The beginning dates and the lengths of thermic seasons in Warsaw (1951-2004) were analysed. These characteristics of seasons are very variable, especially in winter. The changes of winter length influence strongly the dates and length of adjacent seasons, that is, of early winter and early spring. The current warming is best visible exactly during these seasons: winter becomes shorter, while early winter and – to an even longer degree – early spring become longer. The period of spring, summer and autumn is characterised by grater stability. These seasons showed a very weak tendency to change on even its lack, both as regards the length and the beginning date.
This paper aims at presenting changes in everyday air temperature values, triggered by the contemporary warming process. The analysis has been based on the mean, maximum, and minimum daily temperature values measured in Warsaw between 1951 – 2003. The mean daily temperature in that period was between −24.6 and 28.4°C, absolute minimum temperature was −30.7°C, absolute maximum temperature amounted to 36.4°C.
Calculations indicate that the number of days with mean temperature ≤ −5.0°C (minimum < 0.0°C, maximum < 0.0°C) in the last several years decreased. This trend slowed down at the beginning of 21st century, nevertheless, the number of days with mean daily temperature > 20.0°C and maximum temperature > 25.0°C was growing, particularly in the 1990’s and even more so in early 2000’s. Also since 1990’s, there has been increasingly more nights with minimum temperature > 15.0°C, which has been particularly apparent in 2001. Contemporary warming is then marked with an increasing frequency of the hottest days and decreasing frequency of the coldest days. These changes were asymmetrical beyond 1950’s, yet, in late 1990’s they coincided.