Hadley Cell

The hadley cell refers to a somewhat idealized vertical circulation of air in the Earth's atmosphere and comprises the principal component of the general circulation pattern of the Earth's atmosphere. The Hadley Cells are comprised of a trough of low pressure girdling the globe in an equatorial and tropical band (the intertropical convergence zone, or ITCZ) and its associated rising air and a ridge of high pressure (the subtropical highs) where the air subsides back to the surface. A Hadley Cell thus circulates roughly from between 0 degrees latitude and 30 degrees latitude, north and south, although the actual latitudes will shift over the course of the year as the subsolar point passes between the Tropics of Cancer and Capricorn over the course of the year. There are thus two Hadley Cells; both sharing the ITCZ as the zone of lifting, but separating into two separate circulations as the air settles into both the northern and southern hemisphere subtropical highs.

Solar energy drives the system, with the most intense heating of the Earth's surface occurring at the latitude receiving the vertical rays of the sun (the subsolar point). The heating of the surface causes the air above to warm and rise (convectional lifting), creating low pressure. The low pressure draws in surface winds from the higher latitudes (the northeasterly trade winds from north of the ITCZ, and the southeasterly trade winds from the south of the ITCZ). The northeast and southeast trade winds converge on the trough of low pressure, and the collision of [Page 845]these air masses forces the air upwards (convergent lifting), which further decreases pressure. At the surface, the air in the vicinity of the low pressure trough is warm and humid; as the air rises, it cools and the atmospheric moisture condenses into precipitation.

The rising air eventually reaches the tropopause (occurring at roughly 18 kilometers in altitude over the tropics, but descending in altitude to roughly 12 kilometers in the midlatitudes), which is the upper boundary of the lowest region of the atmosphere (the troposphere) and the stratosphere, the region of the atmosphere containing the ozone layer. The ozone is heated by the sun, and above the tropopause, air temperature begins to increase with altitude. The rising air from the ITCZ, having cooled while rising from he surface, encounters warmer atmospheric air (the stratosphere) upon reaching the tropopause; this temperature inversion prevents further lifting of the air.

The circulating air then spreads out along the tropopause, both latitudinally and longitudinally. The longitudinally spreading air becomes accelerated and contributes to the subtropical jet stream. The air spreading toward the higher latitudes along the tropopause are termed antitrade winds; the air now is cool and dry (having the moisture removed through condensation and precipitation), and settles back to the surface, forming the subtropical high pressure systems. As the air subsides, it warms such that the air reaching the surface is warm and dry.

The subsiding air spreads out along the Earth's surface, with the winds spreading toward the equator from the subtropical highs feeding back into the ITCZ as the easterly trade winds, and the winds spreading poleward from the subtropical highs being termed the westerlies and contributing to midlatitude circulation and the formation of extra-tropical cyclones. Technically, the Hadley Cell circulation strictly refers to the air rising over the ITCZ, circulating poleward as the antitrade winds, subsiding to the Earth's surface as the subtropical highs, and circulating back into the ITCZ as the easterly trade winds. Although the westerlies are functionally tied to the subtropical highs, they are not technically considered to be part of the Hadley Cell circulation system.

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