DESERTS & GRASSLANDS: Deserts cover about one-third of the Earth's surface and occur where rainfall is less than 20 inches per year. Although most deserts, such as the Sahara of North Africa and the deserts of the southwestern U.S., Mexico, and Australia, occur at low latitudes, another kind of desert, cold deserts, occur in the basin and range area of Oregon, Utah and Nevada and in parts of western Asia.