Małgorzata Czarnecka

Abstract The aim of the study presented here was to identify distinct Polish regions from the point of view of thermal winter, and in line with two developed criteria, i.e. average monthly and daily values for air temperature. The research in question was based on data series for the November-April periods in the years 1960/61–2014/15 inclusive, as obtained from 36 weather stations of the Institute of Meteorology and Water Management (IMGW). Start and end dates of thermal winter, defined as the period with mean daily temperatures below 0°C, were identified by reference to mean monthly values for air temperature calculated with the commonly-used calculation by Gumiński. For the sake of simplicity, dates and durations of thermal winters calculated by reference to mean daily temperature values are referred to simply in terms of their being “the winter period”. The beginning of such a winter period is thus marked by the first (and the end by the last) occurrence of at least a three-day series of mean daily air temperature below 0°C. Such characterisations of thermal winters and winter periods for the different stations (relating to start and end dates, durations and durational coefficients of variability) were inputted into cluster analysis, with the result that three general regions of Poland featuring similar patterns for negative air temperature were identifi ed. In the identified western region, the period of negative air temperature is the shortest and is also characterised by the highest inter-annual variability, particularly when it comes to winter days in its north-western sub-region. By contrast, the longest thermal winter period, along with the lowest inter-annual variability is found to characterize the so-called eastern region. In comparison with the western, this region differs particularly significantly in terms of mean air temperature values, given that winter lasts almost twice as long there, and is recorded almost every year. The western region displays the highest inter-annual variability where features characteristic for the occurrence of negative temperature are concerned, while the eastern region shows the lowest variability. Over the analysed period, there was no significant linear trend noted for the start and end dates of both thermal winter and the winter period. However, it was possible to note a tendency for thermal winters in the central and eastern regions of Poland to shorten. Only in the eastern region was it possible to note a decrease in the frequency of occurrence of winter days. In the central region, the durations of thermal winter periods determined using mean monthly air temperatures are indeed the same as the actual numbers of days with 24-hour air temperatures below 0°C in an average multi-annual period. In contrast, in the western region the true length of the thermal winter is underrepresented by 7 days (i.e. shorter), while in in the eastern region it is overrepresented by 6 days (i.e. longer). However, over the analysed 55-year period, almost every region reported instances of thermal winters being overrepresented as well as underrepresented in terms of duration, with the deviation in many seasons being in the 20-40–day range. In the western region, durations of thermal winters were seen to be underrepresented twice as frequently, when comparisons with actual numbers of winter days were made, while in the eastern region there was overrepresentation. Mean monthly air temperature, as the criterion adopted most typically in identifying thermal winters in Poland, does offer a best reflection of the durations of winters in the central region. In contrast, in the cases of the western and eastern region identified, mean 24-hour air temperatures would seem better suited to determining thermal winters.

Małgorzata Czarnecka

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