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|The interaction of basic climate-forming
factors over Tajikistan is such that the main climate characteristics in most of the
territory are aridity, abundance of heat, and significant annual mean variability of
almost all climatic elements.
Tajikistan is divided into two large climatic areas: Upper Asian and Central Asian, which are characterized by a varying annual precipitation. Within these areas, some climatic zones of humidity and thermal resources stand out specifically.
Solar radiation is the main source of thermal energy for almost all processes developing in the atmosphere, hydrosphere and biosphere. Solar radiation moving to the terrestrial surface is one of the basic climate-forming factors. It, in turn, strongly depends on the circulation of the atmosphere, whereas in mountainous areas it depends on the height of location above sea level, steepness and exposition of slopes.
Due to southerly latitudinal situation and abundance of solar light in the republic, large values of solar radiation and duration of solar light are observed.
Total duration of sunshine ranges from 2100 to 3170 hours per year. The least total duration of sunshine is seen in mountainous areas, for which significant cloudiness is characteristic throughout the year. Foothills and mountains are affected by the exposition of slopes.
The greatest duration of sunshine is observed in the plain areas of northern Tajikistan, Gissar and Zeravshan valleys, southwest Tajikistan and in the Pamirs. Because of cloud cover and mountain relief, the duration of sunshine available is 50-70 % of the possible light available.
In the territory of Tajikistan, atmospheric transparency is not uniform and depends on weather conditions. The air is clearest after precipitation. The lowest visibility is caused by advective dry fogs and sandstorms. The dust content is highest in dry spring and summer periods, when the dust rises 4 km and more into the atmosphere and remains few months.
The average monthly values of intensity of direct solar radiation moving at midday hours to the perpendicular surface in foothill areas range from 0,35 up to 0,79 kW/m2 in cloudless days. The intensity of solar radiation grows in parallel with the increased height of location and improved transparency of the atmosphere. At the heights of 4000-5000 meters it reaches 1,06 -1,15 kW/m2.
Cloud cover reduces descending solar radiation and radiation balance. During the year as a whole, clouds reduce receipt of direct radiation by 32-35% in the plains and by 50% in the mountainous parts.
Strong turbidity and dust content in the atmosphere, which reduce direct solar radiation, increase dispersed solar radiation, whose intensity under such conditions reaches high values (in summer up to 0,60 - 0,72 ccal/cm2*min).
Total solar radiation is determined by the overall receipt of direct and dispersed radiation on the horizontal surface. Total solar radiation reaches the maximum intensity during May - July. The intensity of total radiation changes in foothill areas from 280 - 925 MJ/m2. In high altitude areas, it fluctuates from 360 - 1120 MJ/m2.
The radiation balance in the plains of Tajikistan remains positive throughout the year, i. e. the air and surface of the Earth absorb more radiation than they emit, and only in some years does the balance become negative in winter. In mountainous areas at the altitude of 2000-3000 m, the negative balance is observed during 3-4 months on the slopes of southern orientation and during 4-5 months on those of northern orientation. At the altitude of 4000 m and higher, the period of negative balance continues for about six months.
The thermal regime of air in Tajikistan is formed under the influence of different factors. The temperature of the warm period of the year is steadier, whereas in the cold period it depends on the prevalence of intruding air masses from the north or south, as well as on their frequency. In the former, the winters are cold, and in the latter, they are warm. Taking into account the diversity of the climate conditions in Tajikistan, a number of areas with similar physiographical conditions have been earmarked to describe the thermal regime of the territory.
Wide valleys and plains at the height of up to 1000 m are primarily agricultural and cotton growing areas. They cover the southwest part of the republic, Gissar, Vakhsh, lowland Kafirnigan and Kulyab valley, as well as Ferghana valley and its surroundings of Sogd region. The summer heats are characteristic to wide valleys and plains when the summer thermal depression is dominant.
In summer, clear and hot weather is characteristic, during which the maximum temperature can reach +43°C +47°C. The monthly mean temperature of the hottest month - July - reaches +28+30°C. In the winter, Tajikistan is under the influence of the Siberian anticyclone.
The cold period of the year is characterized by the intrusions of cold Arctic air, during which even in the extreme south of the republic the temperature of air on some days can lower to -24-30°C below zero. The monthly mean temperature in January - the coldest month - essentially remains around 0,3-2,5°C, while in the northern part of the Republic (Khujand city) it falls to 0,3°C below zero.
Large fluctuations in winter temperature above and below 0°C are peculiar to this zone. The last spring frost for the majority of areas disappears at the end of March, on average, and the first autumn frost starts in the second half of October. For low valleys of southwest Tajikistan, the most continuous frost-free periods of 260 days are characteristic.
Zeravshan valley and the mountainous areas of Central Tajikistan and Western Pamir form a transitional area from valleys high altitudes up to 2500 m. As in the previous case, rare clouds and dry, but cooler, weather persist here in summer. For this zone, a consecutive decline in temperature is characteristic, depending on the altitude. The influence of the forms of relief on the thermal regime in this zone is extremely strong. On open slopes and mountain passes, the temperature in winter months is much higher than in valleys and closed depressions where strong cooling occurs. In summer, there are inverse relations in the thermal regime in the convex and concave forms of the landscape.
The monthly mean temperatures in January fluctuate here within a wide range: from -1°C in the lower reaches of Zeravshan valley down to -7°C in the mountains of Central Tajikistan. Western Pamir is distinct by quite higher temperatures. Thus, the monthly mean temperature in Kalaikhum in January is 0,2°C. The absolute minimum reaches -34°C in Tavildara.
