Hadley circulation

The Hadley cell, named after George Hadley, is a tropical atmospheric circulation that features air rising near the equator, flowing poleward at 10–15 kilometers above the surface, descending in the subtropics, and then flowing equatorward near the surface. This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams.

In each hemisphere, there is one primary circulation cell known as a Hadley cell and two secondary circulation cells at higher latitudes, known as the Ferrel cell, and Polar cell. It can consist from 30 to 40 degrees north and south and is mainly responsible for the weather in the equatorial regions of the world.

The major driving force of atmospheric circulation is solar heating, which is greatest near the equator and lesser 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 differs in tropical and extratropical latitudes.

Hadley cells exist on either side of the equator. Each cell encircles the globe and acts to transport energy from the equator to 30th latitude. This is the result of the buoyancy of warm air rising from the equator to the tropopause; its movement as it is pushed poleward by air rising from below; its cooling as it moves poleward at high altitude; its sinking near subtropics (30th parallels); and finally its equatorward return near the surface, completing the loop. In higher latitudes, the energy transport is also accomplished by cyclones and anticyclones that cause relatively warm air to move polewards and cold air to move equatorwards. The large scale 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 dynamical meteorology.

Near the tropopause, as the air moves polewards in the Hadley cell, it is turned eastward by the Coriolis effect, 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.

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