Secular changes (1826-2021) of human thermal stress according to UTCI in Krakow (southern Poland)

The set of meteorological data from the Jagiellonian University station in Krak?ow is one of the longest-recorded in central Europe. Measurement records date back to 1826. Based on the average monthly values of air temperature, total cloudiness, water vapour pressure and wind speed at 1200 UTC, the UTCI values were calculated and the occurrence of cold stress (UTCI < -13C) and heat stress (UTCI > 32 degrees C) was estimated. The variability of the applied mea-sures of thermal stress conditions was studied both annually and in individual seasons (MAM, JJA, SON, DJF). Statistically significant changes in the studied characteristics of the bioclimate were found. Average annual UTCI values increased at the rate of 0.27 degrees C/10 years. In individual seasons, the trend of UTCI changes ranged from 0.10 degrees C/10 years in summer to 0.43 degrees C/10 years in winter. The number of days with cold stress decreased by 1.8 days/10 years and the number of days with heat stress significantly increased (0.5 days/10 years). Two subperiods with different trends of changes were determined. The first covers the years 1826-1940, and the second, 1941-2021. The former indicates a negative UTCI trend of -0.17 degrees C/10 years, and the latter, an increasing trend of 1.09 degrees C/10 years. The last eight decades have brought a decrease in cold days of 6.5/10 years and an increase in heat days of 2.6/10 years.