Joshua Tree National Park - Deserts Defined

Weather patterns often create short, violent downpours. Flash floods, characteristic of some desert areas, are produced. Much of the water runs off before it can soak into the soil. A lot of moisture is also lost to evaporation. Many deserts lie in areas of high pressure systems where there is little cloud cover. At least 90 percent of the sun’s rays reaches Earth’s surface, producing seasonal hot temperatures. (For comparison, the surface of more humid lands, covered with more vegetation, receives only 40 percent of possible solar radiation.) The hot, dry air causes any available water to evaporate quickly. When temperatures are extremely hot, rain can evaporate before it reaches Earth.

The conditions producing high daytime temperatures reverse the process after sundown. Approximately 90 percent of the day’s accumulated heat radiates back toward the sky. In moister climates only about 50 percent of this heat is lost. These conditions produce the wide range of daily temperatures characteristic of deserts. This range is often 50 degrees or more. The rapid heating and cooling of air create another characteristic of most deserts: strong winds. These winds, circulating air that is often hot and dry, increase the already high rate of evaporation. Evaporation in American deserts ranges from 70 to 160 inches per year. A desert then is not so easily defined. All these characteristics: seasonal, high temperatures; low, sporadic rainfall; a high rate of evaporation; wide temperature ranges; and strong winds are part of the definition.