Excerpt from The Zonally Symmetric Motion of the Atmosphere When viewed from outer space, the earth's atmosphere, as evidenced by the cloud patterns, shows no signs of symmetry or regularity However, analysis of wind and temperature data over long periods of time shows that on the average the atmosphere behaves in a regular manner. Prevailing westerlies, trade winds and the Hadley Circulation are gross features of the motion which are observed from average data.
In 1735, George Hadley initiated the idea that solar heating at the equator forced the air there to rise, and hence he conjectured that the air, once aloft, travelled to the poles, where it sank back to the surface to journey toward the equator again. Subsequent observations by Ferrel around the 1880's suggested that the air rising at the equator did not travel to the poles, but sank back to the surface at about 300 north or south latitude. In addition, the cold air sinking at the poles rose again at about 600 north or south latitude. In between these cells was a single cell rising at 600 and sinking at 300 in each hemisphere. The conjectures mechanism to drive the temperature cell was friction between the adjacent thermally driven cells; while the eastward winds therein were the result of the rotational (coriolis) force interacting with the meridio nal motions. Because of the appearance of three cells in each hemisphere, mechanisms to explain the motions by explaining the presence of each cell have been called tricellular theories.
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