Fale mrozów w Poznaniu i ich cyrkulacyjne uwarunkowania

Abstract

The article aims to determine the long-term variability of frosty days and frost waves occurrence in Poznań, as well as to define the pressure conditions triggering off frost waves. A frosty day was defined as a day with the maximum temperature below the 5th percentile of the daily maximum temperature from a whole year (-1.7°C). A frost wave was defined as a sequence of at least 5 frosty days. In the analysed period, there were 44 frost waves which in total took 406 days. The most frequent were 7-day waves which represented 25% of all the frost waves. These tended to occur between November and March, with a majority of them (as much as 54.5%) in January. The occurrence of frost waves in Poznań in the discussed multiannual period was related to a ridge of high pressure above Eastern and Central Europe. During frost waves, the pressure over the analysed area was higher than the average in the winter season. According to the Grosswetterlagen (GWL) classification of weather types, frosty days forming frost waves were mainly associated with the anticyclonic circulation (54.0%).