Desert Zones of Tropical Belts

natural zones in the tropical belts of the northern and southern hemispheres whose dominant landscapes are deserts. These zones lie in the interior and along the western coasts of most continents. The largest tropical desert zones are found in Asia and North Africa, where they form a distinct east-west belt that includes the Sahara, the world’s largest desert, the Arabian Desert, and the deserts of India and Pakistan (Thar). In North America, tropical deserts occur in narrow strips extending from north to south along the west coast of Lower California and the western edge of the Mexican Meseta Central. In the southern hemisphere, tropical desert zones are clearly evident in Australia. They include the Great Sandy, the Great Victoria, and the Simpson deserts. In South Africa are found the interior tropical Kalahari Desert and the coastal Namib Desert. In South America, the tropical Atacama Desert extends along the Pacific coast.

Desert zones in tropical regions have a highly diverse relief, shaped by physical weathering and eolian accumulation. Land-forms include uplands (for example, Ahaggar and Tibesti in Africa), inselbergs (such as the Macdonnell and Petermann ranges in Australia), ancient alluvial plains (particularly in the Thar Desert), enormous accumulations of eolian sands (called ergs in the Sahara), numerous lake basins, both dry and filled with water, and structural plateaus.

Tropical desert zones have a hot and dry climate, especially in North Africa and on the Arabian Peninsula, where the mean July temperature reaches 35°-38°C in places and a maximum temperature of 50°C has been recorded. The sands heat up to 90°C in the daytime. Winters are mild and warm, with January temperatures of 20°-25°C, dropping to 7°C in places (soil frosts may occur). In oceanic sectors the annual temperature variations level out owing to precipitation (summer 19°-25°C; winter 12°-20°C). The annual precipitation usually does not exceed 50-100 mm, and some areas may receive no precipitation for several years.

The deserts of Australia, West Hindustan, South Africa, and North America (California) are more conducive to plant growth because the annual precipitation here usually exceeds 100 mm. The coastal Namib and Atacama deserts are extremely arid. In view of insignificant cloudiness, the annual solar radiation in tropical desert zones is the highest on earth, about 750-920 kilojoules per sq cm (180-200 kilocalories per sq cm).

The surface runoff is negligible. The rivers usually are not perennial, carrying water only during rains. Only such large through-flowing rivers as the Nile carry water throughout the year. The few lakes are usually saline. The main sources of water are groundwater and sometimes artesian water.

Soils in tropical desert zones are red-brown and tropically primitive. In low-lying places saline soils are common and salt crusts are widespread.

In the most arid deserts (Sahara, Arabian), vast gravelly, sandy, stony, and saline expanses are almost completely devoid of vegetation, which is concentrated along the intermittent streams and in the foothills of mountain ranges. The vegetation is much more abundant in the Thar and Namib deserts and in the interior of Australia, especially on the sandy stretches occupied by xerophilic shrub and semishrub associations. The vegetation is used for pasture.

The animal world is represented by only a few species. The most common ungulates are antelopes (gazelles), and predators include hyenas, jackals, and, in Australia, marsupial moles. There are many kinds of rodents (gerbils, jerboas) and reptiles (lizards, snakes). The fauna is most diverse on stabilized sands, and the fewest species are found in stony deserts.