Frost periods and frost-free periods in Poland and neighbouring countries

The main objective of this article is to analyse the multiannual variability of frost occurrence and the length of the frost-free period in Poland and neighbouring countries between 1971 and 2010. A frost day was defined as a day on which the minimum temperature was below 0 degrees C and the maximum temperature was above 0 degrees C. On average, between March and November, there were 30 frost days in Poland and neighbouring countries, and their number ranged between 12 and 61. In the analysed multiannual period, there was a statistically significant tendency for the last spring frost to be earlier. Conversely, the first autumn frost occurred increasingly late, although the changes were not statistically significant. All but three stations (Augsburg, Kaliningrad, Olsztyn) recorded an increase in the length of frost-free periods, which was statistically significant in 51% of stations. The pressure condition conducive to the occurrence of frosts was anticyclonic circulation, as confirmed by the SLP and z500 hPa maps which were drawn up, and the Grosswetterlagen (GWL) classification types which were distinguished.