Desert (literally, 'a solitary place') is the geographical name applied to a barren and uninhabitable portion of the earth's surface. Four classes of desert may be distinguished.
(1) Ice-wastes occupy the central plateau of Greenland, the islands of the Arctic Sea, and probably the entire Antarctic continent. The ground is covered to an unknown depth by a vast ice-cap, rough and hummocky on the surface, and moving outwards to the coasts in a continual procession of glaciers.
(2) Tundras are flat plains, little elevated above sea-level, fringing the Arctic shores of the northern continents, and specially characteristic in Siberia (q.v.). The soil remains frozen to a great depth all the year round, thawing superficially when the snow melts in summer, and becoming covered with coarse moss and dwarfed Arctic plants.
(3) Arid wastes, or deserts in the popular sense of the word, occur mainly in two zones encircling the world, and corresponding to regions of minimum rainfall (see CLIMATE). The more extensive extends from near the equator in an east-north-east direction across the whole breadth of North Africa, as the Great Sahara, Libyan and Nubian deserts, over the peninsula of Arabia, through Persia, Turkestan, the Gobi or Shamo Desert, in about 52° N. lat., to the Pacific Ocean (see ASIA). The great Indian Desert in the Punjab is the only extension of this belt south of the Himalayas. The ring is completed by the Great Basin of North America in 40° N. lat. The southern zone, less complete, comprises the Kalahari Desert in south-west Africa, the interior of Australia, and small districts in the Argentine Republic and in the Andes. Deserts occur at all elevations, from considerable depths beneath sea-level to many thousand feet above it, and with all varieties of surface, from a flat expanse of sand, where the view for days of travel is bounded by a sharp circle as at sea, to rocky mountains rent by rough defiles all bare and chiselled by the driving sand. The essential character of an arid waste is its rainlessness, and the scarcity of water on the surface and of water-vapour in the atmosphere. Radiation in the clear air is intense, and desert climate is consequently of an exaggerated continental type. The sand in the Sahara becomes heated to over 150° F. during the day, and chilled below the freezing-point at night, while the diurnal and seasonal extremes in the lofty deserts of Central Asia are much greater. Thus desert-regions are most effective in producing land and sea breezes and monsoon winds in consequence of the marked periodical changes in atmospheric density. Another effect is the Mirage (q.v.), a phenomenon which, combined with the indescribable horror of loneliness that oppresses the occasional traveller, probably accounts for the widespread superstitions peopling deserts, above all other places, with evil and malicious spirits. The dreaded sand-storm or simoom is a kind of tornado or whirlwind which raises the sand in tall rotating columns sweeping over the surface with tremendous velocity. Sand-dunes, sometimes several hundred feet in height, are raised by steady winds, and gradually shift their position, extending the bounds of the desert to leeward. Desert vegetation is extremely scanty, consisting mainly of hard prickly plants of the cactus, euphorbia, and spinifex kinds, whose glazed surface exhales little of the hardly-won moisture. Animal life is correspondingly restricted both in variety and number of individuals. The Camel (q.v.) is par excellence the beast of burden for conducting traffic across arid wastes. When an overflowing river, such as the Nile, traverses a desert, the land becomes richly fertile in its immediate neighbourhood, and wherever a spring bubbles up through the sand there is an Oasis (q.v.) bearing palm-trees and grass. Artificial irrigation, especially the sinking of artesian wells, has done much to reclaim tracts of desert for agriculture in the Sahara (see ALGERIA), and to a less extent in Australia.
Geological considerations show that arid deserts are not permanent features of the earth's surface. The most level expanses have once formed part of the ocean-bed, or at least great inland seas. The orographical changes which cut off these seas and created inland drainage areas probably at the same time modified the rainfall of the locality. Excessive evaporation dried up the great lakes, leaving at present a series of diminishing salt lakes without outlet, receiving rivers which dwindle down by evaporation as they flow. The only commercial commodities yielded by deserts are the salts (common salt, borax, sodium carbonate, and sometimes sodium nitrate) left in the dried-up lake-beds. These salt lakes are subject to alternate long periods of desiccation and flooding; during the former the area of the desert extends, during the latter it contracts. These periods have been traced out in the case of the Great American Basin by a series of most interesting researches on the part of the United States Geological Survey.
(4) Temporary deserts, or steppe-lands, border the Asiatic deserts to the north and west. The saline steppes of the Caspian are true arid wastes; but the typical steppes in South Russia are luxuriantly clothed with verdure and flowers in spring. In summer they form a dusty plain of withered herbage, and in winter are wind-swept wastes of snow. The grassy llanos of the Orinoco present a similar desert appearance in the dry season; in the wet season they are inundated marshes which burst into blade and bloom as the water subsides. The pampas of the Plate river-system under similar but less marked seasonal changes form a natural transition to the fertile prairie lands which, although naturally treeless, are always richly grassed.