Celem opracowania jest charakterystyka odwilży i dni odwilżowych w Hornsundzie. Odwilże rozpatrywano podczas 34 sezonów zimowych od 1 grudnia 1982 roku do 30 kwietnia 2016 roku. Pod pojęciem odwilży rozumiano okres co najmniej jednodniowy z temperaturą dobową maksymalną ≥ 0°C, który następuje bezpośrednio po co najmniej dwudniowym okresie mroźnym, czyli z temperaturą dobową maksymalną < 0°C. Dni odwilżowe definiowano jako wszystkie dni z dodatnią temperaturą maksymalną ≥ 0°C w sezonie zimowym, po co najmniej dwóch pierwszych dniach mroźnych po rozpoczęciu tego sezonu. Dokonano oceny zmienności wieloletniej występowania odwilży i dni odwilżowych, określono ich częstość w sezonie zimowym, daty początku, długość ciągów dni, uwarunkowań cyrkulacyjnych ich występowania i warunków meteorologicznych im towarzyszących. Stwierdzono wzrost częstości i długości trwania odwilży oraz przesunięcie zwartego okresu mroźnego z grudnia i stycznia na luty i marzec. Tendencje te są szczególnie widoczne od początku XXI wieku. Występowaniu odwilży sprzyja adwekcja ciepłego powietrza z południa.
Long-term variability in the occurrence of thaw periods can be used as an indicator of climate change in the polar zone due to the snow and ice melting processes associated with them. The study looked at the thaw period and days with thaw parameters at Hornsund and included the long-term variability, frequencies, onset timing, permanence, associated atmospheric circulation and accompanying weather conditions. The study limited itself to looking at thaw periods during the winter season defined by Marsz (2007), which runs from 1 December to 30 April. Thaw was defined as a period of at least one day with a maximum daily temperature equal or greater than zero degrees (TMAX ≥ 0°C), which followed immediately after at least two days of sub-zero temperatures (TMAX < 0°C). Days with a thaw were defined as days with an above-zero daily maximum temperature (TMAX ≥ 0°C) following after at least the first two sub-zero days of a given winter season. The study employed records of daily measurements of air temperature, depth of snow-cover, precipitation totals and wind speed and direction made at the Polish Polar Station in Hornsund during 34 winter seasons from 1 December 1982 to 30 April 2016. During that period, there were on average eight thaw periods per season. The highest number of thaws (15) was recorded in 2015/2016 and the lowest (4) in 1987/1988, 1993/1994 and 2001/2002. The timing of thaw onset varied between seasons, but was most frequent in December (27), followed by January (5) and February. Typical thaws lasted between one and three days in length, but there were many warm spells even exceeding ten days. These longest thaw periods only appeared in Hornsund in 1996, as previously they had never lasted for more than nine days. The longest such warm spell during the study period was recorded between 31 January and 18 February 2014. It was accompanied by a cyclonic situation with air advection from the south-east and south (Niedźwiedź 2016), a peak air temperature of 4.4°C, a wind speed of more than 10 m/s, and a snow cover shrinking rate of 10 cm in four days. Since the beginning of the 21st century, there has been a steady increase in the frequency of days with a winter thaw. In the 2005/2006 season there were 67 such days, while three other seasons also had more than the until-then unheard of 50 days of thaw. The least days with a thaw (6) were recorded in the 1987/1988 season which had the lowest air temperature during a thaw and the lowest number of thaw periods (4), among which there was one three-day spell in January and three isolated days in February, March and April. A total of 18 seasons during the study period, had thaw days in each month, while in the other seasons there would be unbroken monthly or longer periods with freezing temperatures, typically in March, but also in December, January, and February. Winter season weather in Hornsund is primarily determined by atmospheric circulation due to the limited or non-existent solar radiation. The most frequent are cyclonic situations with air advection from the east (Ec peaking in January), north-east (NEc peaking in December) and south-east (SEc peaking in February). April stands out with relatively frequent (more than 8%) anticyclonic situations (Ka and NEa). There are almost no incidences of anticyclonic situations with western or north-western advection (Wa, NWa – less than 1%). Thaws are most likely in the following situations: December – SWc and Sc, January – SWc, Wc and Sc, February – SWa and SWc, March – SWa, Wc ad SWc, and April – SWa, Sc and SWc. In general, the situation favouring a thaw involves air advection either from the south-west regardless of the pressure system, or from the south and west in cyclonic situations.