Deserts can conveniently be classified into three kinds:
- High-latitude deserts. At high latitudes, temperatures are so low most of the year, and sources of atmospheric moisture are so far away, that rainfall is very low. Water present near the surface is frozen for most or all of the year. In a sense, the surface of the northern part of the Greenland Ice Sheet and the interior part of the Antarctic Ice Sheet are deserts!
- Mid-latitude deserts. In temperate latitudes, deserts form only where for one (or both) of two reasons rainfall is scanty over large areas. Rainfall can be low because an area is located in the interior of a large continent, far away from marine sources of atmospheric moisture, like the interior of Asia. Or rainfall can be low because mountain ranges located upwind extract most of the moisture from the air. Such deserts might be called rain-shadow deserts. The deserts of the southwestern United States are of this kind.
- Low-latitude deserts. In the subtropical high-pressure belts of both hemispheres (the so-called horse latitudes), the atmosphere undergoes widespread subsidence as a consequence of broad patterns of the general circulation of the atmosphere, so cloudiness and precipitation is uncommon. Most of the great deserts of the world, in North Africa and the Middle East, are of this kind.