Hourly data of global and diffuse solar radiation in combination with cloud type, cloud cover amount and sunshine duration records are analysed for two sites in Germany. The global solar radiation for overcast conditions at urban, maritime influenced Hamburg and at the rural mountain station Hohenpeissenberg decreased by an average rate of 8 per cent per decade between 1964 and 1990, whereas no significant trend was found under clear skies at either station. However, under partly cloudy conditions the diffuse portion of global solar radiation also declined. Although the total cloud cover amount changed little during the interval analysed, the frequency of cirrus cloud increased in Hamburg and Hohenpeissenberg by 12 per cent and 14 per cent respectively. This may have led to the decline of diffuse solar radiation in cloudy conditions. A shift from stratiform to more frequent convective clouds also occurred at both stations. At Hamburg and Hohenpeissenberg the mean direct solar radiation income was 10 W m(-2) higher during hours with 100 per cent sunshine duration and observed clouds than during hours with totally clear skies. Possible reasons for the observed cloud changes include natural weather variability of convective and stratiform clouds, increase of aircraft traffic increasing the occurrence of cirrus clouds and an indirect aerosol effect on the maritime stratiform clouds. (C) 1997 Royal Meteorological Society.
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