Biometeorological conditions during cold spells in south-east Poland and west Ukraine.

The aim of this research is to analyze the biometeorological conditions, based on the Physiologically Equivalent Temperature (PET) thermal index, during cold spells (CSs) in south-east Poland and west Ukraine during the years 1966-2021. The research shows a high variability of the occurrence of CSs in the study period and a clear increase in the frequency and total duration of CSs in the east of the study area. The number of CSs in the analyzed years varies from 6 cases in the west (in Katowice) to 34 in the east of the study area (in Shepetivka). The total duration of CSs varied from 26 days (in Raciborz and Katowice) to 166 days (in Rivne). At the majority of stations, CSs occurred most frequently in the first two decades (1966/1967-1975/1976, 1976/1977-1985/986) and in the last full decade (2006/2007-2015/2016). The average PET values at 12:00 UTC during CSs decreased eastwards throughout the study domain and were generally lower than -20.0 °C in the west of Ukraine, while in south-east Poland varied between -18.1 and -20.0 °C. At 40% of stations across the study domain, the lowest average PET values were recorded during a cold spell in January 1987, with PET values varying from -28.0 °C in Chernivtsi to -12.7 °C in Yaremche. The longest or one of the longest spells in most stations (in 77% of stations across the study domain) was the cold spell of 2012 and characterized by mean PET values ranging from -25.4 °C in Rivne to -19.5 °C in Zakopane.