Seasonal and long-term variations in the atmospheric extinction in crimea on the basis of the 1979-2002 observations
Seasonal and long-term variations of atmospheric extinction are investigated using several observational programmes in standard UBV photometric system during 24 years (about 1300 measurements for 592 nights). The seasonal curve has a wide maximum from the end of March up to the middle of September and a narrower minimum from the middle of November to the beginning of February. These features are connected apparently with motions of the air masses due to the global atmospheric circulation. In summer there is an additional narrow maximum, which is more strongly expressed in ultraviolet. It is connected apparently to enhanced content of dust. On average, the values of extinction coefficients for V, B and U are equal to 0.19, 0.35, 0.74 magnitudes at minima and 0.43, 0.65 and 1.12 at maxima respectively. The scatter of individual values is very great; during any season, almost from the minimally possible (Rayleigh scattering) up to approximately twice the average maximum. All numerical values correspond to extinction on clear nights since they are obtained from the measurements of stars. The relation between the long-term variations and the cycles of solar activity is very weak. However, some improvement in transparency which probably is connected to the decrease in industrial emissions in the atmosphere in adjoining territories, has been recorded in the last 7-9 years.
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