Long-term variations of available solar radiation on seasonal timescales in 1955–2006 at Tartu-Tõravere Meteorological Station, Estonia

The time series of the daily sums of global and direct irradiance recorded at Tartu-Toravere Meteorological Station site (58°16′N, 26°28′E, 70 m a.s.l.) in 1955–2006 have been analyzed in seasonal timescales. The average daily ratio G/G clear of available global irradiance to its local climatic clear-sky value in the summer half-year corresponds to 65.5%, while that of the direct irradiance on the horizontal surface I′/I′ clear was 41% of the climatic clear-sky value. In the case of dry Rayleigh atmosphere as a reference, these ratios are 53.5% and 28%, respectively. The time series of the summer season totals reveal a longer interval of reduced values in 1976–1993 as well as two periods of frequent sunny summers in 1967–1975 and since 1994. The probability density distribution of the summer season totals during the observed period is strongly asymmetric; in spring, however, it is close to the normal distribution. In winter, there is a moderate negative correlation between the G/G clear and the North Atlantic Oscillation as well as the Arctic Oscillation indices.