The hottest month of the year is July, when the monthly mean temperature changes from 25°C in Zeravshan valley to 18°C in the mountains of Central Tajikistan. The absolute maximum reaches 36-40°C. The first frost is observed in the second half of October and the last one in the spring - in the second half of April. The frost-free period in valleys averages to 200 days, decreasing to 150 days at the altitude of 2500 m.
The high-altitude areas above 2500 m include Central and Eastern Pamir and mountain ranges. The climate of these areas is continental. Rarefaction of the atmosphere in high altitude areas, although causing increased solar radiation, at the same time causes loss of the heat. Therefore, temperature fluctuations from winter to summer and from day to night are rather significant and increase in the east.
Eastern Pamir is distinct for especially severe climatic conditions. Winter is long and cold here. The monthly mean temperature in January falls from -14°C down to -26°C. The absolute minimum is extremely low at -63°C (Bulunkul lake). Summer is short and cool. The July mean air temperature does not exceed +15°C (Irkht). The absolute maximum changes within 20°C (Fedchenko glacier) and +34°C (Irkht).
The biggest duration of the frost-free period continues for 111 days (Irkht), and in a number of the coldest areas (Shaimak, Karakul, Bulunkul, Fedchenko glacier) there is no frost-free period at all.
Atmospheric precipitation in the territory of Tajikistan is basically determined by two zones. The dry climate zone covers the valleys of southwest and northern Tajikistan, foothills of Turkestan range, as well as extensive high-altitude area of Eastern Pamir (50-300 mm per year). The rest of the territory is classified as a zone of insufficient humidity (up to 900 mm). Exceptions are the windward southern slopes of Gissar range, where a damp climate zone stands out (above 1500 mm).
The annual amount of precipitation over the republic varies from 73 mm in Eastern Pamir up to 1500 mm on the southern slopes of Gissar range.
The observed distinctions in the distribution of annual amount of precipitation in the territory of the republic are caused, essentially, by the orographic features. The Gissar range resists western and south-west humid air masses and on the southern slopes falls the largest amount of precipitation in the republic (Sioma, Haramkul - more than 1500 mm).
In spite of the fact that the slopes of all mountain ranges are well-humidified, internal areas of the extensive mountain massifs of Tajikistan, especially deep closed depressions, closed and narrow valleys among mountains, receive little precipitation. For example, in the valley of Surkhob River the precipitation is three times less than that at the same altitude of the southern slopes of Gissar range. It is dry in the Valley of Zeravshan River, where the annual amount of precipitation totals to 190 - 340 mm. It is very dry also in southern Tajikistan with annual precipitation of no more than 150 - 200 mm.
Annual distribution of precipitation in various areas of Tajikistan is not uniform. For most of the territory, the annual distribution of precipitation is lowest in summer months. The maximum precipitation occurs in March - April in valleys and foothills, and in April - May in high-altitude areas.
The period of maximum rainfall in most of the territory changes within the range of 30-100 mm per month; in some areas within the range of 200-300 mm. In the north of Tajikistan and Eastern Pamir, the precipitation maximum decreases 12-20 mm per month. The months of minimum precipitation all over the territory usually does not exceed 5 mm, and only in some high-altitude areas reaches 10-20 mm per month.
In the foothills of Tajikistan, 15-20% of the average annual precipitation falls in the form of snows. The quantity of hard precipitation increases up to 50-70%, with the altitude mounting, to reach a maximum of 85-90% in the Pamirs, including Fedchenko glacier (100%). The number of days with precipitation of 0,1 mm and more in the plains changes within the range of 50-80 days, in the foothills, 80-100 days, up to about 125 days as the altitude heightens. The least number of days with precipitation is observed in Eastern Pamir - 50 days only.
Analyzing the amount of precipitation and humidity deficit, it is possible to note that in the spring, all over the territory of Tajikistan, the availability of moisture exceeds its loss. In the summer, on the contrary, the loss of moisture all over Tajikistan considerably exceeds its availability.
The snow cover, both per height and duration of standing, varies considerably in the territory of Tajikistan.
In the southern areas of the republic (Gissar, Vakhsh, Kulyab and lowland Kafirnigan valleys) and in the northern plain areas, steady snow cover is absent in 90% of winters, and in 15% of winters, the snow cover is not formed at all.
Snow cover on the huge area of Western and Eastern Pamir stockpiles at the height of 3500-4000 m and above almost all year round.
In the mountainous areas, the distribution of the snow cover height is closely connected to the relief conditions and distribution of atmospheric precipitation. The snow cover reaches larger heights in areas of maximum precipitation. So, on the southern slopes of Gissar range, in the area of Haramkul station, the maximum height of snow can exceed 2,5-3 meters, while at the same time the average of the biggest decadal heights in the drought-prone Eastern Pamir reaches 4-5 cm, and the maximum is 20 cm.
In many areas, as the location altitude grows, the height of the snow cover increases. The average of the biggest heights of the snow cover at the altitude of 1500 m reaches 40 cm while at the altitude of 1850 m it increases up to 90 cm and at the altitude of 2800 m - to 250 cm (Haramkul).
The height of the snow cover varies from year to year and can considerably deviate from the average long-term value. For example, on Anzob pass with the average height of snow of 169 cm, it's the maximum reaches 264 ñm, and minimum 92 cm.
Mean monthly distribution of the number of days with snow cover in Tajikistan is scattered.
In the south of the country and in Ferghana valley (in the north), the snow cover is less than 20 days annually. To the north and south from Ferghana valley, in Gissar valley and in Karategin and Darvaz the number of days with snow cover gradually increases, with the altitude increasing, and reaches the maximum of 244 days on Anzob pass. In Western Pamir (Pyanj River), the number of days with snow covers changes upward, depending on the altitude, from 40 to 160 days. In Eastern Pamir, the number of days with snow covers decreases, and ranges between 40 - 90 days.
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