Physical Geographic Belts
Physical Geographic Belts
the largest zonal subdivisions of the “geographic shell.” The distinctive features of a physical geographic belt include heat and moisture conditions, air masses, and the characteristics of the air masses’ circulation. Consequently, biogeochemical and geomorphological processes, growth of vegetation, migration of animals, the cycle of matter, and so on are expressed in a distinctive form and rhythm in each belt.
Depending on climatic factors, particularly the relationship between heat and moisture from season to season, sectors and geographic zones are identified within each physical geographic belt. A succession of belts, sectors, and latitudinal and vertical zones is found from the equator to the poles and from the bases to the peaks of mountains—in connection with the decrease in heat. Such a succession is also exhibited from permanent areas of low atmospheric pressure with frontal-cyclonic activity and abundant precipitation to areas of high atmospheric pressure, where anticyclonic weather conditions and arid landscapes predominate.
Heat and moisture can be compared by means of various indexes of temperature and precipitation or indexes of the radiation balance and coefficient of moisture (or dryness), which is derived from the ratio of the quantity of heat and the mean annual precipitation. For identifying the hydrothermal conditions of landscape differentiation, it is preferable to use the ratio of the total (productive) moisture W and the radiation balance R, where W is in mm per year and R is in kilocalories per square centimeter per year (kcal/cm2-yr). When the hydrothermal coefficient W/R is greater than 10, forest landscapes develop; when it is less than 7, grass and shrub landscapes are found; and when it is in the range from 7 to 10, transitional landscapes occur (see Table 1). This rule is observed in all physical geographic belts except the polar, subarctic, and subantarctic belts, where the acute lack of heat (R is less than 20 kcal/cm2-yr) is of greater importance than differences in moisture.
|Table 1. Hydrothermal coefficients of the principal landscape types (according to A. M. Riabchikov)|
|Hydrothermal coefficient (W/R)||Principal zonal landscape types|
|Less than 2||Deserts|
|4–7||Dry savannas, steppes, subtropical and tropical scrub|
|7–10||Savannas, prairies, forest steppes, open tropical woodlands|
|10–13||Taiga, mixed and broad-leaved forests, subequatorial and tropical monsoon forests, forest savannas, subtropical forests with summer and winter moisture|
|13–20||Tropical rain forests (and equatorial swamps), permanently moist forests, semitropical rain forests, forest tundras, polar deserts|
Scientists have identified several belts on land: one equatorial, two subequatorial, two tropical, two subtropical, two temperate, two subpolar (subarctic and subantarctic), and two polar (arctic and antarctic).
Similar geographic belts can be traced in the World Ocean, although they are usually less clearly expressed there owing to the mobility of the water mass. Their position is determined by heat, evaporation, cloud cover, and the salinity and density of the water, which are basically functions of the radiation balance. Other determining factors are the prevailing winds and sea currents; the vertical water circulation; and the content of oxygen, plankton, and higher organisms in the water. These conditions usually change gradually with latitude, but the sea currents, in conformity with the Coriolis force and the shore outlines, go beyond the belts of prevailing winds and exert a considerable influence on the characteristics of other belts. Natural boundaries are therefore more important for determining the limits of geographic belts in the ocean. Such natural boundaries are the lines of convergence of the principal water masses, the edges of perennial (in the summer) and seasonal (in the winter) ice in the polar regions, and the latitudinal axes of centers of high and low pressure.
Equatorial belt. The equatorial land belt is characterized by a high radiation balance—from 70 to 90 kcal/cm2-yr. The mean monthly air temperature at sea level varies from 24° to 27°C. The gross moisture supply—that is, precipitation minus surface runoff—is 1,400 mm/yr, and the hydrothermal coefficient reaches 20. There is no seasonal rhythm. The drainage network is dense, and rivers are always at a high stage. The groundwater is ultrafresh and lies close to the day surface. Bi-ogeochemical processes are exceptionally intense throughout the year. As a result, a thick zone of weathering and special types of soils and vegetation are formed. The zone of tropical rain forests (gileias) is characterized by a rich flora and a high production of phytomass, reaching 40–50 tons (t) of dry matter per hectare (ha). Dead vegetable matter is completely mineralized. Red-yellow ferralitic and boggy soils develop. In the animal population, herbivorous forms predominate; the number of carnivores is limited, and many birds, reptiles, and insects are found.
