Semiarid regions average 10 to 20 inches of annual precipitation . Typically, desert moisture occurs in brief intervals and is unpredictable from year to year. Evaporation is also an important factor contributing to aridity. In some deserts, the amount of water evaporating, exceeds the amount of rainfall. Areas with few clouds, bodies of water and little vegetation absorb most of the sun's radiation, thus heating the air at the soil surface.
More humid areas deflect heat in clouds, water and vegetation, remaining cooler.
High wind in open country also contributes to evaporation. Locations of deserts have changed throughout geologic time as the result of continental drift and the uplifting of mountain ranges. Modern desert regions are centered in the horse latitudes, typically straddling the Tropic of Cancer and the Tropic of Capricorn, between 15 and 30 degrees north and south of the equator. Some deserts, such as the Kalahari in central Africa, are geologically ancient. The Sahara Desert in northern Africa is 65 million years old, while the Sonoran Desert of North America reached its northern limits only within the last 10,000 years.