Vladimir Kotlyakov

Climate is a long-term regime of atmospheric conditions, typical for a specified area. Climate is formed as a result of processes running in the atmosphere and fluctuates over time. The knowledge of climate is based on statistical analysis of the long-term observations of meteorological elements: atmospheric pressure, wind speed and direction, heat budget of the earth's surface, air temperature and humidity, cloudiness, and atmospheric precipitation. In this case, not only mean values of these elements are determined, but also their annual and diurnal course, extremes, and deviations from average quantities. Occurrence of certain weather phenomena, as well as the average and extreme terms of their appearance, are also noted. Complex indices, such as humidification, continentality, and others, are also used.

Russia is located within four climatic zones: Arctic, sub-Arctic, moderate, and subtropical. Cyclone and anticyclone motions in zonal and meridional flows of the atmospheric circulation transport air masses from the regions where they are formed into other territories. The air masses are also transformed by the influence of various underlying surfaces having different features and different conditions of solar radiation income and expenditure.

Depending on the geographical latitude, the amount of solar radiation annually received by the earth's surface, varies over the Russian territory from 2,510 megajoules per square meter (MJ/m²), or 60 kilocalories per square centimeter (kcal/cm²)  in the latitude 80 N to 5,440 MJ/m² (130 kcal/cm²) in the latitude 42 N. Maximum radiation totals come in May-June, when the sun is at its highest altitudes and the longest day ensues. During this period, the differences between radiation totals in different latitudes are smaller since in northern regions the lowering of the Sun's altitude is compensated for by the increasing length of the day.

The proportion of the direct solar radiation in the annual totals of global radiation under real cloudiness conditions amounts to 35–45% in the north, and about 50% in middle latitudes. Over most of the country, during cold seasons the diffuse solar radiation slightly exceeds direct radiation, or it is roughly equal to it. During warm seasons, the direct radiation is significantly predominant. The Arctic is the only exception, as the diffuse radiation remains predominant there in summer, too.

Reflectivity of a surface (albedo) plays an important role in the formation of net radiation during the warm season when the maximum solar radiation comes to the surface. Within the tundra zone, albedo changes from 10% to 13%; in the coniferous forest zone, from 12% to 13%; in the mixed forest zone, from 12% to 14%; in the hardwood forest zone, from 20% to 23%; and in the icy and snowy Arctic areas, up to 60% to 70%.

Net radiation for a year is positive over the whole Russian territory, changing from 210 MJ/m², or 50 kcal/cm², down to values close to zero in the central Arctic. In January, the net radiation is negative everywhere. Because of significant cloudiness and a shorter duration of snow cover existence, net radiation is greater in the European part of Russia than that in the Asian part at the same latitudes.

In middle latitudes, the radiating heat is expended mainly for water evaporation from the earth's surface and for direct heating of the soil; additional radiation heats the air from the earth’s surface. But, a ratio of the energy expenditures for these processes is not equal in different regions of the country. The largest part of this heat (70–80 MJ/m², or 28–32 kcal/cm²) is spent for evaporation in the central part of the forest zone of European Russia and in the south of the Far East; it is a little less (23–24 kcal/cm²) in the forest and forest-steppe areas of West Siberia.

Over Russia's territory, the west-to-east air transport is predominant throughout the whole troposphere. Cyclonic activity is great and favors the meridional exchange of air masses and the fall of atmospheric precipitation. Russian climates are mainly formed under the influence of the continental air of middle latitudes, particularly in the Asian part of the country. At the same time, the climatic features of the western regions are formed under the prevailing influence of maritime air masses coming from the Atlantic ocean. The cyclonic activity is the most intensive to the north and west of the European part of Russia, in the north of West Siberia, and in the Far East. Over the larger part of the Asian territory of the country, high pressure systems are predominant (Siberian high).

Joint action of circulation and radiation along with orographic factors condition a degree of humidification. A dividing line goes through Samara, Ekaterinburg, and Novosibirsk. To the west of this boundary the humidification is sufficient and the precipitation mainly exceeds the evaporation, while to the south the deficient humidification makes it necessary to irrigate the fields in some regions.

