Usłonecznienie w miastach na podstawie wybranych stacji w Europie
(Sunshine duration in urban areas based on selected stations in Europe

Summary/Abstract: The aim of the study is to characterize the variability of sunshine duration in selected Europeancities based on the long-term series of heliographic data. The paper is also a voice in the discussion onthe methodological concerns regarding homogeneity of climatological series available in online databases,which are used for the analysis of climate change. A review of the long-term heliographic seriesin Europe was performed and, after a laborious verification of the data from different online databasesfor further analysis, 12 stations were selected with certain reservations: Krakow, Vienna, Potsdam, Groningen, Basel, Lugano, Geneva, Bradford, Durham, Oxford, Sheffield, Southampton.Long-term course was presented based on the longest possible series for each station. For comparisonsof monthly and annual values of sunshine duration and relative sunshine duration, the data fromthe common period covering the years 1929–1991 was used. For the detailed characteristic the longestheliographic series in Europe were used: Krakow and Potsdam series (common period: 1893–2014).An analysis of the correlation of monthly sums of sunshine duration showed that all relationshipsare statistically significant at the level of < 0.05. The strongest relationships exist between the Britishstations, especially between Oxford and Southampton (0.85). The heliographic series of Swiss stationsshow a strong correlation as well (Geneva and Lugano – 0.74). The series of Krakow has the highestcorrelation coefficients with the sunshine duration values from Vienna (0.73) and Potsdam (0.61) andthe lowest with the British stations. The long-term and annual course of sunshine duration is verysimilar in the selected European cities. Common periods of increase and decrease of sunshine durationcan be noted, as well as extreme values occurring in the same years (max – 1921, 1943; min – 1980).It is worth noting that similar trends in long-term course of annual sums occurred not only in periodsof “global dimming” and “global brightening” but also at the turn of the twentieth century and in thefirst half of the twentieth century. It is difficult to determine the cause of long-term sunshine durationvariability in European cities. Similar trends over such a large area evidence the global determinants ofsolar radiation, modified only by local factors. Macro scale conditions might have both circulation andanthropogenic origin (e.g. the industrialization in the 50s to 80s of the twentieth century).In all the cities the annual variability of sunshine duration depends mainly on the position of thestation – latitude and circulation conditions, affecting the size and type of cloud cover. Major pollutionin cities certainly affects the reduction of air transparency and reduction of sunshine duration. However,the impact of this factor, with the dominant role of cloudiness, is very difficult to grasp and requiresfurther study.

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