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The Desert Environment
An area having an annual rainfall of 10 in. (25 cm) or less is considered to be a desert. Some deserts have no rain for intervals of several years. Deserts and semideserts exist in some regions having up to about 20 in. (50 cm) of rainfall where evaporation is very high and loss by runoff is great. The largest desert regions of the world lie between 20° and 30° north and south of the equator, either where mountains intercept the paths of the trade winds or where atmospheric high-pressure areas cause descending air currents and a lack of precipitation. Other factors contributing to the formation of deserts include the amount of sunshine, rate of evaporation of water, and range of temperature. Temperature ranges in deserts are often extreme.
Plants of the desert have leaves and stems adapted to lessen their loss of water, and individual plants are more widely spaced than those in more humid regions; their roots form a spreading network sometimes penetrating to 50 ft (15 m) underground. Among the animals living in deserts of North America are species of squirrels, mice, bats, foxes, rabbits, and deer; reptiles, e.g., the Arizona coral snake, species of rattlesnakes, the desert tortoise, and the horned toad, gila monster, and many other lizards; a number of birds, e.g., the cactus wren, the road runner, species of owls, sparrows, and hawks; and spiders, scorpions, termites, and beetles. See dune ; oasis .
The Deserts of the World
Europe is the only continent without deserts; there are, however, semiarid portions around the Black and Caspian seas, in parts of kraine andU the N Caucasus. In Asia a great desert, the Gobi , exists in the middle latitudes chiefly because of its remoteness from water. Also in central Asia are the Kara Kum and Kyzyl Kum deserts. Farther south there are desert areas in NW India and through S Pakistan, Afghanistan, Iran,Iraq, and Arabia; these are largely the result of their situation in a subtropical high-pressure belt and of the distribution of pressure areas that produce cold, dry winds in winter and hot, dry winds in summer.
The Sahara , the largest desert in the world, is in Africa. Second only to the Sahara in area is the desert region of central and W Australia, lying in a high-pressure belt and in the path of the trade winds (which lose much of their moisture on the windward slopes of the east-coast mountains). South America has deserts on the coast and interior of Chile and E of the Andes in Argentina and Patagonia. In North America, deserts are found from N Mexico northward through parts of the SW and W United States. Extreme desert conditions exist in the Mojave Desert , the Imperial Valley , and Death Valley . The northern plateau region of Mexico and the adjacent portions of Texas, Arizona, and New Mexico have less extreme desert conditions with a quite abundant growth of mesquite, greasewood, creosote bush, yucca, and various species of cactus . Middle-latitude deserts are found in parts of the Great Basin.
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