As a result, central and northern parts of Tibet have been in a "wind shadow", and the Early Pleistocene (750-500 thousand years ago), there is finally formed arid climatic regime. The Western and especially the southern outskirts of Tibet were well hydrated and maintained mountain forest (tropical, subtropical and temperate) and prairie landscapes, completely disappeared in the mountainous part Tibet. Asymmetry of atmospheric moisture and orographic motley led to considerable internal heterogeneity of the nature of Tibet. Mountains of Southern Tibet (Gandisyshan – "area of the longitudinal valleys") are large in contrasts in altitude, steep slopes, watersheds glaciation. part of Tibet is characterized by the dominance of broad inter-mountain valleys and lake basins, the bottom of which lies at the absolute height of up to 5,5 km. Modern climate desolate part of Tibet is characterized by extreme aridity and continentality. In this regard, the July isotherm of 0 is located in the desert at altitudes of about 6 km, and the absolute minimum temperature recorded value -55 C, although the average values of the absolute minima at altitudes of 4200-4500 m and varies from – 34 to – 39 C in summer and winter over Tibet remains anticyclonic state of the troposphere. Therefore, the temperature is very low: the average January at the altitude of 4500 m ranges from -20 to – 25 , and July 6-7 .

It is easy to notice the presence of one or more periods of the biological effects of drought, not surprisingly, at low rates of annual precipitation – from 54 to 444 mm in Actually, this amount is sometimes drops to 10 mm / year. In the highlands (above 4200 m) slightly decreases the amplitude of the temperature throughout the year (up to 37 compared with 50 in the basins) and increases the force of wind (average 15 m / sec). Humidification in the mountains Tibet rises to watersheds without inversion precipitation. The snow line in the southern part of Tibet is situated at an altitude of 4800-5000 m, on the northern edge of the arid – at an altitude of 5200 m in the center of Highlands – at an altitude of 6400 m. The high position of the snow line leads to a relative reduction in the area of glaciation, represented mainly by "dead" ice is not melting because of the low temperatures of the summer.

For the development of arid and semiarid landscapes the real climatic conditions, therefore, are only in the lower zones Tseyatralnogo (Changtan), Northern (Kunlun, Nanshan) and Northwest (northern Karakoram) Tibet. On the eastern and southern periphery of this country in the world Deserts do not. The main part of surface runoff in Tibet formed by melting snow. Rivers of Tibet dry, and a large volume of runoff have only river in the humid windward periphery of the country. The largest lake in Tibet – Nam Tso (Tengri-Nur and Siling-Tso (Selling). All of arid and semiarid regions of Tibet treeless. Only in the mountains surrounding the basin grow spruce, aspen, pine and birch forests with an admixture of maple, hazel, Fieldfare and set shrubs, and in the highlands developed a colorful meadow. In Tibet, the rigorous selection of plant and animal species was determined by testing them on a dry, low temperatures, short growing season, harsh ultraviolet insolation, high winds, salt-poor and undeveloped soil. The modern flora of desert part of Tibet is very poor: its strength is estimated at a thousand species, but perhaps this figure is somewhat understated.

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