Zuzanna Bielec-Bąkowska

The paper identifies trends in the change in extreme air temperature characteristics, including the average, maximum and minimum temperatures, at 22 Polish weather stations during the period 1951 – 2006. The statistical analysis included the determination of ≤ 10 and ≥ 90 percentile temperatures, the number of extremely cold days ( Tmin ≤ –30 °C ), extremely hot days ( Tmax ≥ 35 °C ) and hot nights ( Tmin ≥ 20 °C ). It was found that the trends in the air temperature were not only related to changes in thermal conditions observed across Central Europe, but were often synchronised. In the long–term, there was a gradual increase in the temperatures, as was the number of extremely hot days, while the number of extremely cold days was falling. Around the year 2000, there were seasons and entire years that reached very high temperatures to the point, in some cases, of extreme values. Indeed, this was a period with the highest air temperatures since the beginning of instrument–based records. It also involved an increase in the number of extremely hot days and a decrease in the number of extremely cold days thus confirming trends observed earlier in these two types of days in Poland. A comparison of the long-term change in the maximum and minimum air temperatures reveals that their average annual values are always statistically significant, which suggests a warming of the region’s climate. The picture changes, however, when looking at seasonal and monthly values, both average and exceptionally high or low; and the change becomes less clear and mostly statistically insignificant over a large proportion of the country. This is confirmed by the results of long-term studies into the variability of extreme thermal conditions in the southeastern Polish region of Malopolska in the second half of the 20th century ( Bielec-Bąkowska and Łupikasza 2007 ). This may mean that the intensity of the climate warming observed is not high enough to be translated into a visible change in extreme conditions. The sources of variability of the thermal conditions in Poland also remain debatable. Natural factors, prominently featuring atmospheric circulation, seem highly influential, but are also strengthened by antropopressure, including the urban heat island. This latter proposition is confirmed by the fact that the temperature increase observed greatly exceeds the natural climatic variability (IPCC 2007).

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.

The study presents the spatial and temporal variability of warm and cold days occurring in the years 1966–2017 at five meteorological stations representing four peripheral areas and the centre of Poland. The analysis was based on the daily maximum air temperature values. A warm or cold day was considered as a day during which the maximum temperature was respectively higher or lower than the average daily temperature by a value of 1.28 standard deviation for a given calendar day. The obtained results indicate an increase in the frequency of the occurrence of warm days sequences and a decrease in the number of sequences of cold days, especially after 1990. The analysis also found a clear differentiation of the annual course of the considered days depending on the geographical location of the area in question. The most distinctive is the north-western region of Poland, where the highest number of warm days was recorded. In contrast, the area of southern Poland is characterized by a higher number of cold days. Most of the analyzed days occurred as one- or two-day long sequences, constituting approximately 70–80% of all cases. In the analyzed period, short (lasting 3–5 days) waves of warm or cold days were most frequent, and the longest waves (lasting at least 11 days) usually accounted for less than 1% of all cases recorded at a given station.