Air has different types of movements. It moves vertically as well as horizontally. The vertical movement of air is easy to understand. When a part of air becomes warm, its density becomes low, in other words it becomes lighter.
The low density results in upward movement of the warm air. The warm moist air rising up forms clouds. The horizontal movement of air called wind is, not so easy to explain. In a very simple way, we can say that wind is caused due to difference in air pressure at different places.
Air always moves from areas of higher pressure to areas of lower pressure. One may ask how these differences in pressure are created. The answer is, due to an unequal heating of the earth’s surface. As we have discussed above, earth’s surface gets heated by the solar radiation reaching the earth.
The solar energy does not reach everywhere in a uniform way. Some areas receive more energy while others receive less, due to absorption, scattering and reflection of energy by the atmosphere. Where the energy has to travel longer distances, these factors become more effective and important. This is the reason why the areas of the earth, which receive solar radiation, is at an inclination.
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Also, the sea surface and the land surface behave differently the sea surface does not change its temperature quickly while land areas get heated or cooled quickly. This is the reason why during the day the air above the land gets heated and expands while the air above sea does not show much change.
Therefore, the air above the land moves towards the ocean at higher altitude. To compensate it there is a movement of air mass from sea towards land at the lower altitude. Thus, a kind of circulation is created. In the lower part air moves from sea to land and in the upper part from land to sea. This kind of circulation is of local nature and can be seen only in coastal areas.
However, the unequal heating of different areas of earth plays a more important role. This leads to air circulation on a global scale.
To represent the global circulation of air, several models were proposed. Presently, the three-cell circulation model for each hemisphere of the earth is considered to be very useful and explains most of the phenomena. There is one zone between the equators. In the upper part of the atmosphere, flow of air in this zone is towards the poles. This is the zone where trade winds are created. The horse latitudes where not much wind flows, are located at the edge of this zone.
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Next is the zone between 30o and 60o latitudes. In this zone, the surface flow is poleward and the winds have a westerly component. The westerlies are important in maintaining the heat balance of the earth’s surface. They carry the excess heat from the equatorial region towards the poles.
The third zone lies beyond 60o latitudes. In these areas, there are surface flows of wind, which move towards the equator. They form polar easterlies. These cold wind currents meet with the warm westerlies and often cause foggy conditions.
Due to these air currents all over the glove, heat is transferred from one area to another. Quantity of precipitation falling on different parts of the earth is also controlled by air currents.
There is a close relationship between precipitation and distribution of earth’s wing systems. We can easily identify zonal arrangement of precipitation from equator to poles. For example, the equatorial region has convergence of warm and moist air which results in heavy rainfall in the zone, while the areas where subtropical high winds dominate are generally dry.
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Temperature also changes from place to place. Air currents and temperature are the two main factors, which cause different types of climates in different zones of the earth. As said earlier, the climate is the deciding factor for ecosystem. That is found in the equatorial region is different from that which exists in the polar region or in the subtropical region.