less than 20kJcm2; in mountains of Southern Siberia it is about 40kJcm2. The annual sum of the radiation balance does not exceed 40% of the incoming radiation. The natural surface albedo of a bare site during the snowless season has a comparatively low value of 0.15-0.16. Effective radiation is 25-32% of the total radiation. Heat consumption for surface evaporation varies within a wide range compared to total radiation: 1539% in summer and 11-33% in winter. Heat flow into the soil for a thawing season (and in the same way, for a freezing season) and correlations of the flow with radiation balance, vary from west eastwards, and from north southwards in the territory of the former USSR. Over the European Arctic coast (the settlement of Amderma) and north-east of European Russia it makes up about 15% of the value of R, in the north of West Siberia 12%, and in Central Yakutia 10%. Eastwards of Lake Baikal and in the south of Central Siberia the total heat flow into soil makes up 7% and in desert regions of Central Asia 2-4%, of the value of R.

Let us consider the characteristics of atmospheric circulation over the territory of the former USSR which have a substantial influence on the heat-and mass-exchange of ground with the atmosphere. It is known that since there is a difference in heating of the atmosphere at low and high latitudes there exists constantly a meridional gradient of pressure which is a driving force for the Earth's atmospheric circulation. Boundaries of zonal transfer do not coincide with boundaries of the radiation zones as the zonation of air flows is determined not only by radiation processes but also by such factors as blocking processes and Arctic intrusions.

Blocking is due to high anti-cyclones that disturb the western transfer of air masses typical of the temperate latitudes. Most often blocking processes develop above the European territory. In Eastern Europe cyclones come from the Mediterranean bringing warm and humid flows. Warm anticyclones block the usual paths of cyclones directed towards the east and cause them to deviate northwards. It is with these processes that the features of the radiation-thermal regime of European Russia are associated, the shifting northwards of the sum total of absorbed radiation, values of effective radiation and radiation balance, temperature zones, etc. Arctic intrusions are most often encountered in Eastern Siberia. These processes weaken the zonal circulation and are accompanied by the advance into temperate latitudes of high, cold troughs. The almost constant presence of high-altitude troughs is the main characteristic of the atmospheric circulation relevant to the formation of the radiation-thermal regime of the Asian part of the former USSR. As a result of winter intrusions of cold air, the mean temperature of the lower half of the troposphere, at the 40th parallel in the east of Asia is almost 4°C lower than that in North America. The lower temperature is explained by the bigger size of the continent which, together with the orography of its surface creates favourable conditions for the radiation cooling of air, the increase of the latitudinal gradient of the total absorbed radiation, the effective radiation, the temperature of the ground layer of air, etc.

Heat and mass exchange at the Earth's surface with the atmosphere has an altitudinal-zonal or altitudinal-belt pattern in mountain regions, where, with higher altitude the smaller is the radiation balance owing to greater loss of radiative energy by reflection and radiation. However, of importance here is the role of the advection component in the 'Earth-atmosphere' system. A substantial role is played by the aspect of macro-orographic elements of the mountain regions - ranges, ridges and massifs. Usually, windward slopes in the path of air flows bringing heat and moisture (western slopes of the Urals, Putorana Plateau, Anabar massif and Verkhoyansk Range) receive more heat than leeward eastern slopes. In the areas of influence of monsoon air flows of the Pacific, on the contrary, the eastern slopes of the mountain structures receive more heat and moisture (Sikhote-Alin, Bureinskiy, Taykan, Al'skiy ranges and others). Such features of heat and mass exchange may lead to either shifting of altitudinal boundaries of belts or variation of their vertical extent on slopes of different aspects.

Thus, the radiation processes along with zonal components of the atmospheric circulation serve as prerequisites of the most characteristic latitudinal features of heat and mass exchange over the Earth's surface on the territory of the former USSR. Meridional components of the atmospheric circulation give rise to longitudinal and sectorial differences within the limits of the latitudinal zones. Circulation of the atmosphere associated with the formation and shift of cyclones and anti-cyclones both in zonal and meridional directions from the place of their origin, brings about fluctuations of temperature and air humidity. Simultaneously, there is transformation of air masses over the surface which is characterized by a variety of properties and conditions for incoming and outgoing radiation and thermal balance.

The role of the atmospheric circulation and of radiation-thermal processes in the development of weather conditions differs according to the season of the year. The radiation factor has the greatest influence in summer time, as is revealed in the intensification of the process of transformation of the thermal regime of the air in the south-eastern regions of European Russia, Kazakstan and Central Asia (more than a half of the radiation balance is accounted for in the heating of the atmosphere). However, the prevalence of the northern winds in summer over the European North of Russia, West Siberia and Kazakstan brings about a lowering of the general level of air temperature. The summer maximum of precipitation which constitutes 80-85% of the annual amount is associated with cyclonic activity in Eastern Siberia and the Russian Far East. In the Far East cyclones and thick cloud cover with sea winds lead to substantial loss (up to 60%) of solar radiation.

In winter the air temperature regime over the greater part of the former USSR forms largely under the influence of the atmospheric circulation as the incoming solar radiation is drastically reduced. The role of the radiation factor in winter is evident in the radiation cooling of the surface (on account of the effective radiation), which leads to higher temperatures in European Russia than in the east of West Siberia, Central and Eastern Siberia and the Russian Far East, at the same latitude. This difference may be as much as 10°C. It is explained by the fact that cyclonic activity over the former USSR European territory and north-east of West Siberia brings about thick cloud cover that hampers the radiative cooling and reduces the depth of snow cover. Snow covers 70-80 cm thick are observed north of 60° N in the north-east of European Russia and West Siberia, and on windward slopes of the Central Siberian plateau (over 80 cm). On the contrary, in Eastern Siberia, during the cold season of a year, cold air masses prevail that are greatly cooled in the ground layer of air and become even colder than the Arctic masses (in November and March), showing a thermal inversion.

Thus the observed distribution of temperature, precipitation and other climatic elements is the result of a complicated interaction of all climate-forming factors. In accordance with the climatic zonation of B.P. Alisov, four climatic belts are distinguished by their radiation regimes: Arctic, sub-Arctic, temperate and subtropical. In general, the recent climatic conditions over the greater part of the former USSR territory reflect such a heat exchange between the soil surface and atmosphere as to give rise to the existence and formation of permafrost. In the north of European Russia, West and East Siberia, and in the Russian North-East these conditions in Arctic and sub-Arctic belts are associated with long winters and a predominance of Arctic air masses over the territory. In East Siberia and eastwards of Lake Baykal, within the limits of the temperate climatic belt the development of permafrost is associated with formation of a continental regime of atmospheric circulation with frequent Arctic intrusions of cold air masses, an anti-cyclone regime and radiative cooling of the ground surface in winter time. Within the subtropical belt, conditions of perennial freezing are observed in the mountain regions at elevations exceeding 3000 m and are related to intense radiative cooling of the surface and certain other factors.

Since latitudinal zonation and altitudinal belts together with their sectorial and longitudinal distinctions are the main factors in the differentiation of the radiation-thermal regime of the ground surface, then, the same patterns are found in the differentiation of the temperature regime of the ground and, accordingly, the differentiation of the areas of seasonally and perennially frozen ground.

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