This article seeks to analyse the universal thermal climate index (UTCI, °C), in order to characterise hazards associated with high air temperature that may possibly occur in Poland around noon on very hot days. Values for the index (calculated for UTC by reference to air temperature (°C), relative humidity (%), wind speed (v∙ms-1) and cloudiness (%)) related to from the Polish cities of Kołobrzeg, Poznań and Kraków, and to the period 2001–2018. Mean values with standard deviations were calculated, with minimum and maximum values noted, amplitudes, lower and upper quartiles and the skewness coefficient. One-way analysis of variance was deployed to determine whether UTCI values at midday on very hot days differ significantly from month to month. The frequency of occurrence of different intensities of heat stress was also determined. In each case, analysis related to both the overall period and individual years. Hours around noon on very hot days were mainly associated with “moderate” or “severe” heat stress, though instances “very sever heat stress” may also arise. The greatest threat of thermal heat stress could be noted for July and August, with conditions noticeably more severe in Kraków than the other stations studied. Kołobrzeg faced the most-limited hazard associated with the occurrence of heat stress, and only in July may “very severe heat stress” appear there sporadically. In turn, Poznań – located in a region with bioclimatic conditions typical for Poland – was rather characterised by “moderate” or “severe heat stress”. Equally, on a majority of the very hot days studied, all three stations recoded above-average UTCI values, with this fact making it clear that when a high level of thermal stress arises it may be rather a country-wide phenomenon. The most stable, near-average conditions characterised May, while biothermal differentiation peaked in July and August. Analysis of variance showed that, other than in relation to April in Poznań, levels of thermal stress on hot days did not differ significantly from one month to another. Analyses of the variability to values for the multi-year universal thermal climate index revealed an increase over time for maxima, especially in Kraków. This may point to an intensified risk associated with overheating of the body, in the south of Poland in particular. In addition, calculations confirm both spatial and temporal differentiation of biothermal conditions. Years in which hot days proved particularly burdensome were 2005, 2007, 2010 and 2013.
The Universal Thermal Climate Index as an Operational Forecasting Tool of Human Biometeorological Conditions in Europe