Variability in the occurrence of thermal seasons in Poland in 1961–2020

The article analyses the variability of thermal seasons in Poland in 1961–2020. Based on the average monthly air temperature values from 55 stations, the onset and end dates as well as the duration of the six thermal seasons were calculated. Taking into account the thermal thresholds of 0°C, 5°C and 15°C for each year, early spring, spring, summer, autumn, early winter and winter can be distinguished. A significant spatial differentiation of the dates of the beginning and duration of the thermal seasons in Poland was observed. The influence of continentalism (in the east) and oceanic climate (in the west) and the influence of the Baltic Sea are noticeable, and in mountainous regions, altitude above sea level is the main factor. The air temperature in Poland increases at the rate of 0.3°C–0.4°C/10 years. This causes significant changes in the occurrence and duration of the thermal seasons. An early beginning of early spring and spring was observed. Summer arrives earlier and is significantly longer, becoming the longest season of the year (above 110 days in the south of Poland). Autumn and early winter are delayed and get shorter, whereas winter comes much later. Throughout the country, winter is 44 days shorter, and in the north-west it does not occur at all. Changes in the onset of thermal seasons are a clear indicator of progressing global warming. They have also a decisive impact on the environment and human activity.