Atmospheric transmittance and its long-term changes in Estonia

Atmospheric transmittance as a characteristic describing the state of the atmosphere has been studied on the basis of direct solar radiation data obtained from Toravere Actinometric Station (Estonia) for the period 1955 to 1995. Enhanced turbidity is typical to summer, while the air is dearest in winter During the first half-year the attenuation of solar radiation by aerosol dominates over that by water vapour. The analysis of the year-to-year changes in direct solar radiation has revealed a decrease in atmospheric transmittance. The annual mean atmospheric transmittance has decreased by 4.6 % over the period 1955 to 1995. The largest decrease occurred in the monthly means of April (9.1 %). The extinction of direct radiation by aerosol has been observed to have a tendency to increase. The annual averages of solar radiation attenuated by aerosol have increased by 47.0 % over the above-mentioned period.