earth. As the cloud comes closer, the charge on the earth increases.

          We all know that in electricity and magnetism, opposites attract. A plasma channel of ionized air opens between the negative charges in the cloud and the positive charges on earth. The charges in the cloud follow this “leader” and reach for the earth in steps. Likewise, the charges on the ground also reach toward the cloud. The result is a bolt of lightning.

          Negative charges in one cloud can form leaders to the positive charges in another cloud, or even within the same cloud. A strong cell can produce a lightning strike every three seconds.

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          Damage to buildings during a storm obviously varies according to the construction and location of the building, but damage generally occurs above Force 9 or 10. Chimney pots, roofing tiles and slates are the parts of buildings most at risk from storm damage.

          As wind passes over and around a building, two things happen.  First, positive wind pressure applies to building components on the side(s) of the building that face the incoming wind (the “windward” direction).  The windward wind load is essentially the force of the blowing wind pressing on the building.  Second, negative wind pressure applies to building components on the side(s) of the building that face away from the incoming wind (the “leeward” direction).  The negative wind pressure is also known as “suction.”  The suction force will be applied to vertical surfaces such as walls, and also horizontal or sloped surfaces such as roofs.  The suction force can be thought of as acting like a vacuum that pulls on a building and its components.

          During an event with moderately strong winds, building materials such as asphalt shingles or vinyl siding may displace away from the building.  Sometimes, windows break and the broken shards end up outside of the building.  In such cases, a common misconception held by many people in the construction industry is that wind must have gotten under or behind the surface of the material to “blow” it out.  However, acknowledging and understanding the concept of wind suction can explain how those materials were damaged.  Wind does not only apply a direct blowing force on buildings, but it also induces a suction force.  Depending on various factors, the suction force can be significant enough to cause damage to individual building components or the structure of the building itself.

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          During high winds, some bridges may be closed for safety reasons. The structure of the bridges is rarely in doubt, although there have been cases of bridges collapsing in strong winds. The chief concern is for the safety of the vehicles that cross the bridge, particularly high-sided lorries and trucks. Those bridges in especially high positions are most prone to closure.

           When the wind reaches speeds of 65 miles an hour, the bridge closes to traffic.

          A category one hurricane starts at 75 miles per hour.

         The Mackinac Bridge Authority says there was a gust up to 72 miles per hour on Sunday.

          It caused both a camper and a boat on a trailer to tip over on the bridge.

         Typically the bridge would be closed at that wind speed, but that storm front only took a couple of minutes to go from calm winds to hurricane gusts.

         “We’re expected to get some pretty bad storms this week so everybody should take their time driving across the bridge,” said Bob Sweeney from the Mackinac Bridge Authority. “If you’re driving across the bridge during a high wind event, even if it’s only 20 miles per hours, or the winds only 20 miles per hour, drive slow, drive 20 miles per hour and you’ll safely get across the bridge.”

         At 35 miles per hour, the bridge authority starts escorting high profile vehicles like trucks or trailers.

        At 55 miles per hour, it is closed to those high profile vehicles.

        To keep an eye on the wind, the bridge authority watches weather reports online, and they have their own wind meter on the bridge.

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          The Beaufort scale was devised for use by sailors. By observing the wind’s effect on the ship’s rigging and the waves, sailors would know how much sail should be carried or stowed in order for the ship to sail efficiently and safely. The 12 levels of wind strength have since been adapted for use on land.

          Beaufort scale, in full Beaufort wind force scale, scale devised in 1805 by Commander (later Admiral and Knight Commander of the Bath) Francis Beaufort of the British navy for observing and classifying wind force at sea. Originally based on the effect of the wind on a full-rigged man-of-war, in 1838 it became mandatory for log entries in all ships in the Royal Navy. Altered to include observations of the state of the sea and phenomena on land as criteria, it was adopted in 1874 by the International Meteorological Committee for international use in weather telegraphy.

          The Beaufort scale as originally drawn up was calibrated to Beaufort’s assessment of the various effects of the wind on a full-rigged man-of-war. Somewhat arbitrarily, he identified 13 states of wind force on his vessel and ranked them 0 to 12. The scale, however, made no reference to the speed of the wind, and various attempts, particularly during the 20th century, have been made to correlate the two. An attempt made in 1912 by the International Commission for Weather Telegraphers was interrupted by World War I. In 1921 G.C. Simpson was asked to formulate equivalents, which were accepted in 1926 by the Committee. In June 1939 the International Meteorological Committee adopted a table of values referring to an anemometer at a height of 6 metres (20 feet). This was not immediately adopted by the official weather services of the United States and Great Britain, which used the earlier scale referring to an anemometer at an elevation of 11 metres (36 feet). The Beaufort force numbers 13 to 17 were added by the U.S. Weather Bureau in 1955.

          The scale is now rarely used by professional meteorologists, having been largely replaced by more objective methods of determining wind speeds—such as using anemometers, tracking wind echoes with Doppler radar, and monitoring the deflection of rising weather balloons and radiosondes from their points of release. Nevertheless, it is still useful in estimating the wind characteristics over a large area, and it may be used to estimate the wind where there are no wind instruments. The Beaufort scale also can be used to measure and describe the effects of different wind velocities on objects on land or at sea.

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          The strength of the wind varies between gentle breezes and destructive storms. Knowing the strength of the wind and its effect is important for the safety of people and property, particularly for those at sea. In 1805, Sir Francis Beaufort devised a scale by which the strength of the wind could be determined by observing its effect on the environment. This is known as the Beaufort scale.

          Wind has both speed and direction. Anemometers measure wind speed and wind vanes measure wind direction.

          A typical wind vane has a pointer in front and fins in back. When the wind is blowing, the wind vane points into the wind. For example, in a north wind, the wind vane points northward.

A cup anemometer is a common tool to measure wind speed. The cups catch the wind and produce pressure difference inside and outside the cup. The pressure difference, along with the force of the wind, causes the cups to rotate. Electric switches measure the speed of the rotation, which is proportional to the wind speed.

          At wind speeds below about 3 mph, the cup anemometer is prone to error because friction keeps the cups from turning. At wind speeds above 100 mph, cup anemometers often blow away or give unreliable measurements. In freezing rain, the anemometer can literally freeze up and stop turning.

          Propellers also can measure wind speed. The propeller blades rotate at a rate proportional to the wind speed.

          A windsock often is used at airports. A windsock is a cone-shaped bag with an opening at both ends. When it is limp, winds are light; when it is stretched out, winds are strong. Pilots can quickly determine the wind direction and speed along a runway just by observing the shape and direction of a windsock.

          Sonic anemometers use sound waves humans cannot hear to measure wind speed and direction. The instrument determines the wind velocity by measuring the time between when the instrument sends a sonic pulse and when it is received.

          An anemometer looks like a weather vane, but instead of measuring which direction the wind is blowing with pointers, it has four cups so that it can more accurately measure wind speed. Each cup is attached to the end of a horizontal arm, each of which is mounted on a central axis, like spokes on a wheel. When wind pushes into the cups, they rotate the axis. The faster the wind, the faster the cups spin the axis.

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          Windmills usually face into the prevailing wind, but they can also be adjusted should the wind direction change. Some types of windmills can be completely rotated according to the wind direction; in others, the angle of the sails can be adjusted to receive the maximum amount of wind power. Some wooden sails have spring shutters that open and close according to the wind strength. If the wind gusts, the shutters open up, if it drops, they close. In this way, a constant wind force is maintained on the windmill sails.

          Up until recently, people still only had visual impressions of what a windmill is, often associating it with the past and particularly before the industrial revolution. Today, things have come full circle, if you will and there is now a growing demand for large, technologically advanced windmills across the world. The term wind energy or wind power describe the process through which wind turbines convert the kinetic energy in the wind into electrical energy by the use of generator.

         What this introductory guide seeks to do is describe the apparatus in layman’s terms and also outline how they work and what they were intended for originally and the purposes for which they are used today. We begin with a brief definition of what a windmill is.

          It was originally a structure with sails, much like that on pre-industrial ships, and was originally used to produce flour from corn. In order to do this, the wind would have to prompt the sails to turn. They were also originally built by master craftsmen.

         A dictionary definition explains it thus; it is a machine which is propelled by the wind from a horizontal shaft which extended onto sails. Windmills still used today, mainly in parts of the world which have traditionally relied on them, are powered by electricity or water.

       The dictionary expounds this definition further by relating it to a human physical exercise technique which replicates the symbolism and movement of the original windmill. It is also famously symbolic in Cervantes’ classic of Don Quixote. This definition reminds readers that the original mill was also used to pump water and generate power.

       In modern terms, the advanced windmill operates with just three blades mainly to generate sustainable sources of electricity and energy. Today, these windmills are also referred to as wind turbines.

      Winds are produced due to uneven heating of the atmosphere by the sun, the rotation of the earth and the irregularities of the earth’s surface. Wind flow patterns differ from place to place and are modified by bodies of water, vegetation, and differences in terrain. This next section explains briefly but accurately how windmills work. Sourcing more extensive information, readers will learn that understanding technical processes initiated in wind turbines will be easy to follow because the manner in which windmills work follows a simple process. Here we continue to rely on layman’s terms.

       A number of different options were tried when modern wind turbines were first built. Today, the universal mechanizing principle is to operate the turbine by using just three blades placed around a rotor which is connected to a shaft. Note that numbers of variations have been tried, two blades and even one blade. But, three blades work the best.

       As its name states, the windmill’s only source of energy is derived from the wind. The wind turns the blades which spins a shaft, in turn, prompt a generator to produce electricity. These blades are connected to a generator, sometimes through a gearbox and sometimes directly. In both the cases, the generator converts the mechanical energy into electrical energy. Interestingly, most modern turbines turn in a clockwise direction. Depending on wind speed, most modern turbines can operate at speeds from as little as four meters per second to as much as 15 mps.

       Quite a number of green energy advocates and NGO’s describe the wind-generator process more succinctly by correlating it closely with the environmental sustainability initiatives.

       Once the turbine’s blades turns a shaft located inside of a box placed on top of the turbine, gearbox mode is propelled and more speed rotation is given off. A transformer within the turbine then converts electricity into a voltage suitable for distribution to a national grid.

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          Land breezes occur at night, as the land cools down more quickly than the sea. The cold air sinking over the land pushes out to the low-pressure area over the sea. Land breezes tend to be lighter than sea breezes, as the difference in temperature between the sea and the land during the night is only slight. Land and sea breezes help make a coastal climate very different from that inland.

          Land breeze, a local wind system characterized by a flow from land to water late at night. Land breezes alternate with sea breezes along coastlines adjacent to large bodies of water. Both are induced by differences that occur between the heating or cooling of the water surface and the adjacent land surface. The land breeze is typically shallower than the sea breeze since the cooling of the atmosphere over land is confined to a shallower layer at night than the heating of the air during the day. Since the surface flow of the land breeze terminates over water, a region of low-level air convergence is produced. Locally, such convergence often induces the upward movement of air, fostering the development of clouds. Therefore, it is not uncommon to see clouds lying off the coast at night, which are later dissipated by the daytime sea breeze.

          The air over the ocean is now warmer than the air over the land. The land loses heat quickly after the sun goes down and the air above it cools too.  This can be compared to a blacktop road. During the day, the blacktop road heats up and becomes very hot to walk on. At night, however, the blacktop has given up the added heat and is cool to the touch. The ocean, however, is able to hold onto this heat after the sun sets and not lose it as easily. This causes the low surface pressure to shift to over the ocean during the night and the high surface pressure to move over the land. This causes a small temperature gradient between the ocean surface and the nearby land at night and the wind will blow from the land to the ocean creating the land breeze.

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          On a hot and sunny day, coastal areas will experience sea breezes. The land and the sea heat up and cool down at different rates, producing moving currents of air. The land heats more quickly than the sea, producing an area of low pressure, into which the cooler sea air moves. This breeze may move in a completely different direction from the prevailing wind and can blow up to 30km (18 miles) inland.

           A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes are more localized than prevailing winds. Because land absorbs solar radiation far more quickly than water, a sea breeze is a common occurrence along coasts after sunrise. By contrast, a land breeze or offshore breeze is the reverse effect: dry land also cools more quickly than water and, after sunset, a sea breeze dissipates and the wind instead flows from the land towards the sea. Sea breezes and land breezes are both important factors in coastal regions’ prevailing winds. The term offshore wind may refer to any wind over open water.

          Wind farms are often situated near a coast to take advantage of the normal daily fluctuations of wind speed resulting from sea or land breezes. While many onshore wind farms and offshore wind farms do not rely on these winds, a near shore wind farm is a type of offshore wind farm located on shallow coastal waters to take advantage of both sea and land breezes. (For practical reasons, other offshore wind farms are situated further out to sea and rely on prevailing winds rather than sea breezes.)

