Hadley cell
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The Hadley cell is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, poleward flow 10-15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface. This circulation is intimately related to the trade winds, tropical rainbelts, subtropical deserts and the jet streams.
The major driving force of atmospheric circulation is solar heating, which on average is largest near the equator and smallest at the poles. The atmospheric circulation transports energy polewards, thus reducing the resulting equator-to-pole temperature contrast. The mechanisms by which this is accomplished differ in tropical and extratropical latitudes.
Between 30°N and 30°S latitude, this energy transport is accomplished by a relatively simple overturning circulation, with rising motion near the equator, poleward motion near the tropopause, sinking motion in the subtropics, and an equatorward return flow near the surface. In higher latitudes, the energy transport is instead accomplished by cyclones and anticyclones that cause relatively warm air to move polewards and cold air to move equatorwards in the same horizontal plane. The tropical overturning cell is referred to as the Hadley cell. Questions as to why it extends only to 30 degrees latitude and what determines its strength are at the heart of modern dynamical meteorology.
Near the tropopause, as the air moves polewards in the Hadley cell it is turned eastward by the Coriolis force, which turns winds to the right in the Northern hemisphere and to the left in the Southern Hemisphere, creating the subtropical jet streams that flow from west to east. One can also think of a ring of air trying to conserve its angular momentum in an absolute reference frame (one not rotating with the Earth). As the ring of air moves polewards, it contracts closer to the axis of rotation so it must spin faster, which creates jets that rotate more rapidly than the Earth itself, which therefore appear as jets flowing from west to east with respect to the surface. Analogously, near the surface, the equatorward return flow is rotated to the west, or is slowed from the perspective of a non-rotating observer due to its movement away from the axis of rotation. These surface winds, with both an equatorward and a westward component, are referred to as the trade winds.
In the early 1700s, George Hadley, an English lawyer and amateur meteorologist, set out to determine what it was that caused the Northern-hemisphere trade winds to blow west instead of straight south. Edmond Halley, the astronomer noted for the comet bearing his name, had some years earlier proposed a theory of circulation in which the solar heating creates upward motion but he could not account for the eastward component of the trade winds. It was Hadley who realized that a rotating coordinate system would be required to describe the motions of the winds. The work of Gaspard-Gustave Coriolis, a century later, would be required before the dynamical underpinnings of the theory were made more coherent, but Hadley’s description of the tropical circulation cell was sufficiently valid that his name is now universally attached to this phenomenon.
The region of subsidence in the Hadley cell is known as the "horse latitudes". According to the story, in times when ship's captains relied upon the wind to reach their destinations, finding themselves becalmed was usually bad news for any horses aboard, which were thrown overboard in order to conserve precious water.
The region in which the equatorward moving surface flows converge and rise is known as the intertropical convergence zone, or ITCZ, a high-precipitation band of thunderstorms.
Having lost most of its water vapor to condensation and rain in the upward branch of the circulation, the descending air is dry. Low relative humidities are produced as the air is adiabatically heated due to compression as it descends into a region of higher pressure. The subtropics are relatively free of the convection, or thunderstorms, that are common in the equatorial belt of rising motion. Many of the world's deserts are located in these subtropical latitudes.
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