one of the natural geographic belts of the northern and southern hemispheres, located between the tropical and temperate belts. The subtropical belts (the subtropics) are characterized by periodic alternation of tropical and temperate climatic conditions caused by rhythmic seasonal shifts of general atmospheric circulation. In summer they are influenced by the trade winds, and in winter, by the cyclonic circulation of the temperate belt, with the exception of the eastern margins, which receive summer monsoonal precipitation. The average monthly temperature in summer is higher than 20°C, while in winter it is higher than 4°C; early and late frosts and minor freezes, occasionally as low as – 10°C, are possible as a result of intrusions of polar air. Within the land boundaries of the subtropical belts, the amount and regime of atmospheric precipitation vary greatly in the maritime regions and in the interior continental regions. This variation in precipitation, together with an increasingly continental climate in the interior, determines the substantial differences of landforms in natural zones.
Three main sectors are clearly distinguished on each of the continents in the subtropical belts: the western maritime, or mediterranean sector, which has increased humidity in winter; the continental sector, which has low humidity year-round; and the eastern maritime, or monsoonal sector, which has high humidity in summer. The western maritime sector (semiarid subtropics) has mediterranean sclerophyll forests and shrubs on cinnamon-colored soils. In the northern hemisphere sclerophyll forests and shrubs change in the southeast to subtropical steppes on graycinnamon-colored soils, and they, in turn, in the east give way to the arid subtropics, which are subtropical semideserts and deserts of the continental sector on gray-brown and gray soils. In the southern hemisphere subtropical steppes on gray-cinnamon-colored soils are characteristic of sectors in continental areas. In the subtropical belts in the eastern sectors (humid subtropics), evergreen forests predominate; there are deciduous broad-leaved forests with some evergreen species on yellow-brown soils, yellow soils, and red earths in the higher latitudes.
In the USSR the subtropics lie on the extreme northern boundary of the subtropical belt, and therefore the flora and fauna of the region are not typically subtropical. The subtropical belts are subdivided into four regions: humid, semihumid, mediterranean semiarid, and arid subtropics. The humid subtropics occupy the central and southern parts of the Black Sea coast of the Caucasus and the Colchis Lowlands. The semihumid subtropics, which have mainly arid summers, encompass the Lenkoran’ Lowland. The mediterranean semiarid subtropics include the northern part of the Black Sea coast of the Caucasus and the southern coast of the Crimea, and the arid subtropics encompass the Alazan’ River valley, the Kura-Araks Lowland in the Transcaucasus, and the southern margins of the deserts of Middle Asia. In the mountains the subtropical belts feature forest-meadows in humid regions and forest-steppe in arid regions. The forests of the subtropical belts have been to a great extent replaced by fields and large-scale commercial farms. The fauna is characterized by a mixture of species of the temperate and tropical belts.
The ocean within the subtropical belt has a high temperature (15°–16°C) and high salinity. The weak vertical mixing of ocean waters decreases the oxygen and plankton content, which results in an insignificant quantity of commercial fish.
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IU. K. EFREMOV and E. V. MILANOVA