In general, deserts are caused by the presence of dry air. The average humidity (moisture in the air) is between 10 and 20 percent. In some cases, mountain ranges prevent moisture-laden clouds from reaching the area. Mountains can also cause heavy, moisture-filled clouds to rise into the colder atmosphere. There, the moisture condenses and falls in the form of rain, leaving the air devoid of moisture as it crosses the range. In other cases, certain wind patterns along the equator bring air in from dry regions. In another scenario, cold-water ocean currents can cause moist air to drop its moisture over the ocean. The resulting dry air quickly evaporates (dries up) ground moisture along the coastal regions as it moves inland.

Deserts have always existed, even when glaciers covered large portions of the Earth during the great Ice Ages. Although geological evidence is scarce...