A statistically significant decrease in mean annual global solar radiation between 1964 and 1990 under completely overcast skies was found at five out of eight studied locations in Germany. A decrease of global solar radiation is also evident in partly cloudy conditions. The mean annual cloud cover fraction and sunshine duration did not significantly change, while the visually assessed mean annual horizontal visibility increased at six of the eight stations. The authors' findings point to a decrease of the cloud transmissivity, which in turn could be explained by an increased frequency of multilevel cloudiness, changing cloud types, or by indirect aerosol effects on clouds. The decreasing trend of global solar radiation in clear skies was most expressed at high and intermediate solar zenith angles, whereas a slight increase of global radiation was found at low sun zenith angles. A decline of the diffuse component of the global radiation over time was also detected. It is shown that the observed changes in clear-sky radiation were probably related to the recovery from the effects of major volcanic eruptions in the mid-1960s and 1980s. Increase of submicron aerosol particles with simultaneous reduction of aerosol mass concentrations reported by others and increasing absorption by urban aerosol may also contribute to the observed changes. The results are based on statistical analyses of hourly data of solar radiation, sunshine duration, cloud cover, and horizontal visibility stratified by solar zenith angle.
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