Cause

          The sea has a greater heat capacity than land, so the surface of the sea warms up more slowly than the land’s. As the temperature of the surface of the land rises, the land heats the air above it by convection. The warming air expands and becomes less dense, decreasing the pressure over the land near the coast. The air above the sea has a relatively higher pressure, causing air near the coast to flow towards the lower pressure over land. The strength of the sea breeze is directly proportional to the temperature difference between the land and the sea. If a strong offshore wind is present (that is, a wind greater than 8 knots (15 km/h)) and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop.

Effects

          A sea-breeze front is a weather front created by a sea breeze, also known as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, the front can sometimes trigger thunderstorms. If the flow aloft is aligned with the direction of the sea breeze, places experiencing the sea breeze frontal passage will have benign, or fair, weather for the remainder of the day. At the front warm air continues to flow upward and cold air continually moves in to replace it and so the front moves progressively inland. Its speed depends on whether it is assisted or hampered by the prevailing wind, and the strength of the thermal contrast between land and sea. At night, the sea breeze usually changes to a land breeze, due to a reversal of the same mechanisms.

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          The wind can make the air temperature feel colder than it actually is. A thin layer of warm air normally surrounds your body, creating an insulating “blanket” of air. If the wind is strong, this warm air gets blown away, making you feel a lot colder. This is known as the wind-chill factor. In a breeze blowing at 9km/h (5.6mph), an air temperature of 0°C (32°F) will feel like —3°C (27°F). If the breeze increases to around 15km/h (9.3mph), the wind-chill factor will make it feel like —10°C (14°F).

Meteorologists call this phenomenon the wind chill factor.

   Wind chill is what the air temperature feels like on our exposed skin due to wind. It’s always lower than the actual air temperature.

   For example, even though the thermometer may indicate it’s 35° F (1.6° C) outside, a 25-mile-per-hour wind will make it feel like it’s only 8° F (-13.3° C)!

   The opposite effect can occur at temperatures above 50° F (10° C). At higher temperatures, humidity on the skin can make the air temperature feel hotter than the actual temperature. Meteorologists call this effect the heat index.

   It’s important to note that wind chill is a prediction of what experts believe humans will perceive the temperature to be because of the wind. No matter how fast the wind blows, the air temperature is what it is and can be measured by a thermometer.

     The wind chill factor, on the other hand, is calculated using various formulas. There is no one formula that all scientists agree on. Most meteorologists in the United States use a standard formula accepted by the National Weather Service.

    What causes the wind chill effect? It’s a result of the fact that the human body loses heat through a scientific process called convection.

     During convection, heated air molecules rise into the air and are replaced by cooler air molecules. How quickly your body loses heat by convection depends on air flow around your body.

     Your warm body usually loses heat slowly. When it’s windy, though, the wind carries the warm air molecules away from your body more quickly, making you feel colder than the actual air temperature around you.

     The faster the wind blows, the faster your body loses heat by convection. As the air temperature around you falls, the effect of wind is magnified, making the wind chill effect greater the colder it gets.

     If you’ve ever blown on a hot bowl of soup to cool it down before eating, you’ve created your own wind chill effect on your soup!

     Even though the air temperature stays the same, the presence of wind makes us feel like it’s colder outside than it actually is. The wind chill effect isn’t all mental, though.

     Since wind chill speeds up heat loss by convection, our bodies experience heat loss and react as if the temperature were as low as it feels…even if the actual air temperature is much higher than the wind chill factor.

     Wind chill factors are calculated under the assumption that a person is properly dressed and dry. If you’re not wearing suitable outdoor clothing, if your clothes are wet, or if you’ve been outside for an extended period of time, the wind chill factor will be magnified even further.

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          Prevailing Winds are those that blow constantly in certain parts of the world. They are produced by hot air moving north and south from the Equator and by cold air moving away from the poles. The prevailing winds are the Polar Easterlies, found in the extreme north and south; the Westerlies, blowing between 30° and 60° north and south of the Equator; and the Trade winds, which blow north-east and south-east, either side of the Equator.

          Since the atmosphere is fixed to the earth by gravity and rotates with the earth, there would be no circulation if some force did not upset the atmosphere’s equilibrium.  The heating of the earth’s surface by the sun is the force responsible for creating the circulation that does exist.

          Because of the curvature of the earth, the most direct rays of the sun strike the earth in the vicinity of the equator resulting in the greatest concentration of heat, the largest possible amount of radiation, and the maximum heating of the atmosphere in this area of the earth.  At the same time, the sun’s rays strike the earth at the poles at a very oblique angle, resulting in a much lower concentration of heat and much less radiation so that there is, in fact, very little heating of the atmosphere over the poles and consequently very cold temperatures.

          Cold air, being more dense, sinks and hot air, being less dense, rises.  Consequently, the rising warm air at the equator becomes even less dense as it rises and its pressure decreases.  An area of low pressure, therefore, exists over the equator.

          Warm air rises until it reaches a certain height at which it starts to spill over into surrounding areas.  At the poles, the cold dense air sinks.  Air from the upper levels of the atmosphere flows in on top of it increasing the weight and creating an area of high pressure at the poles.

          The air that rises at the equator does not flow directly to the poles. Due to the rotation of the earth, there is a buildup of air at about 30° north latitude. (The same phenomenon occurs in the Southern Hemisphere).   Some of the air sinks, causing a belt of high-pressure at this latitude.

          The sinking air reaches the surface and flows north and south.  The air that flows south completes one cell of the earth’s circulation pattern.  The air that flows north becomes part of another cell of circulation between 30° and 60° north latitude.  At the same time, the sinking air at the North Pole flows south and collides with the air moving north from the 30° high pressure area.  The colliding air is forced upward and an area of low pressure is created near 60° north.  The third cell circulation pattern is created between the North Pole and 60° north.

          Because of the rotation of the earth and the coriolis force, air is deflected to the right in the Northern Hemisphere.  As a result, the movement of air in the polar cell circulation produces the polar easterlies.   In the circulation cell that exists between 60° and 30° north, the movement of air produces the prevailing westerlies.  In the tropic circulation cell, the northeast trade winds are produced.  These are the so-called permanent wind systems of the each. 

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          The moving Air that produces the winds tries to take the most direct route possible between the different areas of pressure. However, it is deflected by the rotating movement of the Earth. This is known as the Coriolis effect. In the Northern Hemisphere, the winds are deflected to the right of the direction in which they are headed; in the Southern Hemisphere, they are deflected to the left.

          The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around  Earth. The Coriolis effect is responsible for many large-scale weather patterns.

          The key to the Coriolis effect lies in Earth’s rotation. Specifically, Earth rotates faster at the Equator than it does at the poles. Earth is wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour. Near the poles, Earth rotates at a sluggish 0.00008 kilometers (0.00005 miles) per hour.

          Let’s pretend you’re standing at the Equator and you want to throw a ball to your friend in the middle of North America. If you throw the ball in a straight line, it will appear to land to the right of your friend because he’s moving slower and has not caught up.

          Now let’s pretend you’re standing at the North Pole. When you throw the ball to your friend, it will again to appear to land to the right of him. But this time, it’s because he’s moving faster than you are and has moved ahead of the ball.

          Everywhere you play global-scale “catch” in the Northern Hemisphere, the ball will deflect to the right.

          This apparent deflection is the Coriolis effect. Fluids traveling across large areas, such as air currents, are like the path of the ball. They appear to bend to the right in the Northern Hemisphere. The Coriolis effect behaves the opposite way in the Southern Hemisphere, where currents appear to bend to the left.

          The impact of the Coriolis effect is dependent on velocity—the velocity of Earth and the velocity of the object or fluid being deflected by the Coriolis effect. The impact of the Coriolis effect is most significant with high speeds or long distances. 

Weather Patterns

          The development of weather patterns, such as cyclones and trade winds, are examples of the impact of the Coriolis effect.

          Cyclones are low-pressure systems that suck air into their center, or “eye.” In the Northern Hemisphere, fluids from high-pressure systems pass low-pressure systems to their right. As air masses are pulled into cyclones from all directions, they are deflected, and the storm system—a hurricane—seems to rotate counter-clockwise.

          In the Southern Hemisphere, currents are deflected to the left. As a result, storm systems seem to rotate clockwise.

          Outside storm systems, the impact of the Coriolis effect helps define regular wind patterns around the globe. 

          As warm air rises near the Equator, for instance, it flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right (east) as they move northward. The currents descend back toward the ground at about 30° north latitude. As the current descends, it gradually moves from the northeast to the southwest, back toward the Equator. The consistently circulating patterns of these air masses are known as trade winds.

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          The wind is created by differences in air pressure and temperature —winds blow from areas of high pressure to those of low pressure. Rising warm air creates a low-pressure area, and the gap created is filled by high pressure produced by cooler air. The greater the difference in pressure, the stronger the wind.

          Bob Dylan says, “You don’t need to be a weatherman to know which way the wind blows”. I say to Bob, “But, it doesn’t hurt!”

          Wind is a rather elusive meteorological variable, especially since we can’t really see it, like we can clouds or precipitation. Wind, during a storm, is something we expect. Wind can be an unpleasant nuisance though, especially on a bluebird day, to cyclists, sailors, paragliders, climbers, etc.

          The atmosphere is constantly adjusting itself, trying to balance the changes in temperature and humidity from one part of the planet to the other. This leads to different areas of high and low pressure that encircle the globe, and the bigger the difference in temperature, and/or humidity, from one area to another, the bigger the difference in pressure, and the faster the wind blows.

          That’s what gets it started in motion, always moving from high pressure towards lower pressure. Friction at the surface, mountains, buildings, etc. can slow the wind down and alter its direction. In the upper levels of the atmosphere, the wind starts moving from high to low, but it gets re-routed, and turned to the right in the northern hemisphere, because the earth is rotating. This is known as the Coriolis Effect.

          When we observe stronger winds, it means that there is a big difference in pressure across the region, or sometimes across the entire country. A big low-pressure center over the mid-western U.S. and a big area of high-pressure along the West Coast, for instance, could result in strong winds in-between, over the Rockies.

          That difference in pressure from Point-A to Point-B is known as a pressure-gradient. A strong pressure-gradient equals strong winds. You can track that each day by looking at a surface weather map, and look for big highs and big lows, and lots of pressure contour lines in-between, as well.

          The other thing that can cause strong winds at the surface is when the jet stream is directly overhead.

Example: The air pressure is higher in an inflated balloon than outside it. If a hole is made in the balloon, the air streams out, creating a wind that blows from the greater pressure in the direction of the lower. The wind settles when the pressure is the same inside the balloon as outside.In the atmosphere the pressure at the earth’s surface reflects the weight of air above it, which in turn is determined mostly by its temperature, and as people generally know from everyday life, hot air is lighter than cold. This fits with the fact that depressions (low pressure systems) usually bring warm air.

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          A cold front is followed by an area of cold air. Thick, dark clouds, heavy rain and sometimes violent storms arrive immediately. If seen from the side, a cold front looks much steeper than a warm front. Cold air pushes beneath the warm air and rising water vapour condenses into clouds and then rain. Showers of rain will often follow as the front passes over.

          The cold front is depicted on a weather map as a blue line with triangles or as simply a blue line (Figure 9.28). A cold front, as discussed in the previous section, is the leading edge of colder air brought southward by winds around an area of low pressure. These fronts are most common during the active weather times of fall, winter, and spring.

          Winds ahead of the cold front are southwesterly in the warm sector of the mid-latitude cyclone. After the cold front passes a point, winds turn to the west, northwest, or north. Since the cold air is very dense it is very effective at displacing the warm air ahead of it. The dense cold runs under the warm air lifting it. The lifting of warm moist air usually causes cloudiness at the least. If the air is moist and unstable enough, rain and thunderstorms can accompany the passage of the front. Air pressure usually falls as a cold front approaches, rising rapidly after passage as the dense cold air moves in. The dew point falls indicating the change to a dry air mass. Usually there is little local observational evidence of a cold front approaching.

          A cold weather front is defined as the changeover region where a cold air mass is replacing a warmer air mass. Cold weather fronts usually move from northwest to southeast. The air behind a cold front is colder and drier than the air in front. When a cold front passes through, temperatures can drop more than 15 degrees within an hour.

          On a weather forecast map, a cold front is represented by a solid line with blue triangles along the front pointing towards the warmer air and in the direction of movement.

          There is usually an obvious temperature change from one side of a cold front to the other. It has been known that temperatures east of a cold front could be approximately 55 degrees Fahrenheit while a short distance behind the cold front, the temperature can go down to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.

          Again, there is typically a noticeable temperature change from one side of the warm front to the other, much the same as a cold front.

          If colder air is replacing warmer air, it is a cold front, if warmer air is replacing cold air, then it is a warm front.

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          As its name suggests, a warm front has an area of warm, moist air behind it. The warm air rises above the cold air, and clouds are formed along the front. From the ground, the first sign of a warm front approaching is the sight of high, wispy cirrus clouds and maybe some light rain. When the warm front has passed, there is usually a short period of dry weather.

          A warm front is the transition zone that marks where a warm air mass starts replacing a cold air mass. Warm fronts tend to move from southwest to southeast. Normally the air behind a warm front is warmer than the air in front of it. Normally when a warm front passes through an area the air will get warmer and more humid. Warm fronts signal significant changes in the weather. Here are some of the weather signs that appear as a warm front passes over a region.

