The Hadley cell is a movement of warm air heated by the radiation from the sun hitting the surface of the Earth near the equator. The movement of air in the Hadley cell results in the formation of the trade winds moving from the northeast toward the west in the Northern Hemisphere at the equator.
The Hadley Cell Theory
English meteorologist George Hadley created the scientific theory of the Hadley cell in 1735. Hadley’s theory attempts to explain the formation of the trade winds, also called termed tropical easterlies, in the region of the equator. The Hadley cell forms through the heating of the Earth’s surface in the region of the equator, where the sun’s rays are most intense. This heats the air around the equator and creates a circular flow of air around the tropics and subtropical regions of the world.
Subtropic Temperature Creation
As the air is heated, the warm air around the equator rises and moves outward toward the cooler air close by. The warm air of the Hadley cell moves north in the Northern Hemisphere and south in the Southern Hemisphere. The warm air moves toward the cooler air of the Earth’s poles, with some of warm air falling to the surface of the Earth approximately 30 degrees latitude in both the Northern and Southern Hemispheres. This creates the warm temperatures of the subtropics.
Near Equatorial Trough and Subtropical Ridge
The Hadley cell creates two weather systems around the equator of the Earth: the near equatorial trough and the subtropical ridge. The near equatorial trough is the area of low pressure that forms around the equator, which is caused by the movement of warm air rising after it is heated by the sun. The second, or subtropical, trough is a band of semipermanent high pressure in the areas around 30 degrees latitude in both hemispheres.
One of the best known effects of the Hadley cell is the formation of strong winds, known as the trade winds, or tropical easterlies. The movement of air in the Hadley cell forms these winds. As the warm air that is moving northward in the Northern Hemisphere falls to the surface at around 30 degrees latitude, it moves to the right, creating a northwesterly wind. The rightward movement is due to the Earth's spin, creating the Coriolis effect. This air also begins to move back toward the warmer air found at the equator, creating a band of warm winds that was used by sailing ships throughout history to move quickly to the Americas.