| Summary: | The main aim of this study was to present the thunderstorms’ spatial and temporal occurrence in Poland in the period 1949—1998. The paper also attempts to assess an impact of atmospheric circulation on the variability of number of days with thunderstorm. At the same time the study fills a gap in Polish climatological research into thunderstorm occurrence and variability based on the long-term series. The research was based on records on thunderstorms from the period 1949—1998, gathered at 56 meteorological stations which belong to the Institute of Meteorology and Water Management. The main indicator analysed in the study was the day with a thunderstorm. In the second part of the study, devoted to the influence of synoptic situations during which the thunderstorms occurred, the types of synoptic situations elaborated by B. Osuchowska-Klein (1978, 1991, and 1998) for the period 1949—1998 were used. Other sources used in the project were the types of synoptic situations by T. Niedźwiedź (1999) for the period 1949—1998, indices of atmospheric circulation by Z. Ustrnul (1999) for the period 1949-1995, index of North-Atlantic Oscillation (NAO) from the period 1949—1997 (North Atlantic Oscillation, 1998) and synoptic maps from the Institute of Meteorology and Water Management published as Biuletyn synoptyczny PIHM (then IMGW) (Biuletyn., 1959—1979), Mapy synoptyczne (Mapy., 1952—1981) and Codzienny biuletyn meteorologiczny (Codzienny biuletyn., 1980—1981) from the years 1952-1981. The analysis of 50-year series of number of days with thunderstorm, carried for stations from all over Poland, revealed some aspects of spatial variability of that element. It also allowed to find areas with different probability of the thunderstorm occurrence and to define the differentiation of noted numbers of days with thunderstorm in shorter observation periods. Spatial analysis of the occurrence of number of days with thunderstorm in Poland showed that even though in Poland there are 24 such days on average, the number varies from 15 in ...
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