Desert

A desert is a place that gets very little rain. Scientists call an area a desert if it gets less than 10 inches of rain in a whole year. Some deserts get less than one inch. Deserts cover about one-third of all the land on Earth. They are found on every continent.

Most people picture a desert as a sea of sand under a hot sun. Some deserts do look like that. The Sahara in Africa has dunes that can rise as tall as a 60-story building. But many deserts are rocky, not sandy. Others are cold. The Gobi Desert in Asia gets snow in winter. Antarctica is also a desert, even though it is covered in ice, because almost no water falls from the sky.

Deserts form for a few different reasons. Some sit in places where dry air sinks down from high in the atmosphere. The Sahara and the deserts of Australia formed this way. Others sit in the "rain shadow" of a mountain range. Wind pushes wet air up the mountain, the air drops its rain on one side, and dry air slides down the other. Death Valley in California is a rain shadow desert.

Living in a desert is hard, but plants and animals have found clever ways. A cactus stores water in its thick stem and grows a waxy skin to keep that water from leaking out. Its leaves have shrunk into spines, which lose less water than flat leaves and also keep animals from biting. Camels can drink 30 gallons of water in about 10 minutes and then go for days without drinking again. Many small desert animals, like kangaroo rats and fennec foxes, hide underground during the day and come out at night when the air is cooler.

Deserts can swing between extreme temperatures. The sand in the Sahara can pass 130 degrees Fahrenheit during the day. The same spot can drop near freezing at night. Sand and rock do not hold heat the way water and plants do, so the warmth escapes fast after sunset.

Deserts are also growing. When forests are cut down or farmland is overused, dry land can spread into places that used to be green. Scientists call this desertification. It is happening now along the southern edge of the Sahara and in parts of China, and it is one of the harder problems facing people who live near deserts today.