Hadley Cell

The Hadley cell is an atmospheric circulation cell, named after the English scientist George Hadley. In a 1735 paper, Hadley described the global circulation as consisting of one thermally direct cell in each hemisphere. Hadley's conceptual model consisted of rising air at the equator and sinking air at the poles. Surface flow was from the poles toward the equator, while upper-level flow was from the equator toward the poles, completing the cell. Hadley's early contribution did not consider the rotation of Earth, which results in the Coriolis force, an apparent deflection of moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. His description also did not consider seasonal variations due to the annual revolution of Earth around the sun. In the early 20th century, regular meteorological observations became more widespread, and a clearer picture of the global circulation emerged. In a contemporary context, the term Hadley cell is used to describe the circulation in the tropics and subtropics, which is thermally direct as Hadley had envisioned. When combined with seasonal variability and deflection of air due to Earth's rotation, the Hadley cell is a useful conceptual tool for understanding tropical circulation.

In the simplest sense, the Hadley cell is still the meridional overturning circulation described by Hadley in 1735 but constrained to the tropics so that flow near the surface is equatorward and flow near the tropopause (at a height of approximately 15–17 kilometers in the tropics) is poleward. The poleward extent of the Hadley cell is governed by the position of the subtropical high-pressure belt, or, more appropriately, a broken belt of semipermanent high-pressure systems, which are located, on average, near 30° N and 30° S of the equator. Due to the Coriolis force described above, the near-surface air flowing equatorward from the subtropics is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, resulting in the Northeast and Southeast trade winds, respectively. Similarly, the poleward transport aloft is deflected, and it combines with the converging midlatitude flow, resulting in the formation of a subtropical jet stream at the poleward edge of the Hadley cell. The Hadley cell in each hemisphere is completed by rising motion within the equatorial low-pressure belt that results from intense solar heating and convergence of Northern Hemisphere and Southern Hemisphere trade winds and sinking motion in the subtropics.

The zone of convergence near the thermal equator at the surface is known as the Intertropical Convergence Zone (ITCZ), or equatorial low. It is recognizable on satellite imagery as a bright band of clouds near the equator. The release of latent heat due to the condensation of water vapor within the ITCZ is a major source of energy for the rest of the Hadley cell circulation. Outside the [Page 1396]ITCZ, there is little cloud development with the Hadley cell. Instead, the region between the ITCZ and subtropical highs is characterized by warm, subsiding air. This phenomenon results in a temperature inversion, where cooler air near the surface lies below the warming, sinking air. This is known as the trade wind inversion since it occurs in the region above the surface trade winds. The result is that many areas of the tropics experience conditions where the atmosphere is very humid but precipitation amounts are low. This also means that precipitation receipt in the tropics and subtropics is largely determined by the position of the ITCZ. Indeed, seasonal movement of the ITCZ in response to variations in solar energy received produces the strong seasonal variations in precipitation that are experienced within the tropics, and especially within the subtropics. For example, in July the ITCZ's greatest poleward position is over Northern India, where its interaction with the Himalayas plays an integral role in the summer monsoon rains. Such variations in the poleward position of the ITCZ also vary longitudinally due to surface (land/water) variations. Generally, the ITCZ will extend farthest poleward over land in the summer hemisphere. Some of the rain-producing clouds within the ITCZ exhibit strong organization and have [Page 1397]the ability to intensify. These clouds are generally oriented in North-South bands that migrate from east to west, known as easterly waves, and are often precursors to tropical cyclones in the Atlantic Ocean.

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