| MadSci Network: Environment |
The major climate patterns on earth are determined by air circulation, ocean circulation, the shape of continents and mountain ranges, and the astronomical position of the earth with respect to the sun. Among this host of factors, atmospheric circulation explains most of the variation in the locations of major climate zones such as deserts and tropical forests.
(the illustrations here will be helpful)
The atmosphere circulates around the planet in six giant, donut-like rings called Hadley cells. There are three sets of them: a pair that extends from the equator to about 30° on either side, a pair beyond that extending to about 45°, and a pair that extends from 45° north and south to the poles.
At the equator, energy from the sun hits the atmosphere most directly, and thus with greatest intensity. Heated air rises, carrying water vapor from the oceans. As the air rises, it cools and the water condenses, producing high rainfall zones near the equator. At 30° latitude (north and south), this mass of air begins to sink again. Now it is cold and dry, having dropped most of its water vapor on the lower latitudes. This sinking mass of dry air explains the presence of deserts at 30° north and south.
The polar cells are driven by cold air sinking at the poles, where the sun hits the earth at the most oblique angle and thus with the least intensity. In between the polar cells and the equatorial cells, there are weakly circulating cells dragged along by the stronger effects of their neighbors. At the boundaries of these cells (45° north and south), there are strong and persistent wind currents. The jet stream is the best-known example.
Why are these Hadley cells located precisely where they are? NASA scientists have a theory that the Hadley cells allow optimal circulation of the incoming heat from the sun. “Constructal theory,” as this branch of physics is called, predicts that flows of matter and energy in a system will gradually reach a state of minimal obstruction.
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