In the oceanic regions of the equatorial belt the radiation balance averages 115 kcal/cm2-yr. Equatorial air prevails, and cloud cover is substantial. Convective and frontal rains are abundant—precipitation is more than 2,000 mm/yr. The mean monthly air temperature is 28 °C. The humid air makes evaporation more difficult. The water temperature reaches 27° or 28°C. The salinity of surface waters is about 34 per thousand and is slightly less than the average ocean salinity, which is about 35 per thousand. In the zone of trade-wind convergence, where air is transported from the east, a compensatory westerly counter-current arises in the equatorial belt. Water turbulence is considerable at the boundaries with the tropical belts, a circumstance that causes the water to be rich in oxygen. The fauna is exceptionally varied, with nearly 40,000 species, and quantitatively rich (more than 100 mg/m3 of zooplankton are found). Coral structures develop in the equatorial, subequatorial, and tropical belts, where the water temperature is higher than 18.5°C at depths of 30–45 m.
Subequatorial belts. In the subequatorial land belts a seasonal alternation occurs of dry tropical (trade-wind) and humid equatorial (monsoon) air at constantly high temperatures. The radiation balance is 70–75 kcal/cm2-yr and reaches 80 in the coastal areas. Rivers are at a high level in season, slope runoff is intensive, and a great deal of erosion occurs.
With a hydrothermal coefficient of 10, forest savannas and tall-grass savannas develop on red alferritic soils. For coefficients of 10–13, monsoon forests develop on ferralitic soils. With coefficients of 7–10, typical savannas are found on red-brown soils. Where the coefficient is less than 7, dry savannas develop on reddish brown soils. In the monsoon forests the annual production of vegetation is 20–35 t/ha of dry matter; in the typical savannas it is 12 t/ha, and in the dry savannas it is 5–7 t/ha. Today, more than one-half of the area of the monsoon forests and savannas has been brought under cultivation or built up. The most typical members of the animal population are the ruminant artiodactyls, cornivores, rodents, termites, dipterans, and hymenopterans.
In the oceanic regions of the subequatorial belts the radiation balance averages 120 kcal/cm2-yr and in places reaches 140. The steady trade winds cause easterly equatorial currents. In the western parts of the oceans the equatorial belt of air and water convergence in the northern hemisphere is broken up during the summer. The southern trade winds and South Equatorial Current cross over into the northern hemisphere; they warm it and intensify the monsoons. The mean water temperature is 25°C. The weak vertical mixing of the water results in a deficiency of oxygen and a low plankton content (50–70 mg/m3 of zooplankton). Water salinity is close to normal. When the equatorial belt is disrupted and the trade winds of the two hemispheres converge, an intertropical front with cyclonic activity and gales arises.
Tropical belts. The tropical land belts typically have desert and semidesert landscapes. Only the eastern sectors of the continents are occupied by moist tropical monsoon forests and open woodlands. The radiation balance is 60–70 kcal/cm2-yr. The mean temperature in the coldest month is not less than 10°C, and in the warmest month it ranges from 30° to 35°C. Wet and dry seasons are well defined in the east. The drainage network is for the most part of low density. Violent, brief floods occur in the rainy season; the dry season is characterized by a prolonged low level of precipitation. The water table is usually deep, and the groundwater often saline.
In the western and central arid sectors of these belts, precipitation is 50–200 mm/yr, and the hydrothermal coefficient is not more than 4. Because of the scarcity of moisture, the weathering zone is thin, and the production of biomass is negligible. In the semideserts about 4 t/ha of vegetative mass grows per year, and in the deserts less than 2 t/ha. The biomass of the root parts of the vegetation is significantly greater than the surface part. During the short rainy seasons, biochemical processes are very intensive, as is evidenced by the occurrence of ephemerals. Physical weathering plays a larger role than chemical weathering.
Toward the eastern margin of the continents, deserts develop into semideserts, brush, open woodlands, and, finally, summer-moist tropical monsoon forests. In terms of heat regime and moisture supply, these monsoon forests differ little from the subequatorial monsoon forests. A corresponding change occurs in the soil series, from the gray-yellow skeletal soils of the tropical deserts through the gray-brown soils of the semideserts and red-brown soils of the open woodlands to the red alferritic (sometimes podzolized allitic) soils under the monsoon forests.
In the animal population of the tropical belts, ruminant artiodactyls predominate. Some carnivores (hyenas and foxes), birds, and rodents are found, and reptiles abound.
