FLUCTUATION OF SUNSHINE DURATION IN CENTRAL AND SOUTH-EASTERN EUROPE

Sunshine duration fluctuation in this century is studied from annual values for 26 stations in central and south-eastern Europe. Possible sources of inhomogeneity are identified: ageing of the glass ball in the sunshine recorder, changes in instruments, the recording tape and its evaluation, and location of the station, and exposure to air pollution. Long-term changes of annual sunshine duration are presented using a smoothed Gauss filter and principal component analysis. The main feature of sunshine duration fluctuation in this century is its sharp decrease after a maximum at the end of the 1940s. Principal component 1 shows, with small exceptions, positive loadings for the year and its seasons (for the period of 1931-1985). Principal component 2 divides the region studied into two parts, with positive loadings in the western and northern parts of central Europe and negative loadings over the rest of area analysed. A contradictory situation occurs only in autumn. Statistically significant cycles occur at intervals of about 2.3-2.4 years (western Germany), 3 years (Greece), 3.5-3.7 years (central Europe and Bucharest) and 3.7-4.0 years (Bulgaria). Some of the stations used indicate longer cycles (44-110 years). Comparison of coldest (1901-1920) and warmest (1934-1953) 20-year periods in the Northern Hemisphere shows for the warmest period an important increase in sunshine duration, with the exception of Athens and Bucharest and some other stations in winter. In the warmer period sunshine duration variability is generally higher in spring and winter and less in summer and autumn.