Przegląd Geograficzny (2023) vol. 95, iss. 3

Human thermal stress during exceptionally warm summer months in Kraków (Poland)

Robert Twardosz

Przegląd Geograficzny (2023) vol. 95, iss. 3, pp. 255-269 | Full text

This study aims to characterise heat‑loading upon human organisms during the three hottest summer months recorded in Krakow at the beginning of the 21st century, i.e. July 2006, August 2015 and June 2019. This goal was capable of being reduced to questions as to how far the heat‑loads in question deviated from average conditions and what the impacts of air circulation on that might have been. In the event, the work detailed here is able to demonstrate that the months in question were of a thermally anomalous category whereby the temperature exceeded the long‑term average in a significant manner (by at least 2 standard deviations, SD). The basis for these findings lays in UTCI values calculated by taking account of four meteorological elements recorded at three times of the day (6:00, 12:00 and 18:00 UTC). The overall 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 after T. Niedźwiedź.

The summer months selected for analysis proved to be, not merely extremely hot, but also anomalously dry. The drought accompanying the heatwaves exerted a negative impact on the economy, in particular agriculture and people; and a situation in which such major thermal and rainfall anomalies arise represents one of the greatest natural hazards facing Poland.

July 2006 and June 2019 were the hottest months since the beginning of meteorological measurements in Krakow, i.e. since 1792. This is evidenced by the size of the air temperature anomaly, which exceeded the average by as many as 3 standard deviations (SD) (though across about two‑thirds of Poland, the anomaly of June 2019 even went beyond 4 SD). The size of the August 2015 anomaly was slightly below 3 SD. The unusual intensity of the heat was reflected, not only in the high frequency of occurrence of hot days (tmax >30oC), but also and above all in the appearance of very hot days (tmax >35oC), and tropical nights (tmin >20oC) – both rare phenomena for June.

A consequence of this kind of warming is a clear change in bioclimate, which is determined to the greatest extent by air temperature. In the summer months in Poland at the beginning of the 21st century, this change entailed increased numbers of days on which there is strong or very strong heat stress (by 12:00 UTC). This idea gained clear confirmation in the exceptionally warm months this paper considers. Thus, in July 2006, almost 60% of all days were in the above category, along with 51% and 40% in August 2015 and June 2019 respectively. This can be further taken to indicate that there were 3‑4 times as many such days as there were on average in the given months across the whole 30‑yearperiod of 1991‑2020. In general, days with heat stress (at 12:00 UTC) were record high: from 70% in August 2015 to 84% in June 2019. Furthermore, on only a few days in these exceptionally warm months were no all‑day heat loads for the human body recorded, those numbers ranging between a mere 5 in the case of July 2006, and 10 in August 2015.

An outcome of the study was to show how the greatest intensity of heat load characterised the first half of August 2015, when all‑day heat stress prevailed on most days, reaching strong stress levels coinciding with times of peak human activity, and including instances of severely strong stress conditions on three days of the period. Similar heat‑load conditions arose in July 2006, albeit in the circumstances of two separate spells: 5‑13 July and 18‑23 July. In turn, in June 2019, the heat stress load began to grow at the beginning of that month, culminating in two peaks – at the beginning of the month’s last10‑day period, and again between on 25th‑27th.

When average temperatures and UTCI are looked at together, conditions experienced during the three exceptionally warm months under study are seen to be similar. Of course, the specifics from day to day proved to differ slightly, and from the point of view of direct impact on the human organism the greatest heat stress to the human body would have been experienced in June 2019, given the status as the first summer month, in which the human body has not yet adapted to long‑term circumstances of extremely high air temperatures.

Causes for the occurrence of unusually warm months should not be sought solely in the overall warming of the Earth’s climate, given likely mediation of effects via variability or change in atmospheric circulation. The intensity of heat can be influenced by the nature of the said circulation, by the direction of advection, and by the types of air mass involved. In fact, all the spells of days characterised by the strongest heat stress in the course of the three unusually warm months identified here were found to coincide with anticyclonic circulation from the southern sector (Sa and SWa), acting as a source of input into Poland of hot tropical masses of air.

Keywords: UTCI, heat stress, exceptionally warm month, atmospheric circulation, air masses, Kraków, Poland

Robert Twardosz [], Faculty of Geography and Geology Jagiellonian University Gronostajowa str. 7, 30-387 Kraków: Poland


APA: Twardosz, R. (2023). Obciążenia cieplne człowieka podczas niezwykle ciepłych miesięcy letnich w Krakowie. Przegląd Geograficzny, 95(3), 255-269.

MLA: Twardosz, Robert. "Obciążenia cieplne człowieka podczas niezwykle ciepłych miesięcy letnich w Krakowie". Przegląd Geograficzny, vol. 95, no. 3, 2023, pp. 255-269.

Chicago: Twardosz, Robert. "Obciążenia cieplne człowieka podczas niezwykle ciepłych miesięcy letnich w Krakowie". Przegląd Geograficzny 95, no. 3 (2023): 255-269.

Harvard: Twardosz, R. 2023. "Obciążenia cieplne człowieka podczas niezwykle ciepłych miesięcy letnich w Krakowie". Przegląd Geograficzny, vol. 95, no. 3, pp. 255-269.