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 -20°C (-4°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 30° 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.