Changes in Bioclimatic Indices

This paper assesses changes in bioclimatic conditions in Poland using two indices: the Universal Thermal Climate Index (UTCI) and Physiological Subjective Temperature (PST). The indices were calculated using the BioKlima 2.6 software package on the basis of several meteorological parameters (temperature, humidity, cloudiness, wind speed), measured daily at 12:00 UTC, from the period 1951–2018 at 24 stations representing different biometeorological regions. First, the spatial distribution of UTCI and PST values and the frequency of different thermal stress and thermal sensation categories are presented, with the various thermal stress and sensation categories grouped into 3 for each index. Then, the absolute values and relative percentage trends (compared to 1951) are calculated for ten-year periods and days with specific characteristics. The analysis shows a significant increase in the minimum and mean values of the applied indices during the period under investigation over all areas of Poland, including the mountains, especially in the north-east (where UTCI minimum values rose up to 1.8 °C per decade) and Tarnów in the south-east (up to 2.2 °C per decade). The rise of UTCI summer maximum values was significant only in the north-east and north-west of Poland (0.3–0.5 °C per decade) and in the mountain foothills (0.3–0.4 °C per decade). The changes in PST maximum values were insignificant. On average, in every decade, the number of cold days reduced by 2.7 in the case of PST and 3.2 on the UTCI. However, in the north-east, the reduction was much greater, at 7 days per 10 years (UTCI) and 6 days when considering PST. Regionally, the greatest increase in the mean and minimum values of the indices used and the decrease in the frequency of cold days were observed in north-eastern, eastern and western Poland, i.e. the regions most exposed to frequent changes in air circulation, which is hypothesised to play the main role in shaping bioclimatic conditions in Poland.