The yearly seasons are clearly pronounced over Russia's whole territory. In winter, the Siberian high prevails over a great part of Russia; its ridge of high pressure reaches southern regions of the East European Plain. Owing to this anticyclone, the processes of the land cooling prevail and a dry continental air is formed. The average January temperatures gradually drop down from the southwest to northeast, reaching minimal values in Yakutiya. The northern half of the European part of Russia is influenced by frequently occurring Atlantic cyclones, which usually yield warming and precipitation. During this time, the winter monsoon with its dry and cold west and northwest winds dominates over the Far East territory. Almost everywhere the winter precipitation falls as snow.

In summer, a system of low pressure is formed over the major part of Russia's territory. This results from strong warming of the land surface. Mid-latitude westerlies prevail in the European part of the country during this time, while in the north of the Asian part northern winds prevail. The summer monsoon dominates in the Far East. In the lowland regions, the average July temperatures gradually drop from the south to the north.

Climatic zonality is well manifested in mountains: with elevation the air temperature drops, and the annual total of precipitation grows. Because of the prevailing west air transport, the mountain ridges and large highlands – extending from the north to the south (the Urals, Mid-Siberian tableland) – greatly affect the humidification of the adjacent lowland territories.

Soil climate is a long-term heat, water, and air regime of soils, which greatly determines the substance dynamics in the soil profile, the direction of the soil-forming process, and a degree of the soil fertility. Soil climate is closely connected with climate features, primarily with the near-surface layer of the air.

Mean Annual Temperature

Mean annual temperature characterizes a heat regime of a territory in the most general form. Over the Russian territory, it has a predominantly zonal character and corresponds to the distribution of annual net radiation. Meridional advection, especially that from the Arctic, enhances the zonality, while the latitudinal heat transports complicate it. Since in the summer months the temperature variations along the parallels are small, and in the longer cold part of the year they are very large, the latter is reflected in the annual consideration. Annual isotherms are turned (deviated) to the south, especially intensively in East Siberia, thus reflecting the lower temperature of the continent in comparison with the ocean on the average for a year.

Mean annual temperatures rise from the north to the south from about 0° to 10°C in the European part of Russia, and from –15° to 0°C in East Siberia. From the west to the east, these temperatures drop to meridians 120–130° E. At 70°N, the maximum difference in the annual temperatures between the west and the east of the country reaches 15°C; at 60°N it amounts to 12°C; at 54°N it drops down to 10°C. In the continental part of the country, the lowest mean temperatures are recorded on the north of the Taimir peninsula and in the depression of northeast Siberia.

In the Far East, the mean annual temperatures are only a little higher than those in East Siberia in the same latitudes. Here, advection from the seas, which slightly mitigates the winter, also decreases the summer temperatures. On the average for a year, here, the temperature in all parallels is approximately 10°C colder than it is in the west part of European Russia.

In the mountains, mean annual temperatures drop, and at high elevations they are negative. In the low parts of the Siberian mountain slopes, temperature inversions are formed. Thus, in west Altai at heights of 400–600 m, the annual temperatures are a little higher than those on lowlands to the west of Altai. Despite high summer temperatures, in winter the especially low mean temperatures are recorded in inter-mountain depressions in regions with prevailing anticyclonic circulation.

Mean January Temperatures

In winter, under conditions of negative net radiation over the major part of Russia, the temperature distribution is mainly formed under the influence of air advection and radiative cooling. In the northern half of Russia's territory, the latitudinal zonality of the temperature distribution does not exist. Here, the January isotherms extend almost along meridians from the west boundaries of the country to Central Yakutiya. Mean January temperatures drop from the west to the east from –4° to –10°C and below, changing slightly from the north to the south. Everywhere over the northern borders of the country the climate is a little warmer than that at a distance of several degrees of latitude to the south of them. This is especially noticeable in the north of Middle and East Siberia, where a difference in the mean January temperatures between the northern border and latitudes 60–65°N reaches 8–10°C.

