Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia

Data obtained during 1955—1986 at the Tôravere Actinometric Station (Estonia, USSR) are used to study the long-term variations of the direct (S') and global solar radiation (Q) at the earth’s surface. During these years, a certain decreasing trend was observed both in S' and in Q (ΔS'=- 13%, ΔQ=- 6.8%). The results from Tôravere are compared with the data on global radiation acquired in 1964—1986 at 9 different actinometric stations in northern Europe (8) and western Siberia (1). Decreasing trends have been observed in Helsinki and Stockholm (ΔQ = - 11%), and also in Kaunas (ΔQ = 12%) in the same period. The variations of the cloudiness regime and atmospheric transparency as the main reasons for the decrease of radiation are discussed. According to the measurement data from Tôravere, the mean annual amount of low clouds increased by 11%, whereas the value of the Bouguer atmospheric transparency coefficient decreased by 3.7%.