|Climate change trends and scenarios|
|<< FACTORS DETERMINING THE WEATHER AND CLIMATE IN CENTRAL ASIA AND TAJIKISTAN >>|
|Climate is determined by 3 main factors:
solar radiation, atmospheric circulation, and shape of the landscape.
Surface solar radiation depends on the cloudiness, sandstorms, haze, fog, etc. The input of solar radiation is determined by the processes of global circulation which is mainly a planetary factor. In the complex mountain conditions of Tajikistan atmospheric circulation incurs significant changes, forming local peculiarities.
The types of atmospheric circulation in Central Asia, and particularly in Tajikistan, are diverse: cyclonic, anti-cyclonic, frontal and local (highland-valley, breeze, etc.).
The cyclonic circulation takes place in the areas with low pressure, and is characterized by a turbulent movement of air against the clock. The airflows trajectory inclines towards the minimal pressure, namely, to the storm center. Due to a convergence of airflows, according to the law of conservation of mass, the air is displaced upwards. While getting upwards, the mass of air is expanding, getting into strata with lower pressure; it cools down adiabatically (1°C/100 meters of altitude) and saturates with moisture. Thus, a formation of vast cloud systems and rainfalls constantly occur in the cyclones.
Anti-cyclonic circulation occurs in zones of high pressure. Any area with a high pressure is characterized by a turbulent movement of the air clockwise. At that, the airflows trajectory inclines from the center of maximum pressure towards it periphery. It leads to a divergence of airflows, that give rise to vertical movement of the air top-down. This factor moves air masses into the strata with higher pressure down and warms up adiabatically (1°C/100 meters of altitude). The warming of air leads to an abrupt decrease of the relative humidity, which in turn, leads to a dissipation of cloud systems, and favors to fair weather without rainfall.
Depending on the type of weather, synoptic processes in Central Asia can be classified into four main categories:
À. Cyclonic circulation.
There are three basic types of cyclones: the South-Caspian, the Murgab and the Upper-Amudarya. Snow precipitation throughout Tajikistan is caused mainly by the latter.
B. Cold wave intrusions.
According to areas of origin and direction of movement, cold intrusions are subdivided into three types: western, northwestern, and northern. The most abrupt deterioration of weather occurs when intrusions move in the rear of cyclones. Northwestern cold intrusions in the dry season form sandstorms and haze. Northern cold intrusions in the summertime often cause unstable weather on the northern and northeastern slopes of the Turkestan and Zeravshan ranges.
C. The group of synoptic processes causing unstable weather.
Three types of processes belong to this group: confining cyclones, wave activities, and the southwestern and southern peripheries of anti-cyclones. Wave-activities are typical for highland areas; they take place in cold frontal partitions. Such a process can continue from several days to one week, accompanied by significant rainfall. When an influx of the arctic air masses occurs along the southeastern periphery of anti-cyclones of western and northwestern humid intrusions, it is accompanied by widespread rainfall in the winter and springtime.
D. The group of synoptic processes conditioning fair weather.
The following processes belong to this group: southwestern peripheries of the Siberian anti-cyclone, thermal depressions, small-gradient fields of decreased or increased air pressure. Thermal depression is typical only in for the summer season, and is characterized by a very hot hazy weather. In 2000, it played a major role in forming a long drought in Tajikistan.
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