Because the equatorial regions are more perpendicular to incoming EMR, they receive more radiation per unit area and heat more quickly than areas north or south of the equator. This causes warm equatorial air to move towards the poles. On the way, the air cools and begins to sink. The cooler air descends and moves back towards the equator creating the so-called trade winds of both hemispheres. While heating of air changes its density, causing it to rise or fall, the change in temperature also affects the air's moisture content.
Warmer air can hold more moisture than cooler air.
Hence air warmed at the intertropical convergence carries a huge amount of water with it as it lifts upward in the rising arm of the tropical Hadley cell. As the air cools, its capacity to hold water is reduced and that same mass of water falls as rain. The falling arm of the tropical Hadley cell is composed of cooler air, mostly devoid of moisture. Hence somewhere around 30 degrees north and south of the equator, we find the world's major desert zones. Figure 3.4 illustrates these and other global climate zones resulting from Hadley driven temperature and precipitation.