          First before the warm front arrives the pressure in area start to steadily decrease and temperatures remain cool. The winds tend to blow south to southeast in the northern hemisphere and north to northeast in the southern hemisphere. The precipitation is normally rain, sleet, or snow. Common cloud types that appear would various types of stratus, cumulus, and nimbus clouds. The dew point also rises steadily

          While the front is passing through a region temperatures start to warm rapidly. The atmospheric pressure in the area that was dropping starts to level off. The winds become variable and precipitation turns into a light drizzle. Clouds are mostly stratus type clouds formations. The dew point then starts to level off.

          After the warm front passes conditions completely reverse. The atmospheric pressure rises slightly before falling. The temperatures are warmer then they level off. The winds in the northern hemisphere blow south-southwest in the northern hemisphere and north-northwest in the southern hemisphere. Cloudy conditions start to clear with only cumulonimbus and stratus clouds. The dew point rises then levels off.

          Knowing about how warm fronts work gives a better understanding of how pressure systems interact with geography to create weather. Looking at warm fronts we learn that they are the transition zone between warm humid air masses and cool, dry air masses. We know that these masses interact in a cycle of rising and falling air that alters the pressure of atmosphere causing changes in weather.

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          Four major masses of air lie over different parts of the world. The tropical maritime mass is warm and moist; the tropical continental mass is hot and dry. The polar continental mass is cold and dry, and the polar maritime mass is cold and wet. These air masses are blown around by high-level winds, and their interactions have a major influence on the world’s weather. The kind of weather experienced depends on the nature of the air mass — tropical masses bring warm, humid weather, and the polar masses tend to bring snow. In places where these masses meet, the weather can be very changeable indeed.

          Weather is controlled by a variety of factors. One of the most important is Earth’s air masses. Air masses are huge parcels of air with specific characteristics. What’s interesting about the characteristics of an air mass is that, not only do they describe the air mass, but they also tell you where you can find that air mass on Earth.

         Let’s look at the different types of air masses found on Earth to see how this works. Air masses can be divided into two main categories based on whether they are found over land or water. If the air mass is found over land, this is a continental air mass. If the air mass is found over water, this is a maritime air mass. This makes sense: continental air masses occur over the continents, maritime air masses occur over the water, or marine environments. These categories are represented by a lowercase ‘c’ for continental or ‘m’ for maritime.

          The source region of the air mass helps us classify it even further, and for this, we have three categories. Arctic air masses occur over arctic regions, like Greenland and Antarctica. Polar air masses occur a little bit farther from the poles, like in Siberia, Canada and the northern Atlantic and Pacific Oceans.

          Finally, tropical air masses occur in the tropics, so along the equator and over Mexico and the Southwest U.S. Makes sense, right? These categories are represented by the first letter of the source region, but this time we use an uppercase letter. So, ‘A’ stands for arctic, ‘P’ for polar and ‘T’ for tropical. That’s pretty easy to remember!

          Each source region can also be either continental or maritime, and to represent this, we simply combine the category letters. This gives us six total types of air masses on Earth: maritime arctic (mA), maritime polar (mP), maritime tropical (mT); and continental arctic (cA), continental polar (cP) and continental tropical (cT).

Air Masses and Weather

          You can understand a lot about weather from air masses just by looking at the name. Maritime air masses are going to produce moist weather because they occur over oceans, and oceans are filled with water! The air blowing over the ocean regions, either arctic, polar or tropical, picks up that moisture as it travels along. In maritime arctic and polar regions, this moist air is cool (as you probably expected), and the maritime tropical air mass produces the warm, humid conditions you would expect along the tropics, like Florida and the Caribbean.

          In contrast, continental air masses produce dry weather. This is because the continents just can’t compete with the oceans when it comes to moisture! The continental arctic and polar air masses produce dry, cold weather in the winter and pleasant weather conditions in the summer.

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          When different air masses meet, varying pressure differences cause two things to happen. Warm air either bulges into the cold air, or the cold air pushes into the warm air. The collision causes the warm air to rise rapidly over the cold air, creating an area of low pressure called a frontal depression. The weather in this area becomes very unsettled and is worse when the differences in pressure and temperature are greatest. Depressions cover huge areas but tend to pass over in less than a day.

          Cloud formation occurs when humid or water vapor-filled air rises to the point where cooler temperatures force condensation. This often involves the movement of air masses, which are large bodies of air with similar temperatures and moisture content. Air masses are typically at least 1,000 miles (1,600 km) wide and several miles thick.

Four naturally occurring mechanisms on Earth cause air to rise:

Orographic lifting: This phenomenon occurs when an airflow encounters elevated terrains, such as mountain ranges. Like a speeding car heading toward a hill, the wind simply powers up the slope. As it rises with the topography, water vapor in the airflow condenses and forms clouds. This side of the mountain is called the windward side and typically hosts a great deal of cloud cover and precipitation. The other side of the mountain, the leeward side, is generally less lucky. The airflow loses much of its moisture in climbing the windward side. Many mountain ranges virtually squeeze incoming winds like a sponge and, as a result, their leeward sides are home to dry wastes and deserts.

Frontal wedging: When a warm air mass and a cold air mass collide, you get a front. Remember how low-pressure warm air rises and cold high-pressure air moves into its place? The same reaction happens here, except the two forces slam into each other. The cold air forms a wedge underneath the warm air, allowing it to basically ride up into the troposphere on its back and generate rain clouds. There are four main kinds of fronts, classified by airflow momentum. In a warm front, a warm air mass moves into a cold air mass. In a cold front, the opposite occurs. In a stationary front, neither air mass advances. Think of it as two fronts bumping into each other by accident. In an occluded front, a cold front overtakes a moving warm front, like an army swarming over a fleeing enemy.

Convergence: When two air masses of the same temperature collide and neither is willing to go back down, the only way to go is up. As the name implies, the two winds converge and rise together in an updraft that often leads to cloud formation.

Localized convective lifting: Remember the city example? This phenomenon employs the exact same principle, except on a smaller scale. Unequal heating on the Earth’s surface can cause a pocket of air to heat faster than the surrounding air. The pocket ascends, taking water vapor with it, which can form clouds. An example of this might be a rocky clearing in a field or an airport runway, as both absorb more heat than the surrounding area.

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          Swirling masses of high- and low-pressure air are constantly moving around the Earth. When two masses of air with different characteristics meet, they do not mix, and a boundary develops between them. This boundary is called a front. On the ground, the arrival and departure of a front is felt by sharp changes in the weather.

          A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity.

          Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front’s passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.

          Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts.When a front becomes stationary—and the density contrast across the frontal boundary vanishes—the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. This is most common over the open ocean.

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          Severl Major bands of high-and low-pressure areas exist in different parts of the world. Air moves from the areas of high pressure to the low-pressure areas. The movements between these areas contribute to the world’s winds and weather patterns.

          If an isobar chart is observed, it can be seen that pressure is not distributed uniformly in the atmosphere around our planet: there are areas with a lower pressure than the surrounding areas and areas where the pressure is higher. Due to a characteristic of gases, air tends to move from high pressure areas towards those with low pressure in an attempt to balance the difference. The presence of high and low pressure areas is therefore the principal motor of all meteorological phenomena, in other words, of the ‘weather’. Hence, it is important to understand how air circulates close to these areas (see graph) and how they are distributed in the atmosphere.

Anticyclones

          In high pressure zones, air tends to sink towards the ground causing the air that is present to move away with a divergent movement. The air gets compressed while descending and tends to disperse the clouds, and in fact high pressure conditions are associated with settled and calm weather. As a result of the Coriolis effect, air tends to move away from the high pressure system, clockwise in our hemisphere and anticlockwise in the Southern Hemisphere (anticyclonic circulation).

Cyclones

          A low pressure area, instead, tends to attract air from the surrounding region where the pressure is higher. Near the centre of the cyclone, air tends to rise higher attracting a growing amount of air from the neighbouring areas. On rising, air expands and cools with the subsequent formation of clouds and precipitation: it is for this reason that low pressure areas are usually associated with bad weather. Air tends to converge towards the low pressure centre with an anticlockwise movement in our hemisphere and a clockwise movement in the Southern Hemisphere (cyclonic circulation).

Circulation cells

          Temperature and pressure differences are not distributed casually in the atmosphere but permanent and stable low and high pressure areas can be identified, which are organized so as to form big circulation cells around the world (Mean Annual Isobars, Isobars in the month of July, Isobars in the month of January). This situation, obviously, is not static and unchangeable. During the year the circulation cells move towards the North or the South, depending on the unequal amount of solar energy that the different regions of the Earth receive in each season: in our hemisphere, they move towards the Equator in winter and towards the Poles in summer.
          Three main circulation cells can be identified in each hemisphere that are placed symmetrically respect to the Equator.

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          The equtor receives the greatest amount of the Sun’s heat, making the land very hot. This heats up the air, creating a large area of mainly low pressure. This area is known as the Intertropical Convergence Zone (ITCZ).

          The ITCZ (Intertropical Convergence Zone) play important role in the global circulation system and also known as the Equatorial Convergence Zone or Intertropical Front. It is a basically low pressure belt encircling Earth near the Equator. It is a zone of convergence where the trade winds meet. Here, we are giving the concept, causes and impact of ITCZ (Intertropical Convergence Zone) for general awareness.

          It is a zone between the northern and southern hemisphere where winds blowing equator-ward from the mid latitudes and winds flowing poleward from the tropics meet. It shifts from north and south seasonally according to the movement of the Sun. For Example- when the ITCZ is shifted to north of the Equator, the southeast trade wind changes to a southwest wind as it crosses the Equator. The ITCZ shifts only between 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean.

          ITCZ (Intertropical Convergence Zone) is caused by the convergence of northeast and southeast trade winds in the area encircling Earth near the Equator. For better understanding, we must know about the trade winds and air masses.

1. Trade Winds: Easterly winds that circle the Earth near the equator.

2. Air Masses: A volume of air defined by its temperature and water vapour content. In tropical latitudes this air mass is hot to very hot, with high relative humidity, bringing unstable weather.

         It appears as a band of clouds consisting of showers, with occasional thunderstorms, that encircles the globe near the equator due to the convergence of the trade winds.

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          At the altitude at which many jet aircraft fly, the air pressure is extremely low —less than the pressure inside the human body. This makes it impossible for the body to take in air. There is also very little oxygen, so the air inside the plane has to be pressurized in order to simulate the level of air pressure on the surface.

          Although aircraft cabins are pressurized, cabin air pressure at cruising altitude is lower than air pressure at sea level. At typical cruising altitudes in the range 11 000–12 200 m (36 000–40 000 feet), air pressure in the cabin is equivalent to the outside air pressure at 1800–2400 m (6000–8000 feet) above sea level. As a consequence, less oxygen is taken up by the blood (hypoxia) and gases within the body expand. The effects of reduced cabin air pressure are usually well tolerated by healthy passengers.

Oxygen and hypoxia

          Cabin air contains ample oxygen for healthy passengers and crew. However, because cabin air pressure is relatively low, the amount of oxygen carried in the blood is reduced compared with that at sea level. Passengers with certain medical conditions, particularly heart and lung diseases and blood disorders such as anaemia (in particular sickle-cell anaemia), may not tolerate this reduced oxygen level (hypoxia) very well. Some of these passengers are able to travel safely if arrangements are made with the airline for the provision of an additional oxygen supply during flight. However, because regulations and practices differ from country to country and between airlines, it is strongly recommended that these travellers, especially those wishing to carry their own oxygen, contact the airline early in their travel plans. An additional charge is often levied on passengers who require supplemental oxygen to be provided by the airline.

Gas expansion

           As the aircraft climbs in altitude after take-off, the decreasing cabin air pressure causes gases to expand. Similarly, as the aircraft descends in altitude before landing, the increasing pressure in the cabin causes gases to contract. These changes may have effects where air is trapped in the body.

          Passengers often experience a “popping” sensation in the ears caused by air escaping from the middle ear and the sinuses during the aircraft’s climb. This is not usually considered a problem. As the aircraft descends in altitude prior to landing, air must flow back into the middle ear and sinuses in order to equalize pressure. If this does not happen, the ears or sinuses may feel as if they are blocked and pain can result. Swallowing, chewing or yawning (“clearing the ears”) will usually relieve any discomfort. As soon as it is recognized that the problem will not resolve itself using these methods, a short forceful expiration against a pinched nose and closed mouth (Valsalva manoeuvre) should be tried and will usually help. For infants, feeding or giving a pacifier (dummy) to stimulate swallowing may reduce the symptoms.

          Individuals with ear, nose and sinus infections should avoid flying because pain and injury may result from the inability to equalize pressure differences. If travel cannot be avoided, the use of decongestant nasal drops shortly before the flight and again before descent may be helpful.

          As the aircraft climbs, expansion of gas in the abdomen can cause discomfort, although this is usually mild.

