Abstract
The ongoing climate warming affects, among others, and the variability of thermal conditions in spring and autumn are resulting in earlier dates of the beginning and end of the growing season. The present paper provides detailed characteristics of the phenomenon of ground frosts, addressing the question of whether the risk of frost-related damage in the extending growing season is still present. The assessment of temporal and spatial distribution of ground frosts (5 cm above ground level-AGL) in Poland in the thermal growing season (AT > 5 °C) was conducted on the basis of the results of air temperature measurements at 5 and 200 cm a.g.l. obtained from 52 station of the Institute of Meteorology and Water Management—National Research Institute for the period 1971–2020. The thermal growing season was calculated using the method by Gumiński. The following were calculated: the dates of the occurrence of ground frosts in spring and autumn, duration of the frost-free period, the number of days with frosts of various intensity, according to years, ten-day period and days in a year together with trends of change. The conducted analysis demonstrated that the number of days with frosts in the thermal growing season in the multiannual period under analysis (1971–2020) does not show any statistically significant changes. It was found that in the thermal growing season, the average number of days with ground frosts in Poland amounts to 28 and ranges in spring from 15 to 22, and in autumn from 8 to 12. Most frequently, approx. 49%, slight frosts are recorded, followed by moderate (29%), severe (15%) and very severe (7%). A positive effect of water reservoirs on decreasing the frequency and intensity of frosts, as well as sporadic occurrence of the phenomenon in the second half of June were demonstrated. Statistically significant earlier dates of disappearance of frosts in spring, later dates of occurrence in autumn and the lengthening of the frost-free period from approximately 2 days in the north-east of Poland to approximately 8.0 days over 10 years in the Pomerania region, create increasingly more favourable conditions for the cultivation of plants with high thermal requirements in Poland.
Highlights
IntroductionAccording to the Glossary of Meteorology [1], frost can be defined as: (1) a decrease in air temperature below the freezing point of water during the growing period; (2) a decrease in air temperature below 0 ◦C during the prevailing occurrence of a positive average 24-h period temperature; (3) a decrease in minimum air temperature below 0 ◦C in a 24-h period positive maximum temperature.In the literature on the subject, frosts are distinguished and characterised, for example, due to: the origin i.e., advective, radiation, advective-radiation [2,3,4,5,6,7]; time of occurrence— spring and autumn [3,6,8]; altitude above ground level—air frosts at 2 m a.g.l. [6,9,10] and ground frosts at 5 cm a.g.l. [9,11,12]; as well considering their intensity [8,11,12] or frost-free period [6,8,10,13].The frosts recorded in Poland are generally of advective-radiation nature, owing to the approaching cold arctic air masses [14]
Polish literature on the subject analyses the variability of the number of days with ground frosts on a calendar basis, for example from April to October
As is discussed in popular specialist literature and in petitions submitted by farmers and gardeners [61,62,63] to the Minister of Agriculture, the date 15 April, stated in the definition of “spring frosts”, is questioned as being no longer valid owing to the ongoing global warming and increased thermal resources and the trend of an earlier occurrence of the thermal growing season
Summary
According to the Glossary of Meteorology [1], frost can be defined as: (1) a decrease in air temperature below the freezing point of water during the growing period; (2) a decrease in air temperature below 0 ◦C during the prevailing occurrence of a positive average 24-h period temperature; (3) a decrease in minimum air temperature below 0 ◦C in a 24-h period positive maximum temperature.In the literature on the subject, frosts are distinguished and characterised, for example, due to: the origin i.e., advective, radiation, advective-radiation [2,3,4,5,6,7]; time of occurrence— spring and autumn [3,6,8]; altitude above ground level—air frosts at 2 m a.g.l. [6,9,10] and ground frosts at 5 cm a.g.l. [9,11,12]; as well considering their intensity [8,11,12] or frost-free period [6,8,10,13].The frosts recorded in Poland are generally of advective-radiation nature, owing to the approaching cold arctic air masses [14]. The studies by Kozminski and Trzeciak [15] on the multiannual period 1951–1970 show that, in most regions of Poland, the last spring frosts were generally recorded from 26 April to 5 May. In turn, the analysis by Kalbarczyk [12] on the period 1966–2005 indicates later occurrence by, on average, 15 days. As is indicated by Michalska [16], the occurrence of last spring frosts in Pomerania may extend even to the end of June. The occurrence of ground frosts is recorded on individual days; there may be a series of two or more days [9,17]. The average ground frost-free period in Poland, according to the study by Kalbarczyk [12] on the period 1966–2010, is 120–150 days. In Pomerania, the period can extend to 130–160 days [16] and in north-western Poland it is by far shorter and lasts, on average, 112–145 days [9]
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