The oceanic regions of the tropical belts are characterized by the prevalence of anticyclonic weather conditions. The radiation heat (R is approximately 100 kcal/cm2-yr) is expended on heating the water to 20°C and on intensive evaporation, which raises the salinity to 37 per thousand (to 42 per thousand in enclosed seas). Vertical water circulation is weak. The water contains very little oxygen or plankton. The diverse marine organisms are not numerous in a quantitative sense. The content of zooplankton is 25 mg/m3.
Subtropical belts. The subtropical belts are characterized by a radiation balance of 50–60 kcal/cm2-yr, seasonal alternation of temperate and tropical air, and a complex system of landscape zones connected with differences in moisture supply. The hydro-thermal coefficient ranges from 2—for deserts—to 12—for monsoon forests on the eastern margins of the continents. The sum of active temperatures ranges between 4,000° and 6,000°. The mean temperature of the coldest month is above 4°C, but brief periods of frost are possible. A short period of dormancy is observed in many plants. Sectors are well expressed. In the west, Mediterranean forests and scrub are found on brown and gray-brown soils; there is winter moisture and the permanent drainage network is poorly developed. In the center of the continent, semideserts and deserts with gray-brown soils and sierozems occur, and runoff is meager and sporadic. The east exhibits monsoon forests on red earths and yellow soils, there is summer moisture, and the drainage network is well developed. The subtropical forests have been largely cut and replaced by secondary scrub—including maquis, garigue, and shibliak (deciduous brush formations)—or cultivated types of vegetation, such as fields and gardens.
In the oceanic regions of the subtropical belt, the radiation balance is 75 kcal/cm2-yr, with small fluctuations of mean monthly air temperature—12°C in January and 20°C in July. In the winter, temperate air prevails; a westerly shift and cyclonic rains are observed. In the summer, tropical air, anticyclonic weather conditions, and irregular winds are found—except along the eastern periphery of the continents, where stable southeasterly winds (nonequatorial monsoons) occur. The mean water temperature in the northern hemisphere is 16°C, and in the southern hemisphere 15°. High summer evaporation increases the salinity of the water to 36 per thousand, and in enclosed seas, such as the Mediterranean, the salinity reaches 38 per thousand. The weak mixing of oceanic waters decreases the content of oxygen and plankton; the zooplankton content, for example, is 50–100 mg/m3. Consequently, the quantities of fish are not large.
Temperate belts. The temperate land belts have clearly seasonal temperature regimes with an extended winter period. As a result, a seasonal rhythm is given to vegetation and other biochemical processes. The radiation balance varies from 20 to 50 kcal/cm2-yr, and the hydrothermal coefficient ranges from 3 to 12. A westerly movement of temperate air and cyclonic circulation prevail. Spring and autumn runoff of surface waters is dominant. Only in the eastern coastal areas are summer monsoon circulation, precipitation, and runoff still preserved. They are weak, however, in comparison with the subtropics. The sum of active temperatures, which ranges from 1,500° to 4,000°, favors the growth of coniferous and deciduous forests.
The differences between sectors are well expressed in the northern temperate belt. In the northern parts of the ocean-coast sectors, mixed forests—primarily on podzolic soils—predominate. The southern parts exhibit broad-leaved forests on brown forest soils and prairies with chernozem-type soils. In the continental sector, the spectrum of zones is very broad. Moving from north to south, we find taiga with podzolic soils, mixed forests on soddy podzolic soils, forest steppes with gray wooded soils under oak groves and with podzolized chernozems under meadow steppes, typical steppes on chernozems, southern dry steppes with chesnut soils, semideserts with gray-brown soils, and deserts on sierozems. Owing to the considerable hydrothermal differences between the northern taiga part of the northern temperate belt and the southern part with its open forest-steppe and steppe areas, the former is sometimes singled out as a separate boreal belt. More than one-half of the level areas of the southern part of the temperate belt has been brought under cultivation or built up. The average production of grain crops is about 5 t/ha of dry matter. The fauna is relatively meager and uniform, with forest and steppe forms predominating.
Over the ocean, the value of the radiation balance ranges from 20 kcal/cm2-yr in the north to 60 kcal/cm2-yr in the south. The climate is characterized by comparatively warm winters (2.7°C) and cool summers (15°C). These belts exhibit vigorous cyclonic activity, gales, dense clouds, and abundant precipitation, which varies from 1,000 to 2,000 mm/yr. Atmospheric precipitation and river runoff exceed evaporation; as a result, water salinity is low—about 33 per thousand. The intense water turbulence enriches the water with oxygen and plankton. The quantity of zooplankton exceeds 200 mg/m3 and in places reaches 500 mg/m3. About two-thirds of the world fish catch comes from here.
