Deserts cover approximately one-third of Earth's surface. These extremely dry regions, which support sparse vegetation and few animals, are generally defined by the amount of precipitation they receive. Most people classify Earth's desert regions into three categories: extremely arid lands (those that can experience at least 12 consecutive months without rainfall), arid lands (those that receive less than 250 millimeters of annual precipitation), and semiarid lands (those that receive between 250 and 500 millimeters of precipitation annually). Extremely arid lands and arid lands are most often referred to as deserts while semiarid lands are typically classified as grasslands.
When people think of deserts, they tend to picture hot, barren, sand-covered lands located at or near the equator. In fact, latitude, longitude, and elevation do not determine where deserts form; they can be located anywhere on Earth that is susceptible to dry conditions. Some deserts are located near the North and South poles while others are located near the equator. The highest desert in the world, the Qaidam Depression, is located in the People's Republic of China. The Qaidam Depression is 2600 meters above sea level. Also located in the People's Republic of China is the Turpan Depression, a desert 150 meters below sea level. The Sahara Desert of Africa is the world's largest desert. It has experienced temperatures as high as 135°F.
About 20 percent of the world's deserts are sand-covered. These deserts are often characterized by dune formations that resemble waves in a sea. Other deserts are comprised of exposed bedrock, soil, and sparsely vegetated plains. Many deserts feature oases, which are small areas that contain relatively high water content. Springs, wells, or other underground water sources often provide this water. An extensive root system and the ability to store water in leaves, roots, and stems enable desert plants to adapt to drought conditions. Occasional rainstorms can produce heavy amounts of precipitation during short periods of time. In fact, death by drowning in a desert is more common than death by dehydration.