To the east of Central Yakutiya, the isotherms are also meridionally directed: here the temperature rises to the east, though it is still very low even on coasts of the Far-East seas, along which the isotherms –30, –20°C run. In the southern regions of the country, the isotherms take a parallel direction, and the temperature rises from the north to the south.

In winter, high and middle latitudes of Russia's territory receive heat mainly via air advection from the west. At this time, the absorbed solar radiation is very small because of the low altitude of the sun, the short daytime, and the great surface albedo. Like the values of net radiation, its magnitudes are rather uniform; net radiation is negative everywhere except the southern border of the west half of the country.

In winter, in middle latitudes of Russia, heat is transferred from west to east, carried over the Atlantic ocean as a result of heating in the process of evaporation together with water vapor. Farther to the east, the cyclonic circulation weakens, and along with that the cloudiness and precipitation decrease. Conditions for more intensive heat losses are favored because of radiation by the surface and the low troposphere, and also because of the decrease of latent heat released in the middle troposphere. All those processes result in formation of extremely cold air masses in the low troposphere in the central and northeast regions of Yakutiya.

Over the eastern border of the continent, the influence of the heat advection from the Pacific and associated seas is exhibited only within a band of several hundred kilometers. Normal zonal temperature distribution to the south of 50°N in the European part and roughly from 53–55°N in the West-Siberian sector is conditioned by the effect of the radiation regime and atmosphere circulation.

Mean July Temperatures

The distribution of the July mean temperatures over Russia is zonal. Over all territories where no significant altitude differences and influences of large water bodies are present, the isotherms lie mainly along parallels. Mean temperatures rise in all latitudinal sectors from the northern borders of the country to the south: between parallels 50–70° N in the European part up to 15°C; in West Siberia, by 13°C; and in East Siberia, by 10°C.

The basic factor underlying the rise of the summer temperature from the north to the south is the zonal change of the solar radiation; however, the temperature distribution pattern does not completely agree with the monotone distribution of the net radiation. The main cause of great interlatitude temperature differences is the influence of the Arctic Ocean. Frequent cold advections from the Arctic are caused by the monsoon character of the atmospheric circulation over the northern borders of the continent. The transport of the cold from the Arctic decreases the July mean temperature by approximately 5°C on the north of the European part, and by 6–8°C in the tundra zone of Siberia.

The other factor of the interlatitude temperature differences in the north is related to a rather short period with large values for net radiation. A significant part of this energy is spent on heat exchange in the soil: for its warming and melting. In summer, the heat expenditure for these processes takes about 15–20% of the net radiation. In middle and southern latitudes, these processes are completed mainly in spring and require smaller energy expenditures. In summer, the soil heat exchange does not exceed 5–7% of total net radiation. In middle latitudes, the basic energy of net radiation is spent on evaporation that retards the air heating.

The territory over the eastern border of Russia strongly influenced by the cold advection from the Bering and Okhotsk seas. Here, the temperature of low air layers hardly reaches 12–13°C by the end of summer. As a result, in the Far East the July isotherm extends parallel to the coastal line: in summer the temperature drops toward the seas. Here, in latitudes 50–60°N in East Siberia the July mean temperature is equal to 10–15°C.

Over much of the European part of Russia (except the northern regions), there is a general zonal distribution of temperature in summer that drops from east to west. It is caused by the cold advection from the Atlantic as well as by the west-to-east change of the heat budget structure related to the climate aridity in the southeast of the East-European Plain. In the northern part of Middle and East Siberia, the July isotherms are greatly shifted to the north: in latitude 67–70°N the mean temperature is by 2–3°C higher than it is in the European part. This phenomenon is explained by the continent’s protrusion extending far to the north in this sector.

Duration of Frostfree Period

Over the major part of Russia's territory, a mean duration of the frostfree period does not differ much from the average duration of a period with mean daily temperatures higher than 10°C, although under the Arctic air advection and its night cooling, frosts are still possible everywhere in the beginning and at the end of the vegetative season. The duration of the frostfree period shortens from the west to the east to an even greater extent than that of the vegetative period.

Table 1. Duration of the frostless period, days.

