|MadSci Network: Earth Sciences|
In general, the earth recieves far more radiation from the sun at the equator than at the poles averaged over the year. Therefore, hot air should basically rise at the equators, move north or south, cool and descend at the poles, returning back along the surface to the equator. Because the earth rotates, a single cell of circulation cannot exist in each hemisphere and there are actually three circulation cells in each hemisphere (North and South for six total). See figure 1. Figure 1 Rising air leads to low pressure near the surface and sinking air leads to high pressure near the surface. This leads to semi-permanent pressure zones on the earth because of these three circulation cells. See figure 2. Figure 2 In addition, as air travels along, it experiences the coriolis force because of earth's rotation. This force tends to deflect motion the the right in the northern hemisphere and to the left in the southern hemisphere. The combination of the three (six) major circulation cells, the semi-permanent pressure zones and the coriolis force cause the generalized circulation patterns seen in the figures. (i.e. "Easterlies" and "Westerlies")
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