Long-term variability of cloud cover in Poland (1971–2020)

Abstract

The aim of this study is to characterize the long-term variability of cloud cover (amount and type of clouds) in Poland based on the data from 50 years (1971–2020) at three observation hours (06, 12, 18 UTC). Data from 34 weather stations operating as part of the Polish Institute of Meteorology and Water Management - National Research Institute (IMGW-PIB) were used in the study. In order to explain the temporal changes in the incidence of certain types of clouds, the average monthly values of air temperature as well as specific and relative air humidity at 4 isobaric levels (850, 700, 500, and 300 hPa) were also used. Statistically significant decreasing trends in the amount of low-level clouds (Nh) were shown for the year (Jan-Dec), as well as in the summer (Jun-Aug) and spring (Mar-May). The incidence of low- (St) and mid-level (As + Ns) clouds decreased significantly, while the incidence of high- (Ci + Cc + Cs) and mid-level (Ac) cumuliform clouds increased. A high dependence of the trends in the occurrence of individual types of clouds on thermal and humidity conditions in the troposphere was found. The decrease in the incidence of stratiform clouds was probably more influenced by the increase in temperature than the decrease in the water vapour content in the air.