Northern latitude, degrees	Eastern latitude, degrees					Sea coast of the Far East seas
	30	50	70	90	120
67	70	55	30	50	70
60	120	110	100	95	95	<60
55	140	135	130	120	Mountains	70
50	165	160	145	Mountains	120	90
45	190	–	180	–	–	120

In West Siberia, the frostfree period is shorter by 30-40 days than it is in the same latitudes in the west of the European part of Russia. Only in southern regions of Middle and East Siberia does it reach 100 days. The frostfree period shortens from the west to the east owing to the weakening of the cyclonic activity and the respective reduction of cloudiness. Under conditions of continental climate, the frosts end in spring and when they begin again in the autumn, they take place under higher mean daily temperatures than happens under milder climate conditions.

On the shores of the Far-East seas, the frostless period is 20–30 days longer than the vegetative one (cf. Tables 1 and 2). The proximity of the cold seas decreases the air temperature, especially in spring and in the beginning of summer, so, the period with temperatures higher than 10°C shortens, while the night temperature remains positive owing to frequent fogs and low cloudiness being formed here. For the same reasons, the frostless period is longer than the vegetative one by more than a month near the Baikal Lake. In seaside regions, the frost cessation in spring and its resumption in autumn takes place under mean daily temperatures of 5–6°C, i.e., under temperatures that are twice as low as ones inside the continent.

Duration of Vegetative Period

The duration of the vegetative period varies from zero in the extreme north to 190 days in the south of the European part of Russia; it also essentially varies along the parallels (see Table 2).

Table 2. Duration of vegetative period, days.

Northern latitude, degrees	Eastern latitude, degrees					Sea coast of the Far East seas
	30	50	70	90	120
67	70	55	30	50	70	None
60	120	110	100	95	95	<60
55	140	135	130	120	Mountains	70
50	165	160	145	Mountains	120	90
45	190	–	180	–	–	120

The duration of the vegetative period essentially depends on the cold air advection from the Arctic. In the north of Siberia, the actual duration of this period is 2-3 times shorter than it could be in the absence of the cold advection. From the west to the east as far as the continental border, the vegetative period shortens because summer’s start is delayed and its end also shifts. However, in the vast intermountain depressions and basins of northeast Yakutiya, owing to the earlier temperature transition through the 10°C limit, the vegetative period is longer than it is in the Pechora River basin, and it is twice as long as that in West Siberia.

In all latitudes, the shortest vegetative period is typical for the shores of the Far-East seas; it is caused by the cold advection on shores of the Bering and Okhotsk seas. And, even the proximity of the Japan Sea does not extend its duration: around Vladivostok, the vegetative period lasts as long as it does on the Baltic Seashore, which is by 10–15° to the north.

The duration of the vegetative period becomes shorter with the growth of true altitude, since there is a general drop in the temperature with height in the troposphere. It also depends on both radiative and advective exposition of slopes in mountains as well as on the relief configuration in plains and tablelands.

Sum of Temperatures Higher than 10°C

The sum of temperatures higher than 10°C characterizes the heat resources of the vegetative period. Its distribution is zonal over all large plains of Russia in accordance with the distribution of the summer temperatures. The zonality is disturbed by mountains on the northeast and on the southeast of Siberia, and also over the whole extent of the southern border of the country and in the Far East.

No steady period with mean daily temperatures higher than 10°C exists around the northern border of the country. Even in the southern part of the tundra zone, the sum of temperatures higher than 10°C amounts to only 300–500°C. Advection of cold air from the Arctic basin decreases the heat resources of the Arctic, and the length of the snowy period is increased. This effect is also observed in middle latitudes.

In the southern regions of Russia, sums of temperatures higher than 10°C vary within a wide range, depending on  latitudinal position. In the south of the European part of the country, sums reach 3,000–5,000°C. On the plains of Central and East Asia, located in latitudes 50–55°N and absolute altitudes 500–700 m and higher, these sums are equal to only 1,500–1,800°C. On the south of the Far East, they are 2,000–3,000°C.

