All deserts have two things in common: they are dry, and they support little plant and animal life. If a region receives an average of less than 10 inches (25 centimeters) of rain each year, scientists classify it as a desert. Contrary to what most people believe, all deserts are not hot. Some ice deserts near the North and South Poles are so cold that all moisture is frozen. This chapter will discuss deserts in tropical and temperate areas. (Tropical areas are those near the equator. Temperate areas are between the equator and the North and South Poles).

True deserts cover about 14 percent of the world's land area, or about 8,000,000 square miles (20,800,000 square kilometers). Another 15 percent of the Earth's land...