Observed Intensification of Heat Waves and Heat Stress in Poland: Long-Term Context and Selected Case Studies

Heat waves are among the most relevant manifestations of ongoing climate change in Central Europe, with an increasing frequency of hot days and tropical nights, especially in urban areas. This study synthesizes and updates observational evidence on the intensification of heat waves and heat stress in Poland over the last five decades (1976-2025), with particular attention to changes in frequency, duration, and thermal severity, as well as to differences between temperature-based extremes and human-perceived heat stress. The analysis is based on ERA5 Copernicus data (for background maps), the database of the Polish Institute of Meteorology and Water Management - National Research Institute (IMGW-PIB), records from the University Meteorological Station in the city centre of Lublin, and urban measurements from HOBO sensors.

The intensification of heat stress is reflected in the increasing number of days with strong heat stress according to different biometeorological indices. The fastest increase is observed in southern Poland, especially in the upland region, including Kraków and Rzeszów. Seasonally, the highest frequency of such days occurs in July, followed by August, while June remains the least affected month.

During one of the most intense heat-wave episodes in Poland - the August 2015 mega-heatwave, which produced prolonged heat stress in many Polish cities - the number of hours with strong heat stress (UTCI > 32°C) exceeded 110 in Wrocław (south-western Poland).

Another city in southern Poland characterized by high heat stress is Lublin, which in this study serves as a representative mid-sized Central European city with a heterogeneous urban structure typical of many cities in the region. In Lublin, the observed rise in minimum and maximum air temperature (0.067 and 0.083°C per decade, respectively) has been accompanied by increasing numbers of heat days and tropical nights, a pronounced urban-rural contrast, especially at night, and stronger heat stress in densely built-up districts than in greener or more open urban areas. In the long-term record, the city centre experiences an average of 7.2 heat days per year, compared with 5.6 in the city outskirts. The record values were 26 in 2024 in the city centre and 22 in 2015 in the outskirts. Tropical nights are also much more frequent in the city centre than in the outskirts, averaging 2.1 nights per year in Lublin and 0.2 in the surrounding area.

By combining long-term trends with selected urban case studies, this study shows that changes in extreme heat in Poland are clearly detectable in observational records and should be assessed not only by air temperature, but also by biometeorological indices that better capture the actual human heat burden.