Trends in sunshine duration in Poland (1971-2018)

This study aims to characterize temporal and spatial trends of sunshine duration in Poland based on data from the years 1971-2018 and to make an attempt to explain their causes. Daily sunshine duration came from 31 Polish meteorological stations. Data from 10 stations came only from a traditional Campbell-Stokes heliograph, while in the other 21 synoptic stations Campbell-Stokes heliograph was replaced with an automatic sensor in 2014 and since then only such data are available. The smallest sunshine duration occurs in December (average areal total-39.7 hr), and the highest in July (average areal total-230.1 hr). The spatial distribution of sunshine duration in Poland shows great diversity associated with different day lengths depending on latitude. In the cool part of the year (November, December, January, and February) the isolines of totals of sunshine duration run from the highest values in the south to the lowest in the north of the country, while in the months of the warm part of the year (March to September) the system is reversed. The multi-annual trends in sunshine duration in Poland are similar to the changes taking place in other parts of the world. The years 1971-1980 mark the end of "global dimming," which was manifested in very low values of actual and relative sunshine duration. "Global brightening" confirms a statistically significant growing trend in sunshine duration at all the analysed meteorological stations in Poland from April to September. There is a strong statistically significant relationship between the areal totals of sunshine duration and the type of pressure system, average monthly cloud cover, and atmospheric optical depth.