A desert is a landscape or region of land that is very dry because of low rainfall amounts. Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation. Desert plants must have special adaptations to survive with this little water. Deserts generally receive less than 25 cm of rain each year.
Deserts are part of a wide classification of regions that, on an average annual basis, have a moisture deficit. Measurement of rainfall alone cannot provide an accurate definition of what a desert is because being arid also depends on evaporation, which depends in part on temperature. For example, Phoenix, Arizona receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters (10 in) of precipitation per year and is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high altitude areas in other parts of the world. Polar deserts cover much of the ice free areas of the arctic and Antarctic. An alternative definition describes deserts as parts of earth that don't have a sufficient vegetation cover to support human population.
Deserts are sometimes classified as "hot" and "cold" deserts. Cold deserts can be covered in snow or ice; frozen water unavailable to plant life. These are more commonly referred to as tundra if a short season of above-freezing temperatures is experienced, or as an ice cap if the temperature remains below freezing year-round, rendering the land almost completely lifeless.
In some parts of the world, deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture. Deserts are also classified by their geographical location and dominant weather pattern as trade wind, mid-latitude, rain shadow, coastal, monsoon or polar deserts.
Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalayas, especially in Ladakh region of Jammu and Kashmir, in parts of the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (10,000 ft) and the thermal regime can be hemiboreal. These places owe their profound aridity to being very far from the nearest available sources of moisture. Montane deserts are normally cold. Deserts at higher altitudes are sometimes referred to as "high deserts", particularly in the American Midwest.
Rain shadow deserts form when tall mountain ranges block clouds from reaching areas in the direction the wind is going. As the air moves over the mountains, it cools and moisture condenses, causing precipitation on the windward side. When that air reaches the leeward side, it is dry because it has lost the majority of its moisture, resulting in a desert. The air then warms, expands and blows across the desert. The warm, desiccated air takes with it any remaining moisture in the desert.
In English prior to the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation)—and a desert may be quite heavily populated, with millions of inhabitants. Phrases such as "desert island" and "Great American Desert" in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population. However, the connotation of a hot, parched and sandy place often influences today's popular interpretation of those phrases.
Deserts take up about one third (33%) of the Earth's land surface. Hot deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures, and low nighttime temperatures. In hot deserts the temperature in the daytime can reach 45 °C/113 °F or higher in the summer, and dip to 0 °C/32 °F or lower at nighttime in the winter. Water vapor in the atmosphere acts to trap long wave infrared radiation from the ground, and dry desert air is incapable of blocking sunlight during the day or trapping heat during the night. Thus, during daylight most of the sun's heat reaches the ground, and as soon as the sun sets the desert cools quickly by radiating its heat into space. Urban areas in deserts lack large daily temperature variations, partially due to the urban heat island effect.
Many deserts are formed by rain shadows; mountains blocking the path of precipitation to the desert. Deserts are often composed of sand and rocky surfaces. Sand dunes called ergs and stony surfaces called hamada surfaces compose a minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation. The soil is rocky because of the low chemical weathering, and the relative absence of a humus fraction.
Bottomlands may be salt-covered flats. Eolian processes are major factors in shaping desert landscapes. Polar deserts have similar features, except the main form of precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98% thick continental ice sheet and 2% barren rock). Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that evaporate even ice.
The largest hot desert is the Sahara in northern Africa, covering nine million square kilometres and twelve countries. Deserts sometimes contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Due to extreme and consistent dryness, some deserts are ideal places for natural preservation of artifacts and fossils.
The ten largest deserts (Antarctic not included)
Rank Desert Area (km²) Area (mi²)
1 Antarctic Desert (Antarctica) 13,829,430 5,339,573
2 Arctic Desert (Arctic) 13,726,937 5,300,000
3 Sahara Desert (Africa) 9,100,000+ 3,320,000+
4 Arabian Desert (Middle East) 2,330,000 900,000
5 Gobi Desert (Asia) 1,300,000 500,000
6 Kalahari Desert (Africa) 900,000 360,000
7 Patagonian Desert(South America) 670,000 260,000
8 Great Victoria Desert (Australia) 647,000 250,000
9 Syrian Desert (Middle East) 520,000 200,000
10 Great Basin Desert (North America) 492,000 190,000