Category Science

          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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