Thunderstorms and hails in Poland

This paper is focuses on thunderstorms and hails in an attempt to address one of the most pressing issues in the debate on climate change, which is to understand sources of extreme meteorological and climatological phenomena. The study used records of observations at 24 weather stations covering the period 1949 – 2006 and thunderstorm details from eight stations in the period 1885 – 2008. The study identified no clear trend in the number of days with thunderstorms or with hails during the study period. For both phenomena there is a strong regional variation, both annually and in the long-term. This kind of spatial variation in the occurrence of thunderstorms and hails is characteristic of these phenomena in many areas of the world (Changnon 1988; Brázdil et al. 1998; Kuleshov et al. 2002). In Poland most of the stations that recorded a decrease in the number of days with thunderstorms after 1949 were found north of the line connecting Mt. Śnieżka in the southwest and the town of Suwałki in the northeast and the greatest decrease in this number was recorded in Słubice at 1.3 days per 10 years. A trend to an increase in the number of days with thunderstorms was observed south of the line. The greatest increase in the number of days with thunderstorms was recorded in Włodawa (1.7 days per 10 years), Lesko (1.6 days) and Katowice (1.1 days). Additionally, the study revealed that during the second part of XX century there was an increase in the number of days with thunderstorms in the cool half of the year at most of the stations involved. The pattern was particularly strong to the south of the Śnieżka-Suwałki line and in Szczecin. With the time scale expanded to the period 1885 – 2008, the changes are weaker, which clearly suggests that any patterns that emerge depend on the study period chosen. The spatial and temporal variabilities in the two phenomena are caused by the dominant types of atmospheric circulation and by regional differences in environmental conditions. Numerous authors have also suggested a link between changes in the occurrence of thunderstorms and hails and macroscale changes in the atmospheric circulation (Kamyshanova 1974; Changnon 1985). On the one hand, in some regions the influence of characteristic synoptic situations may be stronger than the dominant large-scale atmospheric circulation (Bielec-Bąkowska 2002, 2003), while on the other, local conditions are potentially an important factor leading to the free convection conducive to thunderstorms and hails. This seems to be a very important consideration in analysing hails, especially when they accompany thermal thunderstorms. An example is provided by an increased occurrence of these phenomena in an area of Poland that was up to 80 metres higher in altitude than an area, from which the air arrived (Zinkiewicz, Michna 1955; Koźmiński 1965). The annual pattern of hails occurrence seems to be its most significant characteristic. A springtime maximum dominates at stations in the north and in the western half of the country, while a far more even distribution with a maximum shifted towards the summer season is found in the south of Poland. The long-term record of days with hails was very even at most of the stations and periods with significantly higher or lower number of days with hail occurred at most of the stations simultaneously are also difficult to find