         Some forms of surgery (e.g. abdominal surgery) and other medical treatments or tests (e.g. treatment for a detached retina) may introduce air or other gases into a body cavity. Travellers who have recently undergone such procedures should ask a travel medicine physician or their treating physician how long they should wait before undertaking air travel.

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          An instrument called a barometer is used to measure air pressure. A mercury barometer consists of a glass tube standing in an open dish of mercury. The air pressure pushes against the mercury and forces it up the tube. The level of the mercury is recorded against a scale. Mercury barometers are clumsy, and mercury is poisonous, so aneroid barometers are more commonly used. A sealed metal box inside the barometer is connected to the pointer on the clock-like face. The vacuum inside the metal box means that an increase in pressure will squash it; a drop in pressure will make it expand. These changes make the pointer move around the dial.

          Even though we can’t see air, it is real and has pressure. The pressure of the atmosphere changes. It is higher at sea level, and lessens as you go higher up in the atmosphere. Some weather systems have slightly higher pressure than others – you may have heard of High pressure and Low Pressure weather system.

          Let’s look at how atmospheric pressure is measured. For a long time, atmospheric pressure has been measured by a mercury barometer. The first was invented in 1643 by one of Galileo’s assistants. A mercurial barometer has a section of mercury exposed to the atmosphere. The atmosphere pushes downward on the mercury (see image). If there is an increase in pressure, it forces the mercury to rise inside the glass tube and a higher measurement is shown. If atmospheric pressure lessens, downward force on the mercury lessens and the height of the mercury inside the tube lowers. A lower measurement would be shown. This type of instrument can be used in a lab or a weather station, but is not easy to move! Measurements from a mercury barometer are usually made in inches of Mercury (in Hg).

          An aneroid barometer can be used in place of a mercury barometer. It is easier to move and is often easier to read. This instrument contains sealed wafers that shrink or spread out depending on changes of atmospheric pressure. If atmospheric pressure is higher, the wafers will be squished together. If atmospheric pressure lessens, it allows the wafers to grow bigger. The changes in the wafers move a mechanical arm that shows higher or lower air pressure (see image).

          Either a mercury barometer or an aneroid barometer can be set up to make constant measurements of atmospheric pressure. Then it is called a barograph (see image). The barograph may constantly record pressure on paper or foil wrapped around a drum that makes one turn per day, per week, or per month. Nowadays, many mechanical weather instruments have been replaced by electronic instruments that record atmospheric pressure onto a computer.

          Atmospheric pressure can be recorded and reported in many different units. This can get a little confusing! As mentioned, a mercury barometer makes measurements in inches of Mercury (in Hg). Pounds per square inch (abbreviated as p.s.i.) is common in the English system of units, and the pascal (abbreviated Pa) is the standard in the Metric (SI) system. Since the pressure exerted by Earth’s atmosphere is of great importance, pressure is sometimes expressed in terms of “atmospheres” (abbreviated atm). In weather, the bar and millibar (mb) describe pressure. You’ll often hear millibar used by meteorologists when describing low or high pressure weather systems.

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          An area of high pressure is created where the air is cold. The cold air sinks, pushing down and creating high pressure. This causes the air molecules to be squashed together, creating heat. As the air warms up, it tends to bring warm and pleasant weather.

          In an anticyclone, air masses drop extensively. At the same time, the air warms itself up, so that no condensation and consequently no cloud formation can take place. Near to the ground, the air flows out of the anticyclone in the direction of depression – it diverges. Hence, there is no formation of fronts in altitude. During the subsidence of the air masses, an inversion forms. That is where the clouds are dissolved.
An anticyclone is builded quiet slowly. The forces of circulation in the subtropic areas lead to stable anticyclones.

          Because of the differences in the origin or development, the anticyclones are divided into three categories:

          A cold anticyclone originates if air cools off, for example, in winter above a cool land mass (e.g., Central Asian high). Then the air has a bigger density and exerts a higher pressure on the base. In the middle latitudes, it can also originate in the form of flat wedges in the back of cyclones as a ridge of high pressure.

          A dynamic anticyclone is generated by the Rossby-waves (Polar front, Jet Stream). The dynamic Azores anticyclone exerts, on this occasion, a big influence on the weather of Central Europe.

         A high anticyclone is an anticyclone which appears at big heights and is thus shown in high weather maps. It is always connected with a ground low-pressure area, because with the warming of the surfaces, the vertical pressure gradient is lowered and reflects itself the relative atmospheric pressure reduction on the ground with increasing height in a pressure relatively higher to the horizontal surroundings. Hence, one can derive the other way around a height low-pressure area also from a ground anticyclone (also thermal anticyclone).

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         AN AREA of warm air can create low pressure because warm air rises, reducing the level of air pressure. If the warm air evaporates water on the surface, clouds may form, producing the rain and bad weather associated with low pressure.

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          Air pressure is created by the effect of gravity pulling the atmosphere towards the Earth. It can vary according to temperature, causing different amounts of pressure in different parts of the world. It also changes according to altitude —pressure is greater at sea level because there is more air pushing down than there is at higher altitudes.

          The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.

          Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

          An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.

          As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.

          Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness.

          Decompression sickness, also called “the bends”, is also a problem for scuba divers who come to the surface too quickly.

          Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.

          Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.

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          The layer directly above the troposphere is called the stratosphere. The stratosphere is warmer than the upper part of the troposphere and this warm, relatively heavy air acts like a lid, trapping clouds in the troposphere. Going up through the layers, the air gets thinner and thinner — only in the lower parts of the troposphere is there enough air to breathe normally.

          The stratosphere is a layer of Earth’s atmosphere. It is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere.

          The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause.

          Ozone, an unusual type of oxygen molecule that is relatively abundant in the stratosphere, heats this layer as it absorbs energy from incoming ultraviolet radiation from the Sun. Temperatures rise as one moves upward through the stratosphere. This is exactly the opposite of the behavior in the troposphere in which we live, where temperatures drop with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.

          The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere. Polar stratospheric clouds (PSCs) are the exception. PSCs appear in the lower stratosphere near the poles in winter. They are found at altitudes of 15 to 25 km (9.3 to 15.5 miles) and form only when temperatures at those heights dip below -78° C. They appear to help cause the formation of the infamous holes in the ozone layer by “encouraging” certain chemical reactions that destroy ozone. PSCs are also called nacreous clouds.

          Air is roughly a thousand times thinner at the top of the stratosphere than it is at sea level. Because of this, jet aircraft and weather balloons reach their maximum operational altitudes within the stratosphere.

          Due to the lack of vertical convection in the stratosphere, materials that get into the stratosphere can stay there for long times. Such is the case for the ozone-destroying chemicals called CFCs (chlorofluorocarbons). Large volcanic eruptions and major meteorite impacts can fling aerosol particles up into the stratosphere where they may linger for months or years, sometimes altering Earth’s global climate. Rocket launches inject exhaust gases into the stratosphere, producing uncertain consequences.

          Various types of waves and tides in the atmosphere influence the stratosphere. Some of these waves and tides carry energy from the troposphere upward into the stratosphere; others convey energy from the stratosphere up into the mesosphere. The waves and tides influence the flows of air in the stratosphere and can also cause regional heating of this layer of the atmosphere.

          A rare type of electrical discharge, somewhat akin to lightning, occurs in the stratosphere. These “blue jets” appear above thunderstorms, and extend from the bottom of the stratosphere up to altitudes of 40 or 50 km (25 to 31 miles).

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          The Troposphere is sometimes called the weather layer. Here, the air is constantly moving as it is heated and cooled in a process known as convection. Clouds form as water in the atmosphere evaporates and then condenses. This movement of air, heat and water creates the world’s weather systems.

          The height of the troposphere varies between different areas of the Earth. At the Equator, for example, it stretches to about 20km (12 miles) above the surface. At the poles, the layer reaches a height of about 10km (6 miles).

          The troposphere is the lowest layer of Earth’s atmosphere. The troposphere extends from Earth’s surface up to a height of 7 to 20 km (4 to 12 miles, or 23,000 to 65,000 feet) above sea level. Most of the mass (about 75-80%) of the atmosphere is in the troposphere, and almost all weather occurs within this layer. Air is warmest at the bottom of the troposphere near ground level. As one rises through the troposphere the temperature decreases. Air pressure and the density of the air also decrease with altitude. The layer immediately above the troposphere is called the stratosphere.

         Nearly all of the water vapor and aerosol particles in the atmosphere are in the troposphere. Because of this, most clouds are found in this lowest layer as well.

          The troposphere is heated from below; sunlight warms the ground or ocean, which in turn radiates the heat into the air immediately above it. Temperature drops off at a rate of about 6.5° C per km (about 3.6° F per thousand feet) of increased altitude within the troposphere. This is why mountaintops are much cooler than lower elevations nearby. Since warm ‘parcels’ of air are less dense than colder air, warmer air is buoyant and tends to rise up from Earth’s surface towards the top of the troposphere. If you’ve ever watched cumulonimbus thunderstorm clouds form and grow on a hot summer day, you’ve seen this rising air in action. The air in the troposphere is ‘well mixed’ because it is constantly churning and ‘turning over’ as warm air at the surface rises and colder, denser air at altitude descends to take its place. This is not the case for all layers in the atmosphere. At the top of the troposphere the temperature drops to a chilly -55° C (-64° F)!

          The boundary between the top of the troposphere and the layer above it, the stratosphere, is called the tropopause. The height of the tropopause (and thus the top of the troposphere) varies with latitude, season, and between day and night. The troposphere is thickest in the tropics, where the top of the layer can be as high as about 20 km (12 miles or 65,000 feet) above sea level. At mid-latitudes, the typical height of the tropopause is around 11 km (7 miles or 36,000 feet), while near the poles it can dip down to as low as 7 km (4 miles or 23,000 feet). The jet stream, a fast-moving “river of air” that can zip along at speeds up to 400 km/hr (250 mph), is located just below the tropopause.

         Air gets ‘thinner’ with increasing altitude. That’s why mountain climbers sometimes need bottled oxygen to breathe, and why it is so easy to get ‘winded’ while hiking in high mountains or even visiting someplace at elevation.

          The lowest part of the troposphere, right next to the surface of Earth, is called the “boundary layer”. Differences in the surface texture (mountains, forests, flat water or ice) affect winds in the boundary layer.

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          Rising temperatures cause sea levels to rise in two ways. A warmer sea is less dense, so its volume increases and the level rises as it expands. A warmer climate can also cause glaciers to melt into the sea, raising its level.

          The term sea-level rise generally designates the average long-term global rise of the ocean surface measured from the centre of the earth (or more precisely, from the earth reference ellipsoid), as derived from satellite observations. Relative sea-level rise refers to long-term average sea-level rise relative to the local land level, as derived from coastal tide gauges.

          Sea levels are highly variable over periods ranging from seconds to decades. Sea-level rise is the rising trend averaged over longer periods, which is observed at many coastal stations since a few centuries. It is almost certain that global warming due to human emissions of greenhouse gases is responsible for steepening this trend since at least a few decades. The most recent projections for future sea-level rise are presented in the Special IPCC Report on the Ocean and Cryosphere in a Changing Climate (2019). This report is an update of the previous IPCC AR5 report (2013), and includes newer insights in the response of the Greenland and Antarctic ice sheets to global warming. It also provides an estimation of the possible sea-level rise up to the year 2030, see Fig. 1. Two scenarios for greenhouse gas emissions are considered in this figure: (1) a “low” scenario, called RCP2.6, with strong reduction of global greenhouse gas emission, such that global warming will probably not exceed 2 oC; (2) a “high” scenario, called RCP8.5, in which no measures are taken to limit greenhouse gas emissions (‘business as usual’). The high scenario can lead to a rise of up to 5 m of the global average sea level in 2300, but with great uncertainty.

          Several phenomena contribute to sea-level rise. On a global scale, sea-level rise is mainly due to an increase of the water mass and water volume of the oceans. This global sea-level rise (often termed Eustatic sea-level rise) has three components:

(1) thermal expansion of ocean waters related to decrease of the density (also referred to as thermo-steric component of sea-level rise, related to increasing temperature),

(2) water mass increase, which is mainly due to melting of mountain glaciers and decrease of the Greenland and Antarctic ice sheets, and

(3) decreasing storage of surface water and groundwater on land.

Other phenomena can substantially influence sea levels at regional scale, inducing either sea-level rise or sea-level fall. Most important are:

(4) vertical earth crust motions – in particular earth crust adjustment to melting of polar ice caps, the so-called isostatic rebound,

(5) land surface subsidence, related in particular to extraction of groundwater and oil/gas mining and compaction of soft deltaic soils,

(6) changes in the earth gravitational field, related in particular to decrease of the Greenland and Antarctic ice sheets,

(7) regional atmospheric pressure anomalies and changes in the strength and distribution of ocean currents, related in particular to ocean-atmosphere interaction, and

(8) regional sea-level change related to changes in seawater salinity.

