Tropical Belt

one of two geographic belts of the earth, located in the northern hemisphere and the southern hemisphere between the subtropical and subequatorial belts.

The tropical belts are characterized by the predominance of desert and semidesert landscapes on the continents and of high water temperatures and high salinity in the ocean. Anticyclonic atmospheric circulation, a low relative humidity, and exceptionally little cloud cover over most of the area are typical. As a result, the values of total insolation are the highest on earth: 140–220 kilocalories per square centimeter per year (kcal/cm²-yr). At the same time, the radiation balance is relatively low (60–70 kcal/cm²-yr), owing to a large radiational heat loss, and the amplitudes of mean daily, mean monthly, and extreme temperatures are large. In lowlands the mean temperatures in the hottest months are 30–35°C, and the mean temperature in the coldest months is not less than 10°C. Extreme temperatures range from 61°C (the highest on earth) to 0°C and lower. Precipitation is 50–200 mm per year; those parts of the eastern littoral monsoon sector with favorable orographic conditions receive 1,000–2,000 mm of precipitation each year.

In the Soviet classification, the following four sectors are identified in the tropical belts on the continents: the eastern littoral sector, which is humid and has predominantly forest landscapes; the eastern transitional sector, which has open woodland and scrub landscapes; and the intracontinental and western littoral sectors, where desert and semidesert landscapes predominate. The western littoral sector, which is narrow and poorly expressed, is characterized by a high relative humidity, frequent fogs, and a regular temperature variation.

On the continents the character of natural processes in the tropical belts changes as one moves from east to west. The runoff decreases from 100 mm to a few millimeters, and the water content of rivers diminishes; that is, rivers are always flowing in the east but intermittent further west. In the intracontinental and western littoral sectors the runoff is sporadic; that is, the permanent rivers are allogenic. In the east, chemical weathering and erosional processes predominate in the tropical belts, but physical weathering and deflation are dominant in the interior and the west. Toward the west, the thickness of the soils decreases, the formation of allitic material is reduced, and the soil series changes from podzolized lateritic soils through the red soils of alternately moist and dry forests to the brownish red and red-brown soils of open woodlands and savannahs. In the intracontinental sectors and in the west, primitive desert soils—such as light carbonate soils, gypsites with developed crusts, and solonchaks—alternate with wind-blown and semifixed sands and accumulations of detritus. As one moves from east to west, the vegetation also changes, from mixed deciduous-evergreen forests through deciduous monsoon forests, open woodlands or savannahs, xeric forests, and thickets of xerophylous scrub to semidesert and desert vegetation. A corresponding change occurs in the composition of the animal population, and the number of animals decreases; the deserts are much more sparsely inhabited than the forests.

Within the tropical belts on land, from east to west, the following zones are identified: moist tropical forests, tropical open woodlands, xeric forests and savannahs, tropical semideserts, and tropical deserts. In the mountains of the tropical belts the forest-meadow (in the east), open woodland-steppe, desert-steppe, or desert-scrub-desert (in the central and western parts) altitudinal zonality exists.

The oceanic regions of the tropical belts are characterized by a high radiation balance (100 kcal/cm²-yr) and intense evaporation, which raises the salinity of the water to 37 parts per thousand; owing to the weak vertical circulation, these waters are poor in oxygen and plankton. The marine organisms are diverse but not numerous; the zooplankton content is 25 mg/cm³. At water temperatures from 18.5° to 23°C reef-forming corals develop at depths down to 30–45 m.

Except for the eastern parts of the continents, the tropical belts have not been heavily cultivated and settled. Extensive livestock grazing and irrigation farming are carried out in the intracontinental and western littoral sectors. In the eastern littoral sector, especially in Asia, there is irrigation and terrace farming, plantation farming, and logging. As a result of these activities the natural landscapes have been almost completely altered by man.