Physical Geography and Rainfall
Deserts are places that get less than 25 centimeters (10 inches) of rain a year. Why do such rain-free areas exist? For a variety of reasons, but mostly because of the wind patterns in the tropics.
Deserts aren't always hot. In the Dry Valleys of Antarctica (above), average annual temperature is -20o C (-4o F) and precipitation can be as low as 10 millimeters (0.4 inches) a year. These deserts are created by cold winds tumbling off the Antarctic plateau and roaring down the valleys.
Why are deserts where they are?
Sahara and Australian deserts: Many deserts are located at about 30o latitude north or south of the Equator. In this region, the Hadley circulation causes dry air to descend from the upper atmosphere, creating regions of little rain.
Patagonia Desert: Moist air from the Pacific drops its moisture as it crosses the Andes mountains, creating the deserts of Patagonia east of the Andes.
Atacama Desert: On some coasts, upwelling of cold water chills the air and makes prevailing winds cold and dry. In parts of Chile's Atacama Desert, no rainfall has ever been measured.