Increasing Frequency of Sultry Conditions in Poland Based on 60 Years of Meteorological Observations

Sultry conditions, resulting from the combined effect of high air temperature and humidity, represent an important component of human thermal discomfort and may intensify under ongoing climate warming. This study evaluates the occurrence and long-term variability of sultry conditions in Poland during the period 1966–2025 using meteorological observations from three synoptic stations representing contrasting geographical settings: the coastal station of Łeba, the lowland station of Sulejów, and the foothill station of Lesko. Four widely used indices (Scharlau criterion, Humidex, equivalent temperature, and apparent temperature) were compared, and long-term trends in the annual number of sultry days and the duration of sultry episodes were assessed using the modified Mann–Kendall test and the Sen’s slope estimator. All indices consistently indicated significant increases in the frequency of sultry conditions, although the estimated number of sultry days differed substantially among methods. Sultry conditions occurred most frequently in Sulejów and least frequently in Łeba, reflecting the moderating influence of the Baltic Sea. Inland stations also exhibited significant increases in the duration of persistent sultry episodes. Among the evaluated approaches, Humidex provided the most balanced representation of regional variability while remaining simple and readily interpretable. The results demonstrate a clear intensification of humid heat stress in Poland over recent decades and highlight the importance of considering both temperature and humidity when assessing climate change impacts on human thermal comfort.