In the European part of Russia and in West Siberia the average latitudinal gradient of sums of the temperatures higher than 10°C to the south of the polar circle amounts to 100°C per 1° of latitude. In East Siberia, it is only 50–70°C per 1° of latitude.

Over vast northern territories of the country, roughly bordered by the 65°N parallel, the temperature sums generally increase from the west to the east to the Verkhoyansky Ridge. On lowlands and in depressions of the northeast of Siberia they remain higher than at similar latitudes in the European part of Russia. To the north of the polar circle, and in some places on the rivers Lena, Yana, Indigirka, and Kolyma, the sums of the temperatures higher than 10°C exceed 1,000°C. These are the largest values of the heat resources for the vegetative period in these latitudes across the entire globe.

At latitudes 50–60°N within the European part of Russia, the temperature sums vary little from the west to the east. A certain shortening of the vegetative period on the east is compensated here by higher summer temperatures. At these latitudes in West Siberia, the sums of temperatures of the vegetative period decrease eastward due to both a shortening of the period’s duration, and also a certain drop of summer temperatures. Nowhere on the Middle-Siberian tableland do sums reach 1,500°C; here they are 500–1,000°C smaller than they are in the European part of Russia.

South of the 50°N parallel the temperature sums increase greatly from the west to the east, despite a certain shortening of the duration of the vegetative period.

Annual Precipitation

Over Russia's territory, annual precipitation decreases from the west to the east, although this trend is complicated by a relief influence. Precipitation has a generally zonal character of distribution, less distinctive on the east of the country. Zonal features appear clearly when one averages annual precipitation totals over 10-degree belts for the whole country. The figures are: at 79–70°N; about 400 millimeters (mm); at 69–60°N; slightly more than 500 mm; at 59–50°N; almost 600 mm; and to the south, the precipitation amount drastically decreases.

The largest amount precipitates on the band from the southwest to the northeast: from the boundary with Belorussia to basins of the rivers Kama and Vychagda, and farther through the Urals to West Siberia at latitudes 58–65°N. Here, the atmospheric circulation is the most favorable for precipitation formation. In this band, in the European part of Russia the annual precipitation exceeds 700 mm, reaching 800 mm and more on the highlands. In West Siberia, the amount ranges between 550 and 700 mm. To the north of this band, the precipitation decreases to 550–600 mm on the Baltic Seashore, and to 400–500 mm on the Kara Seashore. To its south, the precipitation decreases to 500 mm near the Sea of Azov, and to 300 mm on the Low Volga River.

Conditions of the precipitation formation in the warmer seasons, especially in summer months, and, primarily, the atmospheric circulation conditions are what affects the zonal character of the precipitation distribution in sectors of the European part of Russia. The influence of the Urals is great: over their west slopes upward air flows are intensified that lead to the formation of a longitudinally spread band of increased precipitation, reaching 1,000 mm, and – on the highest parts of the Near-Polar and Northern Urals – 1,200 mm. In contrast, the precipitation is decreased in the trans-Ural area.

In East Siberia at all latitudes the precipitation amount decreases from the west to the east right up to the mountain systems behind the Lena River basin. On the Lenskaya lowland and in the lower course of the Aldan River, annual precipitation totals are smaller than 400 mm, while in central Yakutiya they do not reach 300 mm. To the south of the 60°N parallel, the precipitation decrease is complicated by the relief influence, but on the whole the decrease is observed in the Zabaikalje region approximately to the meridian 117–120°N. From here to the west, precipitation increases. This is caused by the intensification of the ocean influence.

Over the whole eastern border of the country from Chukotka to Primorje, the precipitation amount, being in close relationship with the complicated orography, increases toward the seas of the Pacific basin where the activity of the atmospheric circulation is increased at all seasons and the humidity is large. Annual precipitation is about 600–700 mm on the lowlands, and 1,000–1,200 mm in the mountains.

Mountain precipitation is higher owing to the intensification of the upward air motions, growth of the atmospheric fronts, and cyclones activity. For instance, on the west slopes of the Altai, annual precipitation is 1,000–1,500 mm, and on the Katun’ ridge it is up to 1,800 mm.

