Deserts in the Southwester United States are areas of extreme heat and dryness, just as most of us envision them. More scientifically, deserts, also called arid regions, characteristically receive less than 10 inches of precipitation a year . In some deserts, the amount of evaporation is greater than the amount of rainfall. 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.