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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 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.
The Hadley Cell circulation influences many of the Earth’s climate systems and biomes through its effects on precipitation patterns. High levels of annual precipitation are associated with the ITCZ; the subtropical highs have a variable effect on precipitation, although it is generally associated with drier conditions. Where the ITCZ is present throughout the year, annual rainfall (152-254 centimeters) with no dry season, and defines the tropical wet climate and corresponds to the tropical rainforest biome, composed of broadleaf evergreen trees. With increasing latitude, the influence of the subtropical high tends to confer winter dry seasons of increasing length. Tropical monsoon climates (254-508 centimeters precipitation with 1-3 months of winter dry season) give way to tropical savanna climates (90-180 centimeters annual precipitation and 1-6 months winter dry season), and tropical rain forest gives way to tropical deciduous forest, which grades into the mixed trees, shrubs, and grasslands of the tropical savanna biome.
Poleward of these tropical climates, the subtropical high pressure system exerts the greater influence on climates, but the effect on precipitation is variable. Where the dry, warm air subsides over continental interiors, the climates are quite arid. Where the air subsides over the oceans, the warm air has a high capacity for moisture, evaporation increases and the air can become quite humid. Whether this translates into precipitation over the land is then a function of surface winds. Around high pressure systems, winds follow an anticyclonic circulation, which translates into a clockwise rotation in the Northern Hemisphere and an anticlockwise rotation in the south. On the equatorward margins of the subtropical highs, winds follow an easterly path (giving rise to the northeasterly and southeasterly trade winds). Thus, on the western coasts of continents, dry, warm air subsides over land and blows eastward as an offshore flow of wind, and maintains extremely arid conditions.
As one travels poleward from the tropics on the west coast to the center of continents, the tropical savannas give way to the subtropical steppe climates and subtropical desert, and the savannas give way to grasslands and ultimately low-latitude hot desert biomes. Along east coasts, however, the easterly flow of winds around the equatorward margins of the subtropical highs create an onshore flow of winds, and the moisture the subsiding air picks up over the oceans becomes expressed as precipitation overland. Regions thus affected exhibit humid subtropical climates, which support midlatitude deciduous forests. Although not technically part of the Hadley Cell circulation, the westerlies flowing poleward from the subtropical highs affect Mediterranean, marine west coast, midlatitude cold desert, and steppe and humid continental-hot summer climates and their associated vegetation. The Hadley Cell circulation thus governs the precipitation pattern of most of the Earth’s climate systems.
For human societies, the Hadley Cells thus play an important roll in providing food security by supporting agriculture through precipitation. The majority of the world’s developing countries are located in the tropics, are heavily dependent on agriculture, and thus dependent on the rainfall imparted by the ITCZ. In the arid zones affected by Hadley Cells, lack of precipitation in conjunction with growing demand for agricultural production and improved irrigation technology results in a growing dependence on groundwater, with groundwater mining (extracting groundwater at rates quicker than it can be replenished), aquifer collapse, and sea water intrusion being common problems in these regions, such as on the High Plains of the United States and in the Middle East. Severe seasonally flooding often accompanies the arrival of the monsoons in South Asia. Also, tropical cyclones form within the Hadley Cell system.
Additionally, variation in strength of the pressure gradient between the subtropical high and ITCZ across the South Pacific creates the El Nino-Southern Oscillation (ENSO) phenomenon. An increased frequency and intensity of tropical cyclones in the South Pacific, as well as locally severe droughts and flooding accompany these ENSO events.
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- Robert W. Cristopherson, Geosystems (Pearson Prentice Hall, 2006);
- J. de Blij and Peter O. Muller, Geography: Realms, Regions and Concepts (John Wiley & Sons, Inc., 2000);
- Glen M. MacDonald, Biogeography: Space, Time and Life (John Wiley & Sons, 2003);
- Tom McKnight and Darrel Hess, Physical Geography (Pearson Prentice Hall, 2005).