The Hadley cell, named after George Hadley, is a tropical atmospheric circulation which features 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 and hurricanes, 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 gradient. 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 equator wards in the same horizontal plane. The tropical overturning cell is referred to as the Hadley cell. Why it extends only to 30 degrees latitude and what determines its strength are questions addressed by modern dynamical meteorology.
Near the tropopause, as the air moves polewards in the Hadley cell it is turned eastward by the Coriolis effect, 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. Analogously, near the surface, the equatorward return flow is turned to the west by the Coriolis effect. These resulting surface winds, with both an equatorward and a westward component, are referred to as the trade winds.
The Hadley system provides an example of a thermally direct circulation. The thermodynamic efficiency of the Hadley system, considered as a heat engine, has been relatively constant over the 1979~2010 period, averaging 2.6%. Over the same interval, the power generated by the Hadley regime has risen at an average rate of about 0.54 TW per yr; this reflects an increase in energy input to the system consistent with the observed trend in the tropical sea surface temperatures.
History of discovery
In the early 18th century, George Hadley, an English lawyer and amateur meteorologist, was dissatisfied with the theory that the astronomer Edmond Halley had proposed for explaining the trade winds. What was no doubt correct in Halley's theory was that solar heating creates upward motion of equatorial air, and air mass from neighboring latitudes must flow in to replace the risen air mass. But for the westward component of the trade winds Halley had proposed that in moving across the sky the Sun heats the air mass differently over the course of the day. Hadley was not satisfied with that part of Halley's theory and rightly so. Hadley recognized that Earth's rotation plays a role in the direction taken by air mass that moves relative to the Earth, and he was the first to do so. Hadley's theory, published in 1735, remained unknown, but it was rediscovered independently several times. Among the re-discoverers was John Dalton, who later learned of Hadley's priority. Over time the mechanism proposed by Hadley became accepted, and over time his name was increasingly attached to it. By the end of the 19th century it was shown that Hadley's theory was deficient in several respects. One of the first who accounted for the dynamics correctly was William Ferrel. It took many decades for the correct theory to become accepted, and even today Hadley's theory can still be encountered occasionally, particularly in popular books and websites. Hadley's theory was the generally accepted theory long enough to make his name become universally attached to the circulation pattern in the tropical atmosphere. In 1980 Isaac Held and Arthur Hou developed the Held-Hou Model to describe the Hadley circulation.
The region of subsidence in the Hadley cell is known as the "horse latitudes".
Major impacts on precipitation by latitude
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 warmed 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.
Hadley cell expansion
There is some evidence that the expansion of the Hadley cells is related to climate change. The majority of earth's arid regions are located in the areas underneath the descending branches of the Hadley circulation around 30 degrees latitude. Both idealised and more realistic climate model experiments show that the Hadley cell expands with increased global mean temperature (perhaps by 2 degrees latitude over the 21st century ); this can lead to large changes in precipitation in the latitudes at the edge of the cells. Scientists fear that the ongoing presence of global warming might bring changes to the ecosystems in the deep tropics and that the deserts will become drier and expand. As the areas around 30 degrees latitude become drier, those inhabiting that region will see less rainfall than traditionally expected, which could cause major problems with food supplies and livability. In terms of paleoclimate, there is strong evidence for climate change in central Africa's rain forest in c. 850 B.C.  Palynological (fossil pollen) evidence shows a drastic change in rain forest biome to that of open savannah as a consequence of widescale dryness not connected necessarily to destructive drought but perhaps to gradual warming. The contrast with this change to dryness in central west Africa - and the simultaneous increase in precipitation in temperate zones north - fits well with the hypothesis that, after a decline in solar activity, the latitudinal extent of the Hadley Circulation decreased and consequently mid-latitudinal monsoons decreased in intensity. Meanwhile, mid-latitudinal storm tracks in the temperate zones increased and moved equatorward. 
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