Obciążenia cieplne człowieka podczas niezwykle ciepłych miesięcy letnich w Krakowie = Human thermal stress during exceptionally warm summer months in Kraków (Poland)

Abstract

This study aims to characterise human heat loads during the three hottest summer months recorded in Krakow at the beginning of the 21st century, namely in July 2006, August 2015, and June 2019. This goal can be reduced to the questions of how far these loads deviated from the average conditions and what was the impact of circulation. The study showed that the months in question belonged to the thermally anomalous category, where the temperature significantly exceeded (by at least 2 standard deviations, SD) the long-term average. The study was based on the UTCI values calculated taking into account four meteorological elements recorded at three times of the day (6:00, 12:00 and 18:00 UTC). The analysis was referenced against the latest 30-year period (1991-2020) and relied on the calendar of circulation types and air masses for southern Poland by T. Niedźwiedź. The outcome of the study showed that the highest heat load intensity occurred in the first half of August 2015, when the all-day heat stress prevailed on most of the days, it reached strong stress levels at peak human activity time and included severely strong stress condition on three days of that period. Similar heat load conditions occurred in July 2006, but this time in two separate spells: 5-13 July and 18-23 July. In June 2019, the heat stress load started to grow at the beginning of the month and culminated in two peaks: first in the early 20s of the month and then again between 25-27 July. Causes for the occurrence of unusually warm months should be seen not just in the warming of the Earth's climate, but also in the variability of atmospheric circulation. The intensity of the heat was influenced by the nature of the circulation, by the direction of advection and by the type of air masses. All the spells of days with the strongest heat stress during these three exceptionally warm months coincided with anticyclonic circulation from the southern sector (Sa and SWa), bringing hot tropical air masses.