The Nature and Locations of Deserts

What Is a Desert? 

Formally defined, a desert is a region that is so arid (dry) that it supports vegetation on no more than 15% of its surface. In general, desert conditions exist where less than 25 cm of rain falls per year, on average. Because of the lack of water, deserts contain no permanent streams, except for those that bring water in from temperate regions elsewhere.
Note that the definition of a desert depends on a region’s aridity, not on its temperature. Geologists, therefore, distinguish between cold deserts, where temperatures generally stay below about 20C for the year, and hot deserts, where summer daytime temperatures exceed 35C. Cold deserts exist at high latitudes where the Sun’s rays strike the Earth obliquely and thus don’t provide much energy, at high elevations where the air is too thin to hold much heat, or in lands adjacent to cold oceans, where the cold water absorbs heat from the air above. Hot deserts develop at low latitudes where the Sun’s rays strike the desert at a high angle, at low elevations where dense air can hold a lot of heat, and in regions distant from the cooling effect of cold ocean currents. The hottest recorded temperatures on Earth occur in low-latitude, low-elevation  deserts 58C (136F) in Libya and 57C (133F) in Death Valley, California.

Types of Deserts 

Each desert on Earth has unique characteristics of landscape and vegetation that distinguish it from others. Geologists group deserts into five different classes, based on the environment in which the desert forms (figure below). 
Subtropical deserts form because the air that convectively flows downward in the subtropics warms and absorbs water as it sinks.
  • Subtropical deserts: Subtropical deserts (such as the Sahara, Arabian, Kalahari, and Australian) form because of the regional  pattern of air circulation in the atmosphere. At the equator, the air becomes warm and humid, for sunlight is intense and water rapidly evaporates from the ocean. The hot, moisture-laden air rises to great heights above the equator. As this air rises, it expands and cools, and can no longer hold so much moisture. Water condenses and falls in downpours that feed the lushness of the equatorial rain forest. The now-dry air high in the troposphere spreads laterally north or south. When this air reaches latitudes of 20 to 30 C, a region called the subtropics, it has become cold and dense enough to sink. Because the air is dry, no clouds form, and intense solar radiation strikes the Earth’s surface. The sinking, dry air becomes denser and heats up, soaking up any moisture present. In the regions swept by this hot air on its journey back to the equator, evaporation rates greatly exceed rainfall rates, so the land becomes parched. 
  • Deserts formed in rain shadows: As air flows over the sea toward a coastal mountain range, the air must rise (Fig. 17.3). As the air rises, it expands and cools. The water it contains condenses and falls as rain on the seaward flank of the mountains, nourishing a coastal rain forest. When the air finally reaches the inland side of the mountains, it has lost all its moisture and can no longer provide rain. As a consequence, a rain shadow forms, and the land beneath the rain shadow becomes a desert. A rain-shadow desert can be found east of the Cascade Mountains in the state of Washington.
 The formation of a rain-shadow desert. Moist air rises and drops rain on the coastal side of the range. By the time the air has crossed the mountains, it is dry. 
  • Coastal deserts formed along cold ocean currents: Cold ocean water cools the overlying air by absorbing heat, thereby decreasing the capacity of the air to hold moisture. For  example, the cold Humboldt Current, which carries water northward from Antarctica to the western coast of South America, cools the air that blows east, over the coast. The air is so dry when it reaches the coast that rain rarely falls on the coastal areas of Chile and Peru. As a result, this region hosts a desert landscape, including one of the driest deserts in the world, the Atacama (figure below a, b). Portions of this narrow desert received no rain at all between 1570 and 1971. 
  • Deserts formed in the interiors of continents: As air masses move across a continent, they lose moisture by dropping rain, even in the absence of a coastal mountain range. Thus, when an air mass reaches the interior of a broad continent, it has become so dry that the land beneath becomes arid. The largest present-day example of such a continental-interior desert, the Gobi, lies in central Asia. 
  • Deserts of the polar regions: So little precipitation falls in Earth’s polar regions (north of the Arctic Circle and south of the Antarctic Circle) that these areas are, in fact, arid. Polar regions are dry, in part, for the same reason that the subtropics are dry (the global pattern of air circulation means that the air flowing over these regions is dry), and in part, for the same reason that coastal areas along cold currents are dry (cold air holds little moisture).
 The formation of a coastal desert.
Different regions of the land surface have become deserts at different times in the Earth’s history, because plate movements change the latitude of landmasses, the position of landmasses relative to the coast, and the proximity of landmasses to a mountain range. Because of plate tectonics, some regions that were deserts in the past are temperate or tropical regions now, and vice versa.
Figures credited to Stephen Marshak.
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