          Due to these phenomena, sea-level rise is not uniform around the globe, but differs from place to place. Relative sea-level rise is the locally observed rise of the average sea level with respect to the land level. It is the sum of the components (1-8).

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          Large volcanic eruptions can have an almost immediate effect on the world’s weather. The dust that is thrown into the atmosphere creates a kind of screen, which reflects more of the Sun’s energy back into space. As a result, temperatures around the world can drop slightly and weather patterns may be affected for several years.

          When Mount Pinatubo erupted in the Philippines June 15, 1991, an estimated 20 million tons of sulfur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere. The eruption caused widespread destruction and loss of human life. Gases and solids injected into the stratosphere circled the globe for three weeks. Volcanic eruptions of this magnitude can impact global climate, reducing the amount of solar radiation reaching the Earth’s surface, lowering temperatures in the troposphere, and changing atmospheric circulation patterns. The  extent to which this occurs is an ongoing debate.

          Large-scale volcanic activity may last only a few days, but the massive outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sulfate aerosols, sub-micron droplets containing about 75 percent sulfuric acid. Following eruptions, these aerosol particles can linger as long as three to four years in the stratosphere.

         Major eruptions alter the Earth’s radiative balance because volcanic aerosol clouds absorb terrestrial radiation, and scatter a significant amount of the incoming solar radiation, an effect known as “radiative forcing” that can last from two to three years following a volcanic eruption.

          “Volcanic eruptions cause short-term climate changes and contribute to natural climate variability,” says Georgiy Stenchikov, a research professor with the Department of Environmental Sciences at Rutgers University. “Exploring effects of volcanic eruption allows us to better understand important physical mechanisms in the climate system that are initiated by volcanic forcing.”

          By comparing the climate simulations from the Pinatubo eruption, with and without aerosols, the researchers found that the climate model calculated a general cooling of the global troposphere, but yielded a clear winter warming pattern of surface air temperature over Northern Hemisphere continents. The temperature of the tropical lower stratosphere increased by 4 Kelvin (4°C) because of aerosol absorption of terrestrial longwave and solar near-infrared radiation. The model demonstrated that the direct radiative effect of volcanic aerosols causes general stratospheric heating and tropospheric cooling, with a tropospheric warming pattern in the winter.

        “The modeled temperature change is consistent with the temperature anomalies observed after the eruption,” Stenchikov says. “The pattern of winter warming following the volcanic eruption is practically identical to a pattern of winter surface temperature change caused by global warming. It shows that volcanic aerosols force fundamental climate mechanisms that play an important role in the global change process.”

        This temperature pattern is consistent with the existence of a strong phase of the Arctic Oscillation, a natural pattern of circulation in which atmospheric pressure at polar and middle latitudes fluctuates, bringing higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. It is forced by the aerosol radiative effect, and circulation in winter is stronger than the aerosol radiative cooling that dominates in summer.

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          Fossils contained in layers of rock can reveal details about the climate millions of years ago. Rock that contains a large variety of fossils was formed during a time when the climate was warm; fewer fossils indicate a cooler climate. Rocks that show signs of glacial erosion were part of the Earth’s surface during an Ice Age. Geologists can work out the age of the layers, which tells us when the changes took place.

            Many of the techniques used for palaeoclimatic reconstruction discussed in the preceding sections have only a limited time scale open to their period of study. Most ice cores are restricted to the last million years, whilst tree ring analysis can only provide proxy climate information for at best the last 10,000 years. Ocean sediments provide some of the longest proxy records available, and offer a window on palaeoclimates dating back to the age of the dinosaurs, 100 million years ago. Most older sediments, however, will have been subducted beneath overriding tectonic plates as the continents continue to drift about the Earth. To reconstruct climates older than this, therefore, one needs to look elsewhere for the evidence.

           Sediments laid down on the ocean floor become progressively buried by subsequent debris transported from continental interiors. Deeply buried sediments are subjected to considerable pressures from the overlying layers, and after tens to hundreds of millions of years, the sediments are gradually lithified, forming sedimentary rocks. If, through tectonic movements, these sedimentary rocks are uplifted and exposed, scientists may study them, as they do other forms of evidence, to reconstruct past climates.

          Numerous techniques of analysing sedimentary rocks are used for palaeoclimate reconstruction. Principally, rock type provides valuable insights into past climates, for rock composition reveals evidence of the climate at the time of sediment deposition. However, depositional climatic regimes vary not only due to actual climatic changes but also due to continental movements. The Carboniferous limestones and coals (evidence of warm, humid climates) of Northern England (300Ma), for example, were laid down at a time when Britain was located near the equator, whilst large scale glaciation was occurring in the high latitudes of the Southern Hemisphere.

        The study of rock type is geologically known as facies analysis. Facies analysis investigates how the rock type changes over time, and therefore provides a potential tool for investigating past climatic change. A sedimentary formation consisting of a shale layer (fine-grained mudstone) interbedded between two sandstone layers (coarse-grained), for example, provides evidence of a changing sea level, potentially linked to climatic change (caused either by epeirogeny or ice formation). Sandstones are deposited in coastal zones where the water is shallow, whilst mudstones (shales) are deposited in deeper water of the continental shelf region. A change in the rock type in the vertical cross section must therefore reflect a change in sea level and associated coastline movements.

          Other principal marker rock types include evaporites (lithified salt deposits and evidence of dry arid climates), coals (lithified organic matter and evidence of warm, humid climates), phosphates and cherts (lithified siliceous and phosphate material and evidence of ocean upwelling due to active surface trade winds) and reef limestone (lithified coral reef and evidence of warm surface ocean conditions).

         As well as facies analysis, other techniques, including analysis of sedimentation rates, sediment grain morphology and chemical composition provide information on the climatic conditions prevailing at the time of parent rock weathering. In addition, some of the methods used to reconstruct past climate discussed in earlier sections may be equally applied to sedimentary rocks. For example, the type and distribution of marine and continental fossils within fossil-bearing rocks (principally limestones and mudstones, but occasionally sandstones) are valuable palaeoclimate indicators. Microfossil type, abundance and morphology may also be studied, and palaeotemperatures derived from their oxygen isotope analysis.

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          By studying the growth rings in ancient trees, scientists can gather information about climates of the past. This science is called dendroclimatology. In each year of a tree’s growth, new layers are added to the centre of its trunk, producing a growth ring. Warm, wet growing seasons produce several layers, creating a wide growth ring. In a cold, dry period, fewer layers are produced, and the ring will be narrower.

          The characteristics of the rings inside a tree can tell scientists how old a tree is and what the weather conditions were like during each year of that tree’s life. Very old trees can offer clues about what the climate in an area was like long before measurements were recorded.

         But to understand what the trees tell us, we first have to understand the difference between weather and climate.

          Weather is a specific event—like a rain storm or hot day—that happens over a short period of time. Weather can be tracked within hours or days. Climate is the average weather conditions in a place over a long period of time (30 years or more).

          Scientists at the National Weather Service have been keeping track of weather in the United States since 1891. But trees can keep a much longer record of Earth’s climate. In fact, trees can live for hundreds—and sometimes even thousands—of years!

         One way that scientists use trees to learn about past climate is by studying a tree’s rings. If you’ve ever seen a tree stump, you probably noticed that the top of the stump had a series of rings. It looks a bit like a bullseye.

          These rings can tell us how old the tree is, and what the weather was like during each year of the tree’s life. The light-colored rings represent wood that grew in the spring and early summer, while the dark rings represent wood that grew in the late summer and fall. One light ring plus one dark ring equals one year of the tree’s life.

        Because trees are sensitive to local climate conditions, such as rain and temperature, they give scientists some information about that area’s local climate in the past. For example, tree rings usually grow wider in warm, wet years and they are thinner in years when it is cold and dry. If the tree has experienced stressful conditions, such as a drought, the tree might hardly grow at all in those years.

          Scientists can compare modern trees with local measurements of temperature and precipitation from the nearest weather station. However, very old trees can offer clues about what the climate was like long before measurements were recorded.

          In most places, daily weather records have only been kept for the past 100 to 150 years. So, to learn about the climate hundreds to thousands of years ago, scientists need to use other sources, such as trees, corals, and ice cores (layers of ice drilled out of a glacier).

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            The causes of an Ice Age are not clear. One theory is that the Earth’s tilt and its orbit of the Sun have changed. An orbit that took our planet further from the Sun would result in a cooler climate.

          An ice age is a time where a significant amount of the Earth’s water is locked up on land in continental glaciers.

          During the last ice age, which finished about 12,000 years ago, enormous ice masses covered huge swathes of land now inhabited by millions of people.

          Canada and the northern USA were completely covered in ice, as was the whole of northern Europe and northern Asia.

          At the moment the Earth is in an interglacial period – a short warmer period between glacial (or ice age) periods.

          The Earth has been alternating between long ice ages and shorter interglacial periods for around 2.6 million years.

          For the last million years or so these have been happening roughly every 100,000 years – around 90,000 years of ice age followed by a roughly 10,000 year interglacial warm period.

Causes:

Ice ages don’t just come out of nowhere – it takes thousands of years for an ice age to begin.

          An ice age is triggered when summer temperatures in the northern hemisphere fail to rise above freezing for years. This means that winter snowfall doesn’t melt, but instead builds up, compresses and over time starts to compact, or glaciate, into ice sheets.

          Over thousands of years these ice sheets start to build up – it seems to be in northern Canada when that first happens – and then they spread out across the northern hemisphere.

          “It’s a long term trend over thousands of years to colder summers,” Dr Steven Phipps, an ice sheet modeller, said.

          Dr Phipps is also a climate system modeller and palaeoclimatologist with the University of Tasmania.

          The onset of an ice age is related to the Milankovitch cycles – where regular changes in the Earth’s tilt and orbit combine to affect which areas on Earth get more or less solar radiation.

          When all these factors align so the northern hemisphere gets less solar radiation in summer, an ice age can be started.

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          It is thought that Ice Ages occur roughly every 100,000 years. The last one ended around 10,000 years ago, so we may experience another in 90,000 years time. Scientists call the time between Ice Ages an interglacial period.

         An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene interglacial began at the end of the Pleistocene, about 11,700 years ago.

          During the 2.5 million years of the Pleistocene, numerous glacials, or significant advances of continental ice sheets, in North America and Europe, occurred at intervals of approximately 40,000 to 100,000 years. The long glacial periods were separated by more temperate and shorter interglacials.

          During interglacials, such as the present one, the climate warms and the tundra recedes polewards following the ice sheets. Forests return to areas that once supported tundra vegetation. Interglacials are identified on land or in shallow epicontinental seas by their paleontology. Floral and faunal remains of species pointing to temperate climate and indicating a specific age are used to identify particular interglacials. Commonly used are mammalian and molluscan species, pollen and plant macro-remains (seeds and fruits). However, many other fossil remains may be helpful: insects, ostracods, foraminifera, diatoms, etc. Recently, ice cores and ocean sediment cores provide more quantitative and accurately-dated evidence for temperatures and total ice volumes.

          Interglacials and glacials coincide with cyclic changes in the Earth’s orbit. Three orbital variations contribute to interglacials. The first is a change in the Earth’s orbit around the sun, or eccentricity. The second is a shift in the tilt of the Earth’s axis, or obliquity. The third is the wobbling motion of Earth’s axis, or precession.

          Warm summers in the Southern Hemisphere occur when it is tilted toward the sun and the Earth is nearest the sun in its elliptical orbit. Cool summers occur when the Earth is farthest from the sun during the summer. Such effects are more pronounced when the eccentricity of the orbit is large. When the obliquity is large, seasonal changes are more extreme.

          Interglacials are a useful tool for geological mapping and for anthropologists, as they can be used as a dating method for hominid fossils.

          Brief periods of milder climate that occurred during the last glacial are called interstadials. Most but not all interstadials are shorter than interglacials. Interstadial climate may have been relatively warm but not necessarily. Because the colder periods (stadials) have often been very dry, wetter (not necessarily warmer) periods have been registered in the sedimentary record as interstadials as well.

          The oxygen isotope ratio obtained from seabed sediment core samples, a proxy for the average global temperature, is an important source of information about changes in the climate of the earth.

          An interglacial optimum, or climatic optimum of an interglacial, is the period within an interglacial that experienced the most ‘favourable’ climate and often occurs during the middle of that interglacial. The climatic optimum of an interglacial both follows and is followed by phases within the same interglacial that experienced a less favourable climate (but still a ‘better’ climate than the one during the preceding/succeeding glacials). During an interglacial optimum, sea levels rise to their highest values but not necessarily exactly at the same time as the climatic optimum.

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            The world’s climates have been through many changes since the planet was formed over 4000 million years ago. The Earth has been both hotter and colder than it is now. In the age of the dinosaurs, there were no polar ice caps, and tropical and desert climates were predominant. Since that time, there have been several Ice Ages, when the polar ice sheets expanded to cover up to one-third of the planet. The planet will continue to experience such dramatic changes, as well as minor fluctuations in the weather. Many people are concerned that the activities of mankind will have a catastrophic effect on our planet’s weather patterns.

