Title: Desert Content: thumb|300px|Sand and dunes of the Libyan desert A desert is an arid (dry) biome. They get less than of rainfall a year. A source defines it as "any region that can have a moisture deficit over the course of a year. In other words, they can have less rainfall in a year than they give up through evaporation".Universe TodayMarshak 2009. Essentials of Geology, 3rd ed. W.W. Norton. p452. ISBN 978-0-393-19656-6 Including the semi-arid regions, deserts may cover about 33% of the land on Earth. That includes much of Antarctica, where large areas get no snow at all. The largest hot desert is the Sahara desert, in northern Africa, covering nine million square kilometres. Deserts land surfaces are varied. Examples are stones, sand dunes and snow. They may have some animals and plants. Deserts sometimes expand (desertification), and sometimes contract. Deserts are mostly in the western part of the Americas, Western Asia, Central and western Australia, Namibia and North Africa. Some, such as the Sahara, are very hot during the day and have cold nights, but there are also cold deserts such as the Atacama in South America which are frozen day and night. == Cold deserts == Cold deserts can be found close to the poles. That is why they are also called polar deserts. Other regions of the world have cold deserts too, for instance high altitude areas like the Himalayas. These are called montane deserts. Antarctica is the world's largest cold desert. The second largest one is the Arctic desert, but that is warming up a bit now. == Hot deserts == Hot deserts are mostly in the subtropics. They can be covered by sand, rock, salt lakes, stony hills and even mountains. Most non-polar deserts are hot in the day and chilly at night. The temperature in the daytime can reach or higher in the summer, and dip to or lower at night time in the winter. The largest hot desert in the world is the Sahara in North Africa. It is almost as large as Europe or the United States. == Atacama == The Atacama desert may not have had any significant rainfall from 1570 to 1971. thumb|Feral donkey in the Atacama desert It may be the oldest desert on Earth. It has been a desert for at least three million years. It is the oldest continuously arid region on Earth. == Rain == It does rain in the desert, but not often. One place in the Atacama Desert had no rain for 401 years. In other deserts it may rain every year or once every few years. Rains in a desert may bring a great amount of water to the ground in a short time. Some rain passes straight into the dry soil, but the rest may form a temporary river. Wadis, stream channels that are normally dry, can quickly fill after heavy rain, causing a flash flood. People sometimes bring water from wet places to hot deserts so plants can grow. This is called irrigation. == Sandstorm == A sandstorm or dust storm arises when wind blows loose sand and dust from a dry surface. Clouds of sand or dust are often so dense that they obscure the sun. A sandstorm can move whole sand dunes. Sandstorms are common in large, dusty deserts. == Animals and plants == There are not many animals in the desert, but some animals are able to survive. They have different ways to survive the intense conditions of the desert. Examples of animals that live in hot deserts are lizards, small rodents, snakes, and camels. Plants and animals in hot deserts must live with very little water. Xerophytic plants which live in the desert have special adaptations. They may survive by growing roots that are very near the surface to absorb the rain that may fall before it evaporates. Plants such as the cactus have thick, fleshy stems that help them store water. Small animals such as lizards and small rodents often escape the hot rays by digging underground burrows where they live. They only come out at night to search for food. Like the plants, desert animals must live on as little water as possible. Most of the water used by these animals comes from seeds and stems that absorb and hold water. Camels survive in hot deserts by storing water in body fat in their humps. Like other desert animals, the camel loses little water in its wastes (urine and feces). == Biggest deserts == # Antarctica: 14,200,000 sq kms # Arctic: 13,900,000 sq kms # Sahara: 9,200,000 sq kms # Australia: 2,700,000 sq kms # Arabia: 2,330,000 sq kms # Gobi: 1,298,000 sq kms