University of Silesia, Faculty of Earth Sciences, Department of Climatology - Poland
The main aim of this study was to present the thunderstorms' occurrence in Poland in 20th century. The paper also attempts to assess an impact of atmospheric circulation on the variability of number of days with thunderstorm and estimate synoptic conditions conducive to the formation of thunderstorms. The analysis was based on thunderstorm observations from 56 synoptic stations in Poland in the period 1951-2000 and 7 stations for the years 1885-2000. In the second part of the study the types of synoptic situations elaborated by Osuchowska-Klein, the types of synoptic situations by T. Niedwied, indices of atmospheric circulation by Z. Ustrnul, index of North-Atlantic Oscillation (NAO) and synoptic maps were used.
As a result, it was determined that during analysed period no tendency was found in the multi-annual course of mean yearly number of days with thunderstorm in Poland, and the values usually do not differ much from the value typical for the climatic-geographical region in which Poland is situated. Periods with significantly higher number of days with thunderstorm are also difficult to find. The only season in which the number of days with thunderstorm increased was winter. This is in accordance with the tendencies observed in the whole Europe and most probably is due to the increase of the western circulation in winter.
Large differences in the number of days with thunderstorm at particular stations were found, even if the stations were located quite close to each other. Again, it is difficult to find periods when number of days with thunderstorm would be significantly larger than the average value in the regions or at certain stations. Apart from 2 stations no increasing or decreasing tendencies were found. Therefore the hypothesis that the last years of the 20th century were marked with exceptional increase of the number of days with thunderstorm cannot be confirmed.
Synoptic situations have less influence on the thunderstorm occurrence than it is usually suspected. The eastern advection has the largest influence on the occurrence of the days with thunderstorm, especially in southern Poland. The analysis of relations between the variability of number of days with thunderstorm and the changes of the circulation indices proved these regularities.