Deserts occur in specific latitudes ( 5-35° north and south of the equator) because of the general thermodynamics of our planet. Solar radiation hits the earth with highest intensity near the equator. Because the earth's axis is tilted 3.5° with respect to the plane of its orbit, during part of the year the zone of maximum solar interception shifts northwards, towards the Tropic of Cancer, and during part of the year it moves southwards, towards the Tropic of Capricorn. Thus, the warm tropics form a belt around the equator from latitude 3° north to latitude 3° south, where the tropical heat generates rising, unstable air.

As it climbs, the air condenses the moisture evaporated from the warm tropical seas and forests, and produces the heavy downpours that characterize the wet tropics. As it moves away from the equator at high altitudes, the air cools again and eventually starts descending towards the midlatitudes, some 3000 km away from the equator both north and south. The air masses heat in their descent and, having lost their moisture during their tropical ascent, they become extremely dry. Thus, by contrast with the equatorial forests, the mid-latitude arid fringes that run alongside the tropical belt have a more stable atmosphere. These are the "horse" latitudes, where calm, dry air often dominates.

Additionally, because of the stable atmosphere, not only are winds slack, but rainstorms seldom develop. For this reason most of the world's large deserts occur along the belt that separates the tropics from the temperate regions (Goudie and Wilkinson 1977).