A desert is an arid land area where more water is lost through evaporation than is gained from precipitation. Deserts include the familiar hot, dry desert of rock and sand that is almost barren of plants, the semiarid deserts of scattered trees, scrub, and grasses, coastal deserts, and the deserts on the polar ice caps of the Antarctic and Greenland.

Most desert regions are the result of large-scale climatic patterns. As the earth turns on its axis, large air swirls are produced. Hot air rising over the equator flows northward and southward. The air currents cool in the upper regions and descend as high pressure areas in two subtropical zones. North and south of these zones are two more areas of ascending air and low pressures. Still farther north and south are the two polar regions of descending air. As air rises, it cools and loses its moisture...