DESERT, a term used for those lands which produce insuffi cient vegetation to support a human population. Deserts are classified according to the causes which give rise to them. In "cold deserts" the absence of vegetation is consequent on the prevailing low temperature, while in "hot deserts" the causes are high temperatures and deficient rainfall. Cold deserts accordingly occur in high latitudes or high altitudes. Hot deserts are primarily found along the hot, dry tropical belts of high atmospheric pressure, and on their equatorial sides, but the zonal arrangement is considerably modified in some regions by the influence of ele vated land. The northern hemisphere provides examples of this type in the African Sahara, the Asiatic deserts of Arabia and Iran, and in the Great Basin of North America. The southern hemi sphere has the Kalahari in Africa, the deserts of Western Australia, and the Atacama in South America. Where a line of elevated land runs east and west in this zone as in Asia, the desert belt tends to occur eastwards in higher latitudes but where the line runs north and south, as in Africa, America and Australia, the desert zone weakens on the windward side of the elevation and the arid conditions are accentuated on the lee side. Desert conditions occasionally arise from somewhat obscure local causes, but the Indian desert (q.v.) seems to be situated in a region inaccessible to either of the two main branches of the wet south-west mon soon.
Although permanent rivers rising in wetter regions may tra verse deserts (e.g., the Nile) the fundamental physical condi tion of an arid area is that it contributes nothing directly to oceanic waters. The rainfall chiefly occurs during violent cloud bursts (q.v.) and the soluble matter in the soil is carried down by intermittent streams to salt lakes around which deposits of economic value are found on evaporation. Surface erosion is caused by rapid and extreme changes of temperature, while wind action often forms dunes resembling waves. Dry valleys with precipitous sides, and cirque-like heads are probably caused by occasional cloud-bursts. Natural springs in some deserts give rise to oases which make trans-desert routes possible. When a desert-river has low banks (e.g., the Nile) irrigation is made possible.
See Des Gesetz der Wiistenbildung by Walther, Berlin, 1900, for a general account of deserts.