Category Science

          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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