desert

The Negev Desert in Israel. Although superficially barren, arid regions such as the Negev Desert support a remarkable diversity of plants and animals, all of which have developed highly specialized ways of coping with the harsh conditions.

The sand dunes of the Namib Desert, which stretches 1,290 km/800 mi along the coast and covers one-third of Namibia. It is barren except for succulents that survive by absorbing moisture from the fog that often rolls in from the Atlantic Ocean.

Cacti are adapted for desert life in a number of ways. They have fleshy tissue in which to store water and their leaves are reduced to spines to lessen water loss by transpiration. Cactus roots are branching and shallow, adapted to gathering surface water from a large area.

The Arabian camel does not actually store water in its hump, although it can do so in the lining of its stomach. During protracted periods without water, the camel can lose up to 27% of its body weight without causing harm.

In spite of its arid and inhospitable appearance, Death Valley, California, is home to a variety of plants and animals, including the creosote bush and the desert holly; coyotes, foxes, rats, and rabbits all find a habitat in the region.

Arid area with sparse vegetation (or, in rare cases, almost no vegetation). Soils are poor, and many deserts include areas of shifting sands. Deserts can be either hot or cold. Almost 33% of the Earth's land surface is desert, and this proportion is increasing. Arid land is defined as receiving less than 250 mm/9.75 in rain per year.

The tropical desert belts of latitudes from 5° to 30° are caused by the descent of air that is heated over the warm land and therefore has lost its moisture. Other natural desert types are the continental deserts, such as the Gobi, that are too far from the sea to receive any moisture; rain-shadow deserts, such as California's Death Valley, that lie in the lee of mountain ranges, where the ascending air drops its rain only on the windward slopes; and coastal deserts, such as the Namib, where cold ocean currents cause local dry air masses to descend. Desert surfaces are usually rocky or gravelly, with only a small proportion being covered with sand (about 3%). Deserts can be created by changes in climate, or by the human-aided process of desertification.

Characteristics common to all deserts include irregular rainfall of less than 250 mm/9.75 in per year, very high evaporation rates of often 20 times the annual precipitation, and low relative humidity and cloud cover. Temperatures are more variable; tropical deserts have a big diurnal temperature range and very high daytime temperatures (58°C/136.4°F has been recorded at Azizia in Libya), whereas mid-latitude deserts have a wide annual range and much lower winter temperatures (in the Mongolian desert the mean temperature is below freezing point for half the year).

Desert soils are infertile, lacking in humus and generally grey or red in colour. The few plants capable of surviving such conditions are widely spaced, scrubby and often thorny. Long-rooted plants (phreatophytes) such as the date palm and musquite commonly grow along dry stream channels. Salt-loving plants (halophytes) such as saltbushes grow in areas of highly saline soils and near the edges of playas (dry saline lakes). Xerophytes are drought-resistant and survive by remaining leafless during the dry season or by reducing water losses with small waxy leaves. They frequently have shallow and widely branching root systems and store water during the wet season (for example, succulents and cacti with pulpy stems).