You can find tropical deserts in the continental interior of subtropical lands and in the midlatitudes on the side of mountain ranges sheltered from the wind. Major tropical deserts such as the African Sahara or the Mexican Sonoran are easy to remember, but tropical deserts exist all across the Middle East, Central and South America, central Australia and the southwestern United States.
Westerlies and trade winds blow away from the 30-degree latitude belt, causing light surface winds and intense heat that evaporates moisture in the air. Low-latitude or tropical deserts are located between 18 and 28 degrees, the areas centered on the Tropics of Capricorn and Cancer. Since the winds blow north above the equator and south below, the tropical areas seldom experience rain-producing air masses.
Continuous year-round subtropical high pressure is what defines the tropical desert. High pressure is created when cooling air molecules descend and become dense. In the interior tropical regions, the descending air dries out and its moisture does not condense into rain. For a desert located in rain shadow--on the lee-side of mountain ranges--the peaks block precipitation so all the rain falls before it gets to the other side, which causes a high level of aridity.
Tropical deserts have the highest temperature of any climate on the planet. In subtropical areas, the sun shines more than it does anywhere else and at a higher angle. Monthly temperatures do not drop below 64.4 degrees Fahrenheit and usually remain above 90 degrees. Daytime temperatures can reach 120 degrees Fahrenheit. Because of the constant high pressure, tropical deserts are under clear skies. Clear skies let in a lot of heat during the day, and let it all out again at night. Tropical deserts have a large diurnal temperature range: overnight temperatures can drop more than 60 degrees Fahrenheit.
The Earth's driest locations, tropical deserts actually evaporate more water than they receive each year. The yearly precipitation total is less than 1/10 of an inch each year. According to Michael E. Ritter at the University of Wisconsin at Stevens Point, "An entire year's worth of rain may fall in one downpour." The extremely low humidity, 10 percent or less, causes any surface water to evaporate quickly and the atmospheric stability inhibits any additional precipitation.