Precipitation of the Cold Period

During the cold part of the year, almost all precipitation-forming air masses come from outside of the Euro-Asian continent. The Atlantic air plays an important role over the greater part of Russia. Owing to its water content, about 70% of the precipitation of the cold period is formed in the European part of the country; it is larger in the northern half of the territory and smaller in the southern part. In the south, up to 40–50% of precipitation is connected with the moisture of the Mediterranean air masses. The role of evaporation from the continent in forming the humid properties of the air masses is not significant. During the cold period, the internal water circulation does not play an essential role.

Average water content of the air in winter is  several times smaller than that in summer, and, in contrast to summer, it follows the temperature variation and decreases from the west to the east, especially drastically behind the Enisei river. Like it is in summer, during the cold period the basic precipitation is caused by the processes related to the Arctic and Polar fronts: in the European part of the country up to 65% of all precipitation is connected with the Arctic front on the northeast and about 20% in the southwest. Similar figures for the Polar front are 10–20% on the north and 60-65% on the south. In winter, the southern cyclones are the most important mechanisms for forming precipitation on the south of the European part of Russia.

Precipitation of the Warm Period

During the warm period, over Russia's territory and contiguous areas of the continent the air masses are intensively warmed and their moist properties are transformed, usually resulting in a significant growth of their water content. Circulation mechanisms occur under which these masses become the precipitation-forming ones and make the main contribution into the humidification of the country.

In the direction from the north to the south, the troposphere water content increases but it develops significantly more slowly than does the growth of the air water capacity that results in the increase of the moisture deficit. Throughout the entire warm period, the relative humidity is sufficient to form precipitation in middle and high latitudes of the country. This humidity amounts to 70–80%, and only in Central Yakutiya does it fall below 60%.

In the western half of Russia, during the warm period the precipitation is mainly formed from moist air masses coming from the Atlantic, Mediterranean Sea, and the Arctic basin, which are in different degrees transformed over the continent. To the east, the role of evaporation from the continental surface grows. In the Far East, the moisture of the air masses originating in the Pacific becomes the main source of precipitation.

The precipitation resulting in the northwest of European Russia from the moisture originating in the Atlantic amounts to 70–75% of all precipitation of the warm period. It is less than 35% in the southeast, and over the whole European territory, it is slightly less than 50%. In the north of West Siberia, it exceeds more than half of the warm period precipitation, and in the south of West Siberia and in the basins of the rivers Podkamennaya and Nizhnyaya Tunguskas, it is about-one third. Everywhere over Russia's territory the precipitation portion from the moisture of the Arctic air does not exceed 5% of the summer total. The total significance of all air flows as a source of precipitation is very important: not less than a quarter of all precipitation of the warm period is formed over the European part of Russia, because of the water content of those flows.

Over the European part, evaporation from the continental surface produces about 25% of precipitation, and this part increases from the west to the east by more than one and a half times. In European Russia, frontal precipitation amounts to 70–75% of all precipitation of the warm period without any considerable spatial differences.

Evapotranspiration

Evapotranspiration is the total amount of moisture evaporated from the soil surface and delivered by plants during transpiration. It corresponds to the amount of water returned from the land surface into the atmosphere.

Significant differences in the evaporation values and their yearly distribution over Russia's territory are caused by the climate change from the sub-Arctic climate to the subtropical one. In the Arctic zone the evaporation does not exceed 150 mm per year. It is slightly greater only on the northern shore of the Kola Peninsula, owing to the Gulf Stream current influence. In the sub-Arctic, the evaporation increases and on the border with middle latitudes it reaches 250–300 mm; however, in the depressions of Verkhoyansk and Oimyakon it is smaller than midlatitudinal values owing to scanty precipitation.

Over the European territory of Russia, evapotranspiration increases to the south, reaching 650 mm in the West Pre-Caucasus area. In Central Russia it amounts to 400-500 mm. On the west parts of the highlands, the evaporation is greater than on the eastern slopes; that is caused here by the precipitation increase under the dominant west moisture transport.

