Desert biomes cover about one fifth of the Earth's surface and are defined to occur where rainfall is less than 50 centimeters per year. Although most deserts, such as the Sahara of North Africa and the deserts of the southwestern USA, Mexico, and Australia, occur at low latitudes, another kind of desert, cold deserts, occur in the basin and range area of Utah and Nevada and in parts of western Asia. Most deserts have a considerable amount of specialized vegetation, as well as specialized vertebrate and invertebrate animals. Soils often have abundant nutrients because they need only water to become very productive and have little or no organic matter. Disturbances are common in the form of occasional fires or cold weather, and sudden, infrequent, but intense rains that cause flooding.