            The Earth’s climate has changed throughout history. Just in the last 650,000 years there have been seven cycles of glacial advance and retreat, with the abrupt end of the last ice age about 7,000 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives.

            The current warming trend is of particular significance because most of it is extremely likely (greater than 95 percent probability) to be the result of human activity since the mid-20th century and proceeding at a rate that is unprecedented over decades to millennia.

            Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale. This body of data, collected over many years, reveals the signals of a changing climate.

            The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century. Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis of many instruments flown by NASA. There is no question that increased levels of greenhouse gases must cause the Earth to warm in response.

            Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that the Earth’s climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly ten times faster than the average rate of ice-age-recovery warming.

              It is possible to simulate the conditions of certain climates inside a greenhouse. Glass and other materials can be used to create a space within which the heat and light from the Sun is intensified, making it much warmer than it is outside. The temperature, humidity and air movement can be controlled, recreating the atmosphere of a particular climate.

           Climate simulators (or climate models) are complex computer programmers which simulate the Earth’s climate system, including the atmosphere, ocean, land surface and ice, and the interactions between them. The computer programme represents the climate in terms of key quantities such as atmospheric temperature, pressure, wind, and humidity at locations on a three dimensional grid. The atmospheric grid covers the Earth’s surface and extends from the surface to the upper atmosphere. A similar grid for the ocean extends from the ocean’s surface to the ocean floor. By solving the relevant mathematical equations the computer is able to calculate how the state of the atmosphere and ocean evolves in time.

                 At present, a typical simulator of global climate has grid boxes with horizontal dimensions of approximately 100-200 km; this is known as the “spatial resolution”. Simulators used to predict daily weather use much higher spatial resolution, but typically only simulate a specific region (e.g. the UK). Simulators with higher resolution are more accurate, but they also take longer to run and require larger computers.

To test scientific understanding

              Scientists use climate simulators to test and improve their understanding of the climate system. By comparing the simulated climate with observations of the real world, scientists can identify where a simulator needs improvement.

To predict future climate

              Climate simulators are used to predict how climate may change in the future. For example, scientists can implement expected future conditions, such as a higher concentration of greenhouse gases in the atmosphere, and use the simulator to predict how such a change may affect the climate.

              Climate simulators are not perfect and scientists are careful to study, quantify and communicate their accuracy and reliability along with particular results. However, climate scientists are confident that climate simulators can accurately represent many fundamental aspects of the climate system for several reasons.

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            Parts of India and Southeast Asia have a monsoon climate. In these areas, it changes very suddenly from a wet to a dry season, according to the direction of the prevailing wind. The dry period is extremely hot, and the powerful monsoon winds that blow in from the sea bring torrential rain, often without warning. Such violent extremes of weather can make daily life very difficult, with heavy flooding, damage to property and loss of life commonplace.

            A monsoon often brings about thoughts of torrential rains, similar to a hurricane or typhoon. But there is a difference: a monsoon is not a single storm; rather, it is a seasonal wind shift over a region. The shift may cause heavy rains in the summer, but at other times, it may cause a dry spell.

            A monsoon (from the Arabic mawsim, which means “season”) arises due to a difference in temperatures between a land mass and the adjacent ocean, according to the National Weather Service. The sun warms the land and ocean differently, according to Southwest Climate Change, causing the winds to play “tug of war” eventually switching directions bringing the cooler, moister air from over the ocean. The winds reverse again at the end of the monsoon season. 

            A wet monsoon typically occurs during the summer months (about April through September) bringing heavy rains, according to National Geographic. On average, approximately 75 percent of India’s annual rainfall and about 50 percent of the North American monsoon region (according to a 2004 NOAA study) comes during the summer monsoon season. The wet monsoon begins when winds bringing cooler, more humid air from above the oceans to the land, as described above.

            A dry monsoon typically occurs between October and April. Instead of coming from the oceans, the winds tend to come from drier, warmer climates such as from Mongolia and northwestern China down into India, according to National Geographic. Dry monsoons tend to be less powerful than their summer counterparts. Edward Guinan, an astronomy and meteorology professor at Villanova University, states that the winter monsoon occurs when “the land cools off faster than the water and a high pressure develops over the land, blocking any ocean air from penetrating.” This leads to a dry period. 

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          The polar climate is very dry and windy, as well as being exceptionally cold. Inland, it is nearly always below freezing, and temperatures often reach -40°C (-40°F). Only near the coasts do temperatures reach about 10°C (50°F) in the summer.

          Both the Arctic (North Pole) and the Antarctic (South Pole) are cold because they don’t get any direct sunlight. The Sun is always low on the horizon, even in the middle of summer. In winter, the Sun is so far below the horizon that it doesn’t come up at all for months at a time. So the days are just like the nights—cold and dark.

          Even though the North Pole and South Pole are “polar opposites,” they both get the same amount of sunlight. But the South Pole is a lot colder than the North Pole. Why? Well, the Poles are polar opposites in other ways too.

          The Arctic is ocean surrounded by land. The Antarctic is land surrounded by ocean.

          The ocean under the Arctic ice is cold, but still warmer than the ice! So the ocean warms the air a bit.

          Antarctica is dry—and high. Under the ice and snow is land, not ocean. And it’s got mountains. The average elevation of Antarctica is about 7,500 feet (2.3 km). And the higher you go, the colder it gets.

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          Some relatively small areas have their own climate, which differs slightly from the climate surrounding it — a microclimate. Cities often have a microclimate, due to the concentration of buildings, people and vehicles generating heat. This creates a “heat island” — a warm mass of air that sits over the city, making it up to 6°C (11°F) warmer than the surrounding area.

          Microclimate, any climatic condition in a relatively small area, within a few metres or less above and below the Earth’s surface and within canopies of vegetation. The term usually applies to the surfaces of terrestrial and glaciated environments, but it could also pertain to the surfaces of oceans and other bodies of water.

          The strongest gradients of temperature and humidity occur just above and below the terrestrial surface. Complexities of microclimate are necessary for the existence of a variety of life forms because, although any single species may tolerate only a limited range of climate, strongly contrasting microclimates in close proximity provide a total environment in which many species of flora and fauna can coexist and interact.

          Microclimatic conditions depend on such factors as temperature, humidity, wind and turbulence, dew, frost, heat balance, and evaporation. The effect of soil type on microclimates is considerable. Sandy soils and other coarse, loose, and dry soils, for example, are subject to high maximum and low minimum surface temperatures. The surface reflection characteristics of soils are also important; soils of lighter colour reflect more and respond less to daily heating. Another feature of the microclimate is the ability of the soil to absorb and retain moisture, which depends on the composition of the soil and its use. Vegetation is also integral as it controls the flux of water vapour into the air through transpiration. In addition, vegetation can insulate the soil below and reduce temperature variability. Sites of exposed soil then exhibit the greatest temperature variability.

          Topography can affect the vertical path of air in a locale and, therefore, the relative humidity and air circulation. For example, air ascending a mountain undergoes a decrease in pressure and often releases moisture in the form of rain or snow. As the air proceeds down the leeward side of the mountain, it is compressed and heated, thus promoting drier, hotter conditions there. An undulating landscape can also produce microclimatic variety through the air motions produced by differences in density.

          The microclimates of a region are defined by the moisture, temperature, and winds of the atmosphere near the ground, the vegetation, soil, and the latitude, elevation, and season. Weather is also influenced by microclimatic conditions. Wet ground, for example, promotes evaporation and increases atmospheric humidity. The drying of bare soil, on the other hand, creates a surface crust that inhibits ground moisture from diffusing upward, which promotes the persistence of the dry atmosphere. Microclimates control evaporation and transpiration from surfaces and influence precipitation, and so are important to the hydrologic cycle—i.e., the processes involved in the circulation of the Earth’s waters.

          The initial fragmentation of rocks in the process of rock weathering and the subsequent soil formation are also part of the prevailing microclimate. The fracturing of rocks is accomplished by the frequent freezing of water trapped in their porous parts. The final weathering of rocks into the clay and mineral constituents of soils is a chemical process, where such microclimatic conditions as relative warmth and moisture influence the rate and degree of weathering.

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          In the most mountainous regions of the world, the climate will often be very different from that of the land that surrounds them. The freezing climate of the Himalayas, for example, is surrounded by desert, warm temperate, and monsoon climates.

          Lower down, the climate may be milder (temperate), suitable for lots of different plants and trees to grow, which in turn, provide food for a wide variety of animals. Higher up, plants and animals are fewer: they have to be highly adapted to survive, as the climate becomes much harsher. It’s windy and cold. Frozen ground means that there is not much water available and the soil is shallow. Humans also struggle to cope at high altitude (a fancy word for great height), because the air becomes much thinner, meaning that there is less oxygen available for your body to use.

          Mountain weather conditions can change in a split second! Well, maybe not quite that quickly but in just a few minutes clouds can gather and a thunderstorm begins. That’s why mountaineers have to be ready for anything; they pack their rucksacks really carefully and carry emergency kit like tents and extra food. Professional climbers always tell other people what their plans are so, if they go missing, search and rescue teams know where to look!

          Mountains also receive lots and lots of rainfall. This is because air travelling over land is forced up and over any mountains in its path – it can’t tunnel! This air cools as it rises causing the condensation of any water vapour it was carrying into huge clouds (made up of tiny droplets) ready to burst at any moment.

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            Area with a Tropical climate have high temperatures (24°C to 27°C (75°F to 81°F)) throughout the year. The atmosphere is very humid (full of moisture) and the levels of rainfall are very high — at least 150cm (59in) — particularly in those regions close to the Equator.

          Some people assume the word tropical climate refers to their favorite warm vacation spot. However, this is a bit far from the truth since the word tropical is defined differently in meteorology. A tropical climate is identified as a climate characteristic to the tropics; that is from the equator to the Tropic of Capricorn in the south and from the Equator to the Tropic of Cancer in the north. The Koppen climate classification defines a tropical climate as a non-arid climate in which the mean temperature is about 64°F throughout the year. Unlike subtropical regions which are characterized by variations in temperature to different degrees and day length, temperatures in tropical climates, remain relatively constant all year long as variations for different seasons are dominated by rainfall. Tropical climates comprise of only two seasons which are the dry season and the wet season. Changes in solar angle are small in tropical climates which happen to be frost-free. There are different varieties of tropical climates within the tropical climate zone. The different varieties are based on precipitation. Here are the three subtypes of tropical climates.

The Tropical Wet and Dry Climate

          Also known as the Savannah climate, the tropical wet and dry climate experiences a long dry period and less annual rainfall. The driest month in a wet and dry tropical climate has precipitation of less than 2.4 inches and less than 3.9 inches total annual precipitation. The tropical wet and dry climate are mainly found in Lagos, Nigeria; Bangalore, India; Dar es Salaam, Tanzania; Barquisimeto, Venezuela; Darwin, Australia; Honolulu, US; Fort Myers, Florida; Rio de Janeiro, Brazil; and Kupang, Indonesia among others.

The Tropical Monsoon Climate

          A tropical monsoon climate is the type of climate found in the Southern and Central regions of America and the Southeast and Southern parts of Asia as well as parts of Australia and Africa. The tropical monsoon climate is influenced by the monsoon winds which according to the seasons change directions. For this part of the equator, the driest month of the tropical monsoon climate occurs either soon after or at the ‘winter’ solstice. Rainfall is usually less than 2.4 inches but exceeds 3.9 inches total annual precipitation. Examples of the area that experience tropical monsoon climates include Jakarta, Indonesia; Miami, Florida; Abidjan, Ivory Coast; Puerto Ayacucho, Venezuela; Chittagong, Bangladesh; Yangon, Myanmar; Cairns, Australia; and Macapa, Brazil among others.

Tropical Rainforest Climate

          The tropical rainforest climate is found in places that are around the equatorial region usually between 5° to 10° latitude of the equator. However, in several eastern coastal regions, such climates might extend beyond 26° from the equator. Tropical rainforest climates are mainly characterized by low-pressure systems since they are dominated by doldrums thus receiving rainfall all year long. There is no specific season found in the tropical rainforest climate. All 12 months in this type of climate have an average precipitation of at least 2.4 inches. Examples of areas with tropical rainforest climates include Mbandaka, Congo; Singapore; Klang, Malaysia; Hilo, Hawaii; Innisfail, Australia; Apia Samoa, Davao, Philippines; Bogor, Indonesia among others.

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            There are two types of temperate climate — cool and warm. Cool temperate areas have rainfall throughout the year, warm summers, and winters with temperatures often below freezing. The warm temperate climate features mild, wet winters where the temperature rarely gets below 4°C (39°F). The summers are hot and dry, with temperatures averaging 20°C to 27°C (68°F to 81°F).

            In geography, temperate latitudes of the Earth lie between the subtropics and the polar circles. Average yearly temperatures in these regions are not extreme, not burning hot nor freezing cold. Temperate means moderate.

