Air Frosts in Poland in the Thermal Growing Season (AT > 5 °C)

Abstract

Frosts cause damage to plants in field crops and also trees, thus contributing to heavy economic losses in agriculture. One of the consequences of climate warming is the lengthening of the thermal growing season (AT > 5 °C) and acceleration of phenological phases as well as the lengthening of the frost-free period. This favourable element allows the extension of the range of cultivated plants to include plants requiring warmth and a longer development period. The present study concerns the area of Poland. The data on mean and minimum 24-h period air temperature (200 cm above ground level—AGL) were obtained from 52 meteorological stations of the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB) for the period 1971–2020. A day with air frost was identified when the recorded minimum air temperature was below 0.0 °C and the mean 24-h period air temperature was above 0.0 °C. All calculations concerning frosts were limited to the period with mean 24-h period air temperature >5 °C (the growing season) as determined with the Gumiński method. The obtained results show that in the thermal growing season (AT > 5 °C) in Poland, no statistically significant change in the average number of days with air frosts in the period 1971–2020 was found. On average, in Poland, in the years 1971–2020, a lengthening of the thermal growing season by 6.2 days over 10 years was identified. Earlier disappearance of the latest air frosts in spring was identified as 2 to 3 days over 10 years, and the later occurrence of air frosts in autumn as 1 to 4 days over 10 years. The share of severe (−4.1°C ÷ −6.0 °C) and very severe (<−6.0 °C) frosts in the total number of days with air frosts in Poland amounts to, on average, 5.8% in spring and 2.6% in autumn.

Potential Risk of Frost in the Growing Season in Poland

“…Szyga-Pluta et al [29] found an increase in the length of the growing season in Poland by an average of 4.8 days over 10 years between 1966 and 2020. A slightly higher rate of the said changes, on average by 6.2 days over the period of 10 years, however, for another multiannual period (1971-2020), was determined by Koźmi ński et al [4]. In the thermal growing season (air temperature > 5 °C), the average number of days with ground frosts in Poland amounts to 28 and ranges in spring from 10 to 26, and in autumn from 6 to 16 (Figure 2).…”

“…It is noteworthy that there is a marked, statistically significant trend of an earlier ending date of the last spring frosts and increasingly later occurrence in autumn which, in turn, results in the lengthening of the frost-free period. Multiannual variability of the aforementioned characteristics, both local as well as throughout the country, was confirmed by numerous authors [3,4,11,[16][17][18][19][20][21][22][23].…”

“…On average, the earliest end of air frosts i.e., before 20 April, is recorded on the coast, and the latest end i.e., after 5 May, in the Narew River basin. In turn, the average dates of the occurrence of the first autumn frosts fall in the first decade of October in the northeast of Poland, whereas along the coast, in the last pentad of the said month [4]. It is noteworthy that there is a marked, statistically significant trend of an earlier ending date of the last spring frosts and increasingly later occurrence in autumn which, in turn, results in the lengthening of the frost-free period.…”

“…Weather anomalies, increasing temperatures, the shifting boundaries of agroecosystems and increased incidence of extreme weather phenomena such as drought, hail, flood or hurricane necessitate adaptation. However, despite the observed global warming, many regions, for example in Switzerland, Germany, or Poland, face a non-decreasing risk of frost [1][2][3][4]. This is because climate change causes an acceleration of both the beginning of the growing season, as well as the occurrence of the last spring frosts.…”

“…In Poland, frosts occur in every growing season. Spring frosts are recorded most often in April (particularly in the second decade), and autumn frosts in October (most frequently in the third decade) [3,4,10,11,16]. Ground frosts in spring disappear throughout most of the country, on average, between May 10 and May 25, and in autumn their occurrence is recorded between September 20 and October 5 [3].…”