Subarctic and subantarctic belts. In the subarctic and subant-arctic land belts, arctic (or antarctic) air prevails for most of the year. The radiation balance is not more than 20 kcal/cm2-yr. The sum of active temperatures is less than 500°C, and the growing season lasts 1.5-2 months. Forests are absent. Xeromorphic perennials on tundra-gley soils predominate among the grassy-shrub and moss-lichen vegetation. Biochemical processes are slow. Permafrost obstructs the circulation of water and migration of elements and promotes the development of marshes. The average annual growth of vegetation is 3.5 t/ha in the forest tundra and about 2 t/ha in the tundra. The animal population of the subarctic belt is poor in species; entire groups—such as reptiles, amphibians, and orthopterans—have few or no representatives. At the same time, certain species, such as dipterans, bumblebees, lemmings, and reindeer, are represented by large numbers of individuals. Many water birds are found in the summer.
In the ocean, the boundaries of the subpolar belts are fixed by the edges of the perennial (polar boundary) and seasonal ice. The radiation balance is 20-30 kcal/cm2-yr. The summer heat remaining after the ice is thawed is expended on evaporation and warming the water to 5°C. Series of cyclones move out from the centers of low pressure in a westerly transport of air. Air and water turbulence is intense, and precipitation is abundant. Water salinity is 33-34 per thousand. The water is rich in oxygen, and the long day during the summer favors the development of plankton. The zooplankton content is 200 mg/m3. As a result, schools of fish, flocks of birds, and even whales, which have now been largely killed off, are attracted here.
Arctic and antarctic belts. The arctic and antarctic land belts are characterized by a very low radiation balance: 5-7 kcal/cm2-yr. Nearly all the summer heat is expended on partial thawing of the snow and frozen ground (ice), evaporation, and air turbulence. The air temperature is below freezing for 10 to 11 months of the year. Water, the most important source of life and other natural processes, is in a solid state. Only at the height of summer does the air temperature in the arctic belt rise to 5°C. Since biochemical processes are extremely limited, the development of higher plants is virtually precluded. Mosses and crustose lichens predominate. The summer temperature often crosses the freezing point. As a result, frost weathering occurs, and polygonal markings appear in the active layer of the permafrost.
In the regions of vigorous interaction between arctic (or antarctic) and temperate maritime air, more precipitation falls in solid form than is expended for evaporation, thawing, and runoff. Land ice, which covers approximately 11 percent of the land area (16.3 million km2) and has a volume of 30 million km3 (or 27 million km3 of water), has been preserved from the ice age and is developing.
The absolute minimum air temperature on earth has been recorded in Antarctica: —88.3°C at the Vostok station. Annually the temperature of the land ice in the Antarctic drops to a —56°C. In Greenland it goes down to —11°, and the temperature of the sea ice drops to —8.6 °C. The arctic and the northern hemisphere in general have a less severe climate because of the transport of part of the heat from the tropics of the southern hemisphere to the northern hemisphere by means of atmospheric circulation and, especially, sea currents. The southern hemisphere has no powerful warm currents, such as the Gulf Stream and the Kuroshio Current. According to observations by the Soviet drifting stations “North Pole,” the radiation balance in the center of the Arctic Basin is 2-5 kcal/cm2-yr. This heat is entirely expended on the partial thawing of the ice and on evaporation. The mean July temperature is about 0°C, and the mean January temperature is —30°C.
REFERENCESBerg, L. S. Geograficheskie zony Sovetskogo Soiuza, vols. 1-2. Moscow, 1947-52.
Isachenko, A. G. Osnovy landshaftovedeniia i fizikogeograficheskogo raionirovaniia. Moscow, 1965.
Kalinin, G. P. Problemy global’noi gidrologii. Leningrad, 1968.
Grigor’ev, A. A. Tipy geograficheskoi sredy. Moscow, 1970.
Kalesnik, S. V. Obshchie geograficheskie zakonomernosti Zemli. Moscow, 1970.
Mil’kov, F. N. Landshaftnaia sfera Zemli Moscow, 1970.
Budyko, M. I. Klimat i zhizn’. Leningrad, 1971.
Riabchikov, A. M. Struktura i dinamika geosfery, ee estestvennoe razvitie i izmenenie chelovekom. Moscow, 1972.
A. M. RIABCHIKOV