Hadley Cell Circulation and the Trade Winds

Prevailing Wind Belts of Earth. The earth is encircled by several broad prevailing wind belts, which are separated by narrower regions of either subsidence or ascent. The direction and location of these wind belts are determined by solar radiation and the rotation of the earth. The three primary circulation cells are known as the: Hadley cell; Ferrel cell; and Polar cell.

Fig. 1. Primary circulation cells and prevailing wind belts of Earth.

On or near the equator, where average solar radiation is greatest, air is warmed at the surface and rises. This creates a band of low air pressure, centered on the equator known as the intertropical convergence zone (ITCZ). The Intertropical Convergence Zone draws in surface air from the subtropics. When this subtropical air reaches the equator, it rises into the upper atmosphere because of convergence (see below for explination) and convection. It attains a maximum vertical altitude of about 14 kilometers (top of the troposphere), and then begins flowing horizontally to the North and South Poles. This rising air comprises one segment of a circulation pattern called the Hadley Cell (see diagram below). The Hadley cell eventually returns air to the surface of the earth, near 30 deg N and S.

Fig. 2. The Hadley Cell.

The descending portion of the Hadley Cell produces a band of high air pressure at these latitudes called the subtropical high. From this zone, the surface air travels in two directions. Winds are generated between the subtropical high and the equatorial band of low pressure (ITCZ), as air moves from high surface pressure toward low surface pressure. These winds are deflected from east to west as they travel toward the equator by the coriolis force, and are called the Trade Winds or the Tropical Easterlies. The other portion of the surface air moves towards the poles from the subtropical high zone. This air is also deflected by the Coriolis force, producing the Westerlies.

Fig. 3. Global belts of low and high atmospheric pressure.



Convergence occurs when air moving from different directions collides. When convergence happens near the surface of the earth, such as when air spirals into the center of a low pressure cell, air is forced upward, a process called ascent. When convergence occurs high in the atmosphere, for example near the top of the troposphere, air is forced downward toward the surface, a process called descent, or subsidence.

Fig. 1. Vertical movements of air associated with convergence and divergence.