Deserts are arid regions of extreme dryness, characteristically receiving less than 5 inches of precipitation a year. In some deserts, the amount of evaporation is greater than the amount of rainfall, making them very dry. Semi-arid deserts average 5 to 10 inches of annual precipitation. Desert moisture typically occurs in brief intervals and is unpredictable from year to year. One-third of the earth is arid to semi-arid (being either desert or semidesert).
Evaporation contributes greatly to a desert's aridity. In some deserts, the amount of water evaporating exceeds the amount of rainfall making that desert very dry. Rising air cools and can hold less moisture, producing clouds and precipitation; falling air warms, absorbing moisture. 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 the years. 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 very old. The Sahara Desert in northern Africa is quite old, as is the Sonoran Desert of North America. Deserts can also occur at near-polar latitudes, for example the interior of Iceland.
Because they are poised in such harsh extremes of heat and aridity, deserts are among the most fragile ecosystems on the planet.