            Unlike in the tropics, temperatures can change greatly here, between summer and winter. So, most places with a temperate climate have four seasons: summer, autumn, winter and spring. Other areas with a temperate climate can have very unpredictable weather. One day it may be sunny, the next may be rainy, and after that it may be cloudy. This is normal in summer as well as in winter. These are the main types of temperate climate:

• A maritime climate is generally for places near the sea. That includes London, Dublin, Melbourne and Auckland. Most places do not have a rainy season and a dry season. Prevailing winds in the temperate zone are from the west. The western edge of temperate continents usually get this maritime climate. Examples are Western Europe, and western North America at latitudes between 40° and 60° north (65°N in Europe).
• Some parts of the temperate zone have a Mediterranean climate, which have a dry summer – for example Madrid, and Adelaide.
• Some parts of the temperate zone, especially in the northern part of the continental climate, have severe winters – for example Moscow and Minnesota – this is called a hemiboreal climate.
• Some places in the temperate zone have hot summers and cold winters, for example Chicago, Budapest and Almaty.

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            A region’s climate is the general pattern of weather that it experiences over a long period of time. Climate depends on a number of factors. The position of the area on the Earth’s surface, and its height above sea level are two factors. Warmth carried around the world by ocean currents affects the climate on land, and those areas far from the sea will have a different climate from those on the coast. There are eight main types of climate, but there are variations to be found within them.

           Climate is the average weather conditions in a place over a long period of time—30 years or more. And as you probably already know, there are lots of different types of climates on Earth.

            For example, hot regions are normally closest to the equator. The climate is hotter there because the Sun’s light is most directly overhead at the equator. And the North and South Poles are cold because the Sun’s light and heat are least direct there.

            Using this information, in the late 1800s and early 1900s a German climate scientist named Wladimir Koppen divided the world’s climates into categories. His categories were based on the temperature, the amount of precipitation, and the times of year when precipitation occurs. The categories were also influenced by a region’s latitude—the imaginary lines used to measure our Earth from north to south from the equator.

            Today, climate scientists split the Earth into approximately five main types of climates. They are:

A: Tropical. In this hot and humid zone, the average temperatures are greater than 64°F (18°C) year-round and there is more than 59 inches of precipitation each year.

B: Dry. These climate zones are so dry because moisture is rapidly evaporated from the air and there is very little precipitation.

C: Temperate. In this zone, there are typically warm and humid summers with thunderstorms and mild winters.

D. Continental. These regions have warm to cool summers and very cold winters. In the winter, this zone can experience snowstorms, strong winds, and very cold temperatures—sometimes falling below -22°F (-30°C)!

E: Polar. In the polar climate zones, it’s extremely cold. Even in summer, the temperatures here never go higher than 50°F (10°C)!

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            When the Sun is directly overhead at its most northern or southern position, it is called the solstice. The Northern Hemisphere’s summer solstice occurs when the Sun is above the Tropic of Cancer — on 20, 21 or 22 June — and marks the beginning of summer. Its winter solstice (the Southern Hemisphere’s summer solstice) is on 21 or 22 December. The summer solstice is the longest day of the year; the winter equivalent is the shortest.

            Solstice, either of the two moments in the year when the Sun’s apparent path is farthest north or south from Earth’s Equator. The situation is exactly the opposite in the Southern Hemisphere, where the seasons are reversed. At the winter solstice the day is the year’s shortest, and at the summer solstice it is the year’s longest. The term solstice also is used in reference to either of the two points of greatest deviation of the ecliptic (the Sun’s apparent annual path) from the celestial equator.

            At the time of the summer solstice in the Northern Hemisphere, the North Pole is tilted about 23.4° (23°27´) toward the Sun. Because the Sun’s rays are shifted northward by the same amount, the vertical noon rays are directly overhead at the Tropic of Cancer (23°27´ N). Six months later the South Pole is inclined about 23.4° toward the Sun. On this day of the summer solstice in the Southern Hemisphere the Sun’s vertical overhead rays progress to their southernmost position, the Tropic of Capricorn (23°27´ S). Compare equinox. See also season.

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            Many people suffer from the “winter blues”. Feeling tired, run down and a bit sad is a natural response to the long, dark days, cold weather and the effects of colds and flu. A few people experience exaggerated symptoms, which doctors have recognized as a medical condition known as Seasonal Affective Disorder, or SAD. A lack of daylight can cause sufferers of SAD to become very depressed and have problems sleeping and eating.

            Seasonal affective disorder (SAD) is a type of depression that’s related to changes in seasons — SAD begins and ends at about the same times every year. If you’re like most people with SAD, your symptoms start in the fall and continue into the winter months, sapping your energy and making you feel moody. Less often, SAD causes depression in the spring or early summer.

            Treatment for SAD may include light therapy (phototherapy), medications and psychotherapy.

Don’t brush off that yearly feeling as simply a case of the “winter blues” or a seasonal funk that you have to tough out on your own. Take steps to keep your mood and motivation steady throughout the year.

Symptoms

            In most cases, seasonal affective disorder symptoms appear during late fall or early winter and go away during the sunnier days of spring and summer. Less commonly, people with the opposite pattern have symptoms that begin in spring or summer. In either case, symptoms may start out mild and become more severe as the season progresses.

Signs and symptoms of SAD may include:

• Feeling depressed most of the day, nearly every day
• Losing interest in activities you once enjoyed
• Having low energy
• Having problems with sleeping
• Experiencing changes in your appetite or weight
• Feeling sluggish or agitated
• Having difficulty concentrating
• Feeling hopeless, worthless or guilty
• Having frequent thoughts of death or suicide

Fall and winter SAD

Symptoms specific to winter-onset SAD, sometimes called winter depression, may include:

• Oversleeping
• Appetite changes, especially a craving for foods high in carbohydrates
• Weight gain
• Tiredness or low energy

Spring and summer SAD

Symptoms specific to summer-onset seasonal affective disorder, sometimes called summer depression, may include:

• Trouble sleeping (insomnia)
• Poor appetite
• Weight loss
• Agitation or anxiety

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            In the parts of the world that are close to the poles, the way the Earth tilts means that the summer months in those regions are marked by constant daylight. Parts of Scandinavia, for instance, are known as the “land of the midnight Sun”. In mid-winter, these areas experience the opposite — total darkness for 24 hours a day.

            The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight.

            Around the summer solstice (approximately 21 June in the Northern Hemisphere and 23 December in the Southern Hemisphere), the Sun is visible for the full 24 hours, given fair weather. The number of days per year with potential midnight sun increases the closer towards either pole one goes. Although approximately defined by the polar circles, in practice the midnight sun can be seen as much as 55 miles (90 km) outside the polar circle, and the exact latitudes of the farthest reaches of midnight sun depend on topography and vary slightly year-to-year.

            Because there are no permanent human settlements south of the Antarctic Circle, apart from research stations, the countries and territories whose populations experience the midnight sun are limited to those crossed by the Arctic Circle: the Canadian Yukon, Nunavut, and Northwest Territories; the nations of Iceland, Finland, Norway, Sweden, Denmark (Greenland), Russia; and the State of Alaska in the United States. A quarter of Finland’s territory lies north of the Arctic Circle, and at the country’s northernmost point the sun does not set at all for 60 days during summer. In Svalbard, Norway, the northernmost inhabited region of Europe, there is no sunset from approximately 19 April to 23 August. The extreme sites are the poles, where the sun can be continuously visible for half the year. The North Pole has midnight sun for 6 months from late March to late September.

            The opposite phenomenon, polar night, occurs in winter, when the Sun stays below the horizon throughout the day.

            Since the axial tilt of the Earth is considerable (approximately 23 degrees 27 minutes), the Sun does not set at high latitudes in local summer. The Sun remains continuously visible for one day during the summer solstice at the polar circle, for several weeks only 100 km (62 mi) closer to the pole, and for six months at the pole. At extreme latitudes, the midnight sun is usually referred to as polar day.

            At the poles themselves, the Sun rises and sets only once each year on the equinox. During the six months that the Sun is above the horizon, it spends the days continuously moving in circles around the observer, gradually spiralling higher and reaching its highest circuit of the sky at the summer solstice.

            Because of atmospheric refraction, and also because the Sun is a disc rather than a point, the midnight sun may be experienced at latitudes slightly south of the Arctic Circle or north of the Antarctic Circle, though not exceeding one degree (depending on local conditions). For example, Iceland is known for its midnight sun, even though most of it (Grímsey is the exception) is slightly south of the Arctic Circle. For the same reasons, the period of sunlight at the poles is slightly longer than six months. Even the northern extremities of Scotland (and places at similar latitudes, such as St. Petersburg) experience twilight throughout the night in the northern sky at around the summer solstice.

            Observers at heights appreciably above sea level can experience extended periods of midnight sun as a result of the “dip” of the horizon viewed from altitude.

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          As the Earth orbits the Sun, different parts of the planet face towards or away from it, receiving varying amounts of heat. The Earth is tilted at an angle and always tilts the same way. This means that when the Earth is on one side of the Sun, the Northern Hemisphere leans towards the Sun and experiences summer. At the same time, the Southern Hemisphere is leaning away from the Sun and is having winter weather. Six months later, the Earth is on the other side of the Sun and the situation is reversed. Spring begins in a hemisphere at the moment at which it starts to lean towards the Sun; Autumn starts when it begins to lean away from it.

           The seasons have nothing to do with how far the Earth is from the Sun.  If this were the case, it would be hotter in the northern hemisphere during January as opposed to July.  Instead, the seasons are caused by the Earth being tilted on its axis by an average of 23.5 degrees (Earth’s tilt on its axis actually varies from near 22 degrees to 24.5 degrees).  Here’s how it works:

          The Earth has an elliptical orbit around our Sun.  This being said, the Earth is at its closest point distance wise to the Sun in January (called the Perihelion) and the furthest in July (the Aphelion).  But this distance change is not great enough to cause any substantial difference in our climate.  This is why the Earth’s 23.5 degree tilt is all important in changing our seasons.  Near June 21st, the summer solstice, the Earth is tilted such that the Sun is positioned directly over the Tropic of Cancer at 23.5 degrees north latitude.  This situates the northern hemisphere in a more direct path of the Sun’s energy.  What this means is less sunlight gets scattered before reaching the ground because it has less distance to travel through the atmosphere.  In addition, the high sun angle produces long days.  The opposite is true in the southern hemisphere, where the low sun angle produces short days.  Furthermore, a large amount of the Sun’s energy is scattered before reaching the ground because the energy has to travel through more of the atmosphere.  Therefore near June 21st, the southern hemisphere is having its winter solstice because it “leans” away from the Sun.

          Advancing 90 days, the Earth is at the autumnal equinox on or about September 21st.  As the Earth revolves around the Sun, it gets positioned such that the Sun is directly over the equator.   Basically, the Sun’s energy is in balance between the northern and southern hemispheres.  The same holds true on the spring equinox near March 21st, as the Sun is once again directly over the equator. 

          Lastly, on the winter solstice near December 21st, the Sun is positioned directly over the Tropic of Capricorn at 23.5 degrees south latitude.  The southern hemisphere is therefore receiving the direct sunlight, with little scattering of the sun’s rays and a high sun angle producing long days.  The northern hemisphere is tipped away from the Sun, producing short days and a low sun angle.

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          Because the air is warmed by heat rising from the ground, the air temperature at the top of a mountain will always be lower than it is at the bottom.

          The lowering of temperature as you reach higher altitudes is due to the change in atmospheric pressure. You may be aware that the air around us is constantly exerting pressure on us due to there being lots of air above us weighing down on us. It sounds a bit strange to say that air weighs something but it does, we just don’t feel it because it is what we’re used to, just like gravity is constantly pulling us down.

          There is a direct relationship between temperature and pressure. If you increase the pressure of a system then the temperature will get higher. This is why bicycle pumps can get hot after use. Decrease the pressure and the temperature goes down as can be experienced letting the air out of a balloon very quickly, the balloon gets cold.
As you go higher up, the pressure gets less and less due to there being less air above you weighing down on you, therefore the temperature goes down too.

         If heat rises, then why is it so cold at the top of a mountain? Heat does indeed rise. More specifically, a mass of air that is warmer than the air around it expands, becomes less dense, and will therefore float atop the cooler air. This is true at any altitude, and if this were the only factor at play, one would expect the atmosphere to get uniformly hotter with altitude, like the second floor of a house.

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          Rising currents of warm air are called thermals. They are useful to glider pilots, who use them to help lift their craft into the air. Thermals can form over “hot spots” on the ground, such as a freshly ploughed field. Some large birds make use of thermals to circle in the air.

          The warmer air nearer to the surface expands, becoming less dense than the surrounding air. The lighter air rises and cools due to its expansion in the lower pressure at higher altitudes. It stops rising when it has cooled to the same temperature as the surrounding air.

          Associated with a thermal is a downward flow surrounding the thermal column. The downward-moving exterior is caused by colder air being displaced at the top of the thermal.

          The size and strength of thermals are influenced by the properties of the lower atmosphere (the troposphere). Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and can rise like a hot air balloon. The air is then referred to as unstable. If there is a warm layer of air higher up, an inversion can prevent thermals from rising high and the air is said to be stable.

