Hadley Circulation

Hadley circulation is a type of atmospheric circulation following a stereotypical pattern in the para-equatorial region. Atmospheric circulation and ocean circulation together are the major mechanisms for global heat distribution. As ocean circulation refers to movement of waters, atmospheric circulation defines large-scale air movements around the globe. The principle tropical current known as Hadley circulation affects jet streams, subtropical deserts, trade winds, and tropical rainbelts. Hadley circulation provides westward windflow at the Earth’s surface and eastward jet streams at higher altitudes.

The circulating air patterns create convection currents in four global locations—each current is called a Hadley cell. The pattern of air movement is toward the Earth (prograde) at higher latitudes (in the subtropics) and backward (up from the Earth) at lower latitudes (near the equator). The movement occurs near the Earth’s surface—within 6.2–9.3 mi. (10–15 km). Its span across latitudinal markers remains within 30 degrees north or south of the equator. In this way, a Hadley cell moves heat from the equatorial region to regions within 30 degrees of latitude in either direction. Moisture is moved along with the heat.

In a Hadley cell, the air rises to the atmospheric tropopause, which is the region at the top border of the troposphere and thus the bottom border of the stratosphere. The troposphere is the lowest atmospheric region and is where all weather takes place. At the equator, it reaches up to 11 mi. (18 km) from the Earth’s surface.

The next atmospheric layer is the stratosphere, extending to 31 mi. (50 km) from the Earth’s surface. This characteristic air circulation results from the sun’s rays heating the air at the level of the equator. Solar heating is strongest at the equator and weakest at the north and south poles, because of the direction of the sun’s rays, and therefore currents of atmospheric circulation because of solar heating are more prominent at the equator.

Hadley circulation was first described by George Hadley (1685–1768) to explain the science behind the trade winds. It was to replace a flawed model that had been presented by Edmond Halley (1656–1742). George Hadley was an early-18th-century meteorologist by hobby, and lawyer by trade.

In fact, Hadley’s theory was also imperfect. It was corrected by American meteorologist William Ferrel (1817–91) at the end of the 19th century, but by this time Hadley’s name had stuck. The Hadley circulation is traditionally defined as resting on the equator; in fact, it rests on the thermal equator, or the sun’s zenith.