Earth's winds run as three giant heat engines stacked in each hemisphere
The Sun heats the tropics far more than the poles, and the air spends its whole existence trying to even that out. The result is three circulation cells in each hemisphere, one of them working like a heat engine rated at roughly 200 terawatts, and a middle band whose weather never settles.
Seen whole, the atmosphere behaves like an engine fuelled by sunlight whose final exhaust is the darkness of space. Its work moves air, carrying heat soaked up near the equator toward higher latitudes and eventually out to space. The big pattern holds steady from year to year because it depends on Earth's size, spin, heating and air depth, none of which change much. Individual storms are another matter: chaotic enough that useful forecasts stop at about ten days in practice.
George Hadley described the first loop while trying to explain the trade winds. Moist air warmed at the equator rises, flows poleward high up, cools and sinks near the 30th parallel, then returns along the surface. The Coriolis effect bends that returning flow westward, producing the easterly trades. Where the air sinks sit the horse latitudes, known for light breezes, clear skies and little rain. Near the poles a similar but smaller loop, the polar cell, rises at about 60 degrees and descends as cold, dry air over the ice; together with Antarctic terrain it helped Vostok Station reach −89.2 °C in 1983.
Between them turns the Ferrel cell, proposed by William Ferrel, who lived from 1817 to 1891. It has no strong heat source or sink of its own and is dragged along by its neighbours, almost like an eddy. Because it is weak, the westerlies beneath it are easily upset, and a cold front can swing the wind within minutes. That is why the mid-latitudes, home to the endless procession of highs and lows, are called the zone of mixing.
Other cells run east to west. Over the Pacific, warm western water and cool eastern water drive the Walker circulation, named for Sir Gilbert Walker. Trying to predict failures of India's monsoon, he never managed it, but he uncovered the Southern Oscillation, a see-saw of pressure between the oceans.
Source: Atmospheric circulation