          Thermals are often indicated by the presence of visible cumulus clouds at the top of the thermal. When a steady wind is present, thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as “cloud streets” by soaring and glider pilots. Cumulus clouds are formed by the rising air in a thermal as it ascends and cools, until the water vapor in the air begins to condense into visible droplets. The condensing water releases latent heat energy allowing the air to rise higher. Very unstable air can reach the level of free convection (LFC) and, thus rise to great heights condensing large quantities of water and so forming showers or even thunderstorms. The latter are dangerous to any aircraft.

          Thermals are one of the many sources of lift used by soaring birds and gliders to soar.

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         Where cool air lies above a warm area of land, the air will be heated. As the air warms up, it expands, becomes less dense (its molecules become less tightly packed) and it begins to rise. The surrounding cooler air replaces the rising warm air. As the warm air rises, it cools down and its density increases. These currents of warm and cold air are called convection currents.

           A convection current is a process which involves the movement of energy from one place to another. It is also called to as convection heat transfer. What is the reason that makes you feel hotter when placing hands above a campfire or when sitting next to it? Or, why is the movement of liquid so rapid when water is boiled in a pot? These things happen as a result of the Convection Currents.

          The convection currents tend to move a fluid or gas particles from one place to another. These are created as a result of the differences occurring within the densities and temperature of a specific gas or a fluid. Convection is one among the forms of heat transfers, of which the other two are radiation and conduction. Convection process only happens in the fluids i.e. in liquids and gases. This happens due to the reason that molecules within liquids or gases are free to move.

          The heat energy can be transferred by the process of convection by the difference occurring in temperature between the two parts of the fluid. Due to this temperature difference, the hot fluids tend to rise, whereas cold fluids tend to sink. This creates a current within the fluid called as Convection current.

          The mantle within the earth’s surface flows due to convection currents. These currents are mainly caused by a very hot material present in the deepest part of the mantle which rises upwards, then cools, sinks, again and again, repeating the same process of heating and rising.

          Hence Convection Current is defined as “a process of continuous heating up of liquids or gases by the process called as Convection. “

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            THE RELATIVE “shininess” of the Earth’s surface in a certain area will affect the local temperature — this is called albedo. Icy, snowy areas reflect most of the radiation of the Sun and remain cold. Forests and areas of bare soil absorb the radiation and tend to stay warm.

            Albedo can be defined as a way of quantifying how much radiation is reflected from the surface. It is a comparison between the reflection radiations from the surface to the amount of radiation that hits it. This term also refers to the quantity of radiation generated by electromagnetic rays which consequently reflects away.

Seasonal Effects on Albedo

Summer

            To understand albedo better, we look at two scenarios. One, if you walk barefoot on the black soil during summer, you will feel a lot of heat and can even get burnt because the surface is absorbing and retaining more heat. Another person walking on white soil during the same season will not be burnt. This is basically because white surface tends to reflect more heat and absorb very little of it. Equally, if you touch a black car in summer it will feel much hotter than touching a white car. This is because black absorbs and retains heat while white car surface will reflect back the solar rays.

Winter

            During this season, it is generally wet with either water or ice. Water reflects approximately 6% of the light and absorbs the rest. Ice, on the other hand, reflects 50% to 60% of the incoming solar heat, thereby remaining cooler. A snow-covered area reflects a lot of radiation, which is why skiers having a risk of getting sunburns while on the slopes. Albedo diminishes when the snow-covered places start to warm up.

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           Energy from the Sun arrives on the Earth in the form of radiation. Some of the radiation is absorbed or reflected back into space by the Earth’s atmosphere and clouds, but most of it reaches the surface, where it heats up the land and sea. As the Earth heats up, some of this heat is also reflected back into space.

          Conduction is one of the three main ways that heat energy moves from place to place. The other two ways heat moves around are radiation and convection. Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules. Conduction occurs more readily in solids and liquids, where the particles are closer to together, than in gases, where particles are further apart. The rate of energy transfer by conduction is higher when there is a large temperature difference between the substances that are in contact.

             Think of a frying pan set over an open camp stove. The fire’s heat causes molecules in the pan to vibrate faster, making it hotter. These vibrating molecules collide with their neighboring molecules, making them also vibrate faster. As these molecules collide, thermal energy is transferred via conduction to the rest of the pan. If you’ve ever touched the metal handle of a hot pan without a potholder, you have first-hand experience with heat conduction!

             Some solids, such as metals, are good heat conductors. Not surprisingly, many pots and pans have insulated handles. Air (a mixture of gases) and water are poor conductors of thermal energy. They are called insulators.

Conduction in the Atmosphere

            Conduction, radiation and convection all play a role in moving heat between Earth’s surface and the atmosphere. Since air is a poor conductor, most energy transfer by conduction occurs right near Earth’s surface. Conduction directly affects air temperature only a few centimeters into the atmosphere.

            During the day, sunlight heats the ground, which in turn heats the air directly above it via conduction. At night, the ground cools and the heat flows from the warmer air directly above to the cooler ground via conduction.

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          A Sundial shows the time of day by casting a shadow across its face. With the needle — the gnomon — of the sundial pointing north—south, the shadow indicates the time as the Sun passes through the sky from sunrise to sunset.

          When the earth rotates about its axis, the sun appears to “move” across the sky, causing objects to cast shadows. A sundial contains a gnomon, or a thin rod, that casts a shadow onto a platform etched with different times. As the sun changes relative positions over the course of a day, the rod’s shadows change as well, thus reflecting the change in time.

          If a sundial works based upon a rod’s shadow, then why can’t a simple stick in the ground work? As a result of the tilt of the earth’s axis, the visible movement of the sun changes daily.

          This can be accounted for in several ways. In a normal horizontal sundial, the base platform is kept steady, while the gnomon is moved to reflect the changes due to the earth’s axis tilt.

          Another method achieves the same effect by aligning the platform with the latitude and the gnomon perpendicular; mathematically, this is just the projection of the gnomon onto the platform.

          Sundials must be corrected across the span of a time zone. Every zone has a “reference longitude,” and with every degree of longitude away from the reference, the sundial is off by an additional 4 minutes.

          Thus, equation of time correction is employed in order to maintain a uniform time across the zone.

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          The number of hours of sunshine in a day is recorded on an instrument called a parheliometer. A solid glass ball focuses the Sun’s rays on to a strip of card. The intensified rays leave scorch marks on the card, moving along as the Sun moves through the sky. The longer the marks, the longer the period of sunshine.

         A sunshine recorder is a device that records the amount of sunshine at a given location or region at any time. The results provide information about the weather and climate as well as the temperature of a geographical area. This information is useful in meteorology, science, agriculture, tourism and other fields. It has also been called a heliograph.

          There are two basic types of sunshine recorders. One type uses the sun itself as a time-scale for the sunshine readings. The other type uses some form of clock for the time scale.

          Older recorders required a human observer to interpret the results; recorded results might differ among observers. Modern sunshine recorders use electronics and computers for precise data that do not depend on a human interpreter. Newer recorders can also measure the global and diffuse radiation.

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          Sometimes, intense amounts of electromagnetic energy are released from the Sun in the form of solar wind, or flares. The Earth is protected from solar wind — essentially an extremely hot gas — by its magnetic field, which stretches out into space. The particles of solar winds are known to affect satellites and even cause power blackouts on Earth. Scientists are still investigating the possible long-term effects of this activity on the Earth’s climate.

          Mt. Washington, New Hampshire is the windiest location in the United States, with an average wind speed of 35 mph. At this speed, trees start to sway and it becomes difficult to walk. Barrow Island, Australia has the highest wind speed ever recorded on Earth at 253 mph. This is strong enough to blow the roofs off of most buildings and uproot trees and shrubs. That is a pretty strong wind. But this is a drop in the bucket compared to the wind on the Sun.

          Solar wind is more than 4000 times as strong as the wind speed recorded on Barrow Island. Additionally, it reaches temperatures of around 1 million degrees Celsius, almost 15,000 times the hottest recorded temperature on Earth.

         The solar wind refers to the steady stream of highly charged particles that continually blow off the Sun in all directions. It is caused by the solar corona expanding into space. The corona is the outer atmosphere of the Sun. You can see it as a glowing halo around the Sun during a solar eclipse.

          The corona is so hot that the Sun’s gravity cannot hold it in. Instead, it streams off the Sun as protons and electrons shooting through space at speeds of around 400 km/s (about 1 million miles per hour). At that speed, you could travel from New York to Los Angeles in 10 seconds!

          The solar wind causes the Sun to lose more than 1 million tons of mass per second. That may seem like a really big number, but consider this: The Earth’s mass is about 6.5 sextillion tons. If you write that out it would be 6,500,000,000,000,000,000,000 tons. The Sun’s mass is 333,000 times that of Earth. If you think about it like that, 1 million tons per second isn’t actually that much.

          The solar wind escapes from coronal holes, which are generally found at the Sun’s poles. A coronal hole is an area in the corona that is thinner and less dense than the surrounding areas. It appears as a dark spot on the Sun’s surface since it is also a cooler temperature than the surrounding corona.

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          Some Scientists believe that sunspot activity may have an effect on the Earth’s weather. Sunspots seem to occur in cycles of 11 years. Research has shown that major periods of drought have occurred roughly every 22 years, or two sunspot cycles. We have yet to discover the exact relationship between the two.

          A new study in the journal Science by a team of international of researchers led by the National Center for Atmospheric Research have found that the sunspot cycle has a big effect on the earth’s weather. The puzzle has been how fluctuations in the sun’s energy of about 0.1 percent over the course of the 11-year sunspot cycle could affect the weather? The press release describing the new study explains:

          The team first confirmed a theory that the slight increase in solar energy during the peak production of sunspots is absorbed by stratospheric ozone. The energy warms the air in the stratosphere over the tropics, where sunlight is most intense, while also stimulating the production of additional ozone there that absorbs even more solar energy. Since the stratosphere warms unevenly, with the most pronounced warming occurring at lower latitudes, stratospheric winds are altered and, through a chain of interconnected processes, end up strengthening tropical precipitation.

          At the same time, the increased sunlight at solar maximum causes a slight warming of ocean surface waters across the subtropical Pacific, where Sun-blocking clouds are normally scarce. That small amount of extra heat leads to more evaporation, producing additional water vapor. In turn, the moisture is carried by trade winds to the normally rainy areas of the western tropical Pacific, fueling heavier rains and reinforcing the effects of the stratospheric mechanism.

          The top-down influence of the stratosphere and the bottom-up influence of the ocean work together to intensify this loop and strengthen the trade winds. As more sunshine hits drier areas, these changes reinforce each other, leading to less clouds in the subtropics, allowing even more sunlight to reach the surface, and producing a positive feedback loop that further magnifies the climate response.

          These stratospheric and ocean responses during solar maximum keep the equatorial eastern Pacific even cooler and drier than usual, producing conditions similar to a La Nina event. However, the cooling of about 1-2 degrees Fahrenheit is focused farther east than in a typical La Nina, is only about half as strong, and is associated with different wind patterns in the stratosphere.

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          Light from the Sun is made up of several different colours, each of which has its own wavelength. The wavelength of the blue part of the Sun’s light is shorter than the size of an oxygen atom. When the blue light waves hit the oxygen atoms in the Earth’s atmosphere, they are scattered, making the sky appear blue. The light waves of other colours (with greater wavelengths than blue) are also affected, but blue waves are scattered more than most.

          To understand why the sky is blue, we need to consider the nature of sunlight and how it interacts with the gas molecules that make up our atmosphere. Sunlight, which appears white to the human eye, is a mixture of all the colors of the rainbow. For many purposes, sunlight can be thought of as an electromagnetic wave that causes the charged particles (electrons and protons) inside air molecules to oscillate up and down as the sunlight passes through the atmosphere. When this happens, the oscillating charges produce electromagnetic radiation at the same frequency as the incoming sunlight, but spread over all different directions. This redirecting of incoming sunlight by air molecules is called scattering.

          The blue component of the spectrum of visible light has shorter wavelengths and higher frequencies than the red component. Thus, as sunlight of all colors passes through air, the blue part causes charged particles to oscillate faster than does the red part. The faster the oscillation, the more scattered light is produced, so blue is scattered more strongly than red. For particles such as air molecules that are much smaller than the wavelengths of visible light the difference is dramatic. The acceleration of the charged particles is proportional to the square of the frequency, and the intensity of scattered light is proportional to the square of this acceleration. Scattered light intensity is therefore proportional to the fourth power of frequency. The result is that blue light is scattered into other directions almost 10 times as efficiently as red light.

          When we look at an arbitrary point in the sky, away from the sun, we see only the light that was redirected by the atmosphere into our line of sight. Because that occurs much more often for blue light than for red, the sky appears blue. Violet light is actually scattered even a bit more strongly than blue. More of the sunlight entering the atmosphere is blue than violet, however, and our eyes are somewhat more sensitive to blue light than to violet light, so the sky appears blue.