Desert

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Not to be confused with dessert.

This article is about dry terrain. For arid climate, see Desert climate. For the act of abandoning or withdrawing support, see Desertion. For other uses, see Desert (disambiguation).

Valle de la Luna ("Valley of the Moon") in the Atacama Desert of Chile, the world's driest hot desert

Sand dunes in the Rub' al Khali ("Empty quarter") of Saudi Arabia

A desert is a barren area of land where little precipitation occurs and consequently living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one third of the land surface of the world is arid or semi-arid. This includes much of the polar regions where little precipitation occurs and which are sometimes called polar deserts or "cold deserts". Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location.               
                                                                                       
                                                                                        

Etymology

English desert and its Romance cognates (including Italian and Portuguese deserto, French désert and Spanish desierto) all come from the ecclesiastical Latin dēsertum (originally "an abandoned place"), a participle of dēserere, "to abandon".The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies; thus the use of the word desert can cause confusion. In English before 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).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.

Physical geography

A desert is a region of land that is very dry because it receives low amounts of precipitation (usually in the form of rain but may be snow, mist or fog), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside the area.Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation. Deserts generally receive less than 250 mm (10 in) of precipitation each year.Semideserts are regions which receive between 250 and 500 mm (10 and 20 in) and when clad in grass, these are known as steppes.

Classification

Deserts have been defined and classified in a number of ways, generally combining total precipitation, number of days on which this falls, temperature, and humidity, and sometimes additional factors.For example, Phoenix, Arizona, receives less than 250 mm (9.8 in) of precipitation per year, and is immediately recognized as being located in a desert because of its aridity-adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 mm (9.8 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 Himalayasand other high-altitude areas in other parts of the world.Polar deserts cover much of the ice-free areas of the Arctic and Antarctic.A non-technical definition is that deserts are those parts of the Earth's surface that have insufficient vegetation cover to support a human population.

Weathering processes

Exfoliation of weathering rocks in Texas

Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures falling sharply at night. The diurnal range may be as much as 20 to 30 °C (36 to 54 °F) and the rock surface experiences even greater temperature differentials.During the day the sky is usually clear and most of the sun's radiation reaches the ground, but as soon as the sun sets, the desert cools quickly by radiating heat into space. In hot deserts, the temperature during daytime can exceed 45 °C (113 °F) in summer and plunge below freezing point at night during winter.

Dust storms and sandstorms

Main article: Dust storm

Dust storm about to engulf a military camp in Iraq, 2005

Sand and dust storms are natural events that occur in arid regions where the land is not protected by a covering of vegetation. Dust storms usually start in desert margins rather than the deserts themselves where the finer materials have already been blown away. As a steady wind begins to blow, fine particles lying on the exposed ground begin to vibrate. At greater wind speeds, some particles are lifted into the air stream.