In West Siberia the maximum evapotranspiration (greater than 450 mm) is typical for the interstream area of the Ob River and the low course of the Irtysh River. Both to the north and to the south of this area the evaporation decreases, reaching 300 mm in the north of a moderate zone and 350–400 mm in the south of the plain.

In middle latitudes of the Asian part of Russia, evapotranspiration varies from the values; it is smaller than 250 mm on the northern border of the zone, and exceeds 450 mm in the south of the Far East Primorje. In Central Yakutiya about 250 mm is evaporated; in the Predbaikal region, more than 350 mm; while in the steppe regions of Zabaikaliya the evaporation decreases down to 300 mm and less.

On the whole for Russia, the evapotranspiration amounts to about 300 mm per year; it slightly exceeds 60% of annual precipitation and it is almost 1.5 times larger than the annual river runoff.

Continentality of the Winter Climate

Continentality of the winter climate is determined by the ratio of a sum of mean monthly air temperatures lower than zero to a total of hard winter precipitation. This value regularly varies from the west to the east in accordance with the increase of severity of the natural conditions in this direction.

Over Russia's territory, the index of winter continentality varies from 5 in the west part of the European territory to 70 under the most severe conditions of Central Yakutiya and on the Verkhoyanskyi and Cherskyi mountain ridges. Over a significant part of European Russia, this index changes from 5 to 10; in the Pre-Urals area, Pre-Caspian plain, and in the West-Siberian plain it is equal to 10–20; and over the major part of Siberia its values fluctuate from 20 to 60. Only on the Kamchatka peninsula, Sakhalin Island, and in the Primorje does the index of continentality take the values typical for a maritime climate – less than 20 and even 10.

Coefficient of Humidity

Humidification means a degree of the moisture supply of a territory which is necessary for the development of natural processes and for agriculture. Humidification depends mainly on the amount and regime of precipitation and evaporation which are regulated by heat resources and the air moisture deficit.

As an index of humidification, a coefficient is most commonly used, which is equal to a ratio of an annual precipitation total X to an annual evaporativity E₀, or the index of aridity in the form of a ratio of annual net radiation to amount of heat necessary for evaporation of the annual precipitation total, i.e., R/LX. It is not difficult to see that this index is a value inverse to the coefficient X/E_o. Both indices characterize general geographical regularities of humidification over large territories.

Humidification generally decreases from the north to the south. In the north of the country this decrease proceeds slowly. Growth of the evaporativity to the south is followed here by increase of precipitation, although it is less significant. To the south of the band of maximum precipitation, X and E₀ along meridians take different signs and the humidification quickly decreases.

The most remarkable is the boundary band between the forest and forest-steppe zones. Here, both indices are equal to about one, i.e., according to the mean long-term climatic data the precipitation resources correspond here to the heat resources, but, nevertheless this band is a zone of nonstable humidification since a part of precipitation comes down to the runoff: in the European part of Russia, 120–150 mm, and in the Asian part, 50–100 mm.

In the tundra zone, the precipitation annual total is 1.5–2 times greater than evapotranspiration; in the steppe zone the values of the coefficient of humidification are equal to 0.4–0.6. There is no region in Russia having the actual total evaporation under natural conditions equal to the maximum possible – i.e., having a continuous regime of humidification optimal for the vegetation – since in the warm part of a year long rainless periods sometimes happen when the total evaporation is limited by the lack of moisture in soil.

Besides, everywhere on the plains the evaporativity significantly exceeds the precipitation. Therefore, the evaporation deficit in the major part of the forest zone of European Russia is equal to 50–100 mm per year; in the boggy taiga of West Siberia it is smaller. Everywhere in East Siberia to the south of the tundra it exceeds 100 mm, and in Central Yakutiya it reaches 200 mm, i.e., the values typical for the forest-steppe. This is a result of sharply decreased precipitation totals and higher temperatures during summer months.

In mountains, humidification increases with elevation owing to the increase of the precipitation and the reduction of the heat resources; however, the greater steepness of the slopes and close bedding of water impermeable rocks cause the increase of the runoff.

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