Long-term variability of the low-level cloud base height in Poland

The aim of the study is to characterize the long-term variability of the low-level cloud base heights (CBH) in Poland in the years 1971-2020. Data were used from six weather stations belonging to the Institute of Meteorology and Water Management-the National Research Institute (IMWM-NRI), where CBH was first measured using the IWO device and, since the 1990s, using a ceilometer. The analysis allowed for data from eight observation periods (0000, 0300, 0600, 0900, 1200, 1500, 1800, 2100 UTC), which referred to several height ranges (between the ground surface and 2500 m a.s.l.). In the ana-lysed multiyear period, in the cool half of the year, the height of theCumulonimbuscloud bases increased significantly, that is, the number of cases increased at a height of>1000 m and, in the warm half of the year, at>1500 m. In recent years, the number of cases of the Cumulus cloud with abase at a height between 300 and 999 m has clearly decreased, while it has increased at a height of 1000-1499 m (in October-March) and 1500-1999 m(April-September). A similar increase in CBH was found in the case of the Stratocumulus cloud (their greater share between 1500 and 2499 m). It was only in the case of the Stratus cloud that a lowering of the cloud base was noticed, possibly owing to the imperfect measurement of he fractus species.The positive upward trend inthe base heigh to most low-level clouds can be explained by a statistically significant increase in temperature and a decrease in relative air humidity in the lower troposphere.