Long-term changes in the frequency of exceptionally cold and warm months in Europe (1831-2020)

The onset of periods with very high or low air temperatures has aroused general interest for a long time, which is understandable since it has many dangerous effects directly affecting humans. Such perilous climatic phenomena include exceptionally cold and exceptionally warm months, which this study investigates for all Europe and for its five physico-geographic regions over the 190-year period of 1831-2020. Therefore, the research in this paper includes two periods characteristic of the history of climate - the late Little Ice Age (LIA) and present-day warming. The studies are based on average monthly air temperature values from 40 weather stations in Europe. In this paper, exceptionally cold months (ECMs) or exceptionally warm months (EWMs) are considered to have occurred when the average air temperature at a station differed from the respective long-term average by at least two standard deviations. The highlights of the study include the identification of a drop in the number of ECMs by 20 over the entire 190-year period, and a highly statistically significant increase in EWMs by 44 between 1980 and 2020. These changes proceeded with different intensities from one physico-geographic region of Europe to another

Exceptionally hot and cold summers in Europe (1951-2010)

The paper explores exceptional thermal conditions, an area of research that has increased in significance in the context of the changes that are being observed in climate. Specifically, the study addressed the frequency, long-term change and spatial coverage of exceptionally hot summers, and exceptionally cool summers (EHS and ECS) in Europe. The statistical criterion of plus/minus two standard deviations from the long-term average was used to identify EHS and ECS at 60 weather stations over the period 1951-2010. The study has demonstrated that EHS are characterised by greater temperature anomalies than ECS and are approximately twice as frequent. They occurred virtually everywhere within the study area, whilst ECS are concentrated in its northern part. Five EHS (1972, 2002, 2003, 2007, and 2010) and three ECS (1956, 1962, and 1976) were large enough to be recorded by at least 10% of the station

Exceptionally hot summers in Central and Eastern Europe (1951-2010)

The paper focuses on exceptionally hot summers (EHS) as a manifestation of contemporary climate warming. The study identifies EHS occurrences in Central and Eastern Europe and describes the characteristic features of the region’s thermal conditions. Average air temperatures in June, July and August were considered, as well as the number of days with maximum temperatures exceeding 25, 30 and 35 ^{\circ}C, and with a minimum temperature greater than >20 ^{\circ}C, as recorded at 59 weather stations in 1951-2010. Extremely hot summers are defined as having an average temperature equal to or greater than the long-term average plus 2 SD. A calendar of EHSs was compiled and their spatial extent identified. The region experienced 12 EHSs, which occurred in a given year at 5 % or more stations (1972, 1981, 1988, 1992, 1997, 1998, 1999, 2002, 2003, 2006, 2007 and 2010). The EHS frequency of occurrence was found to be clearly on an increase. Indeed, only one EHS occurred during the first 30 years, but these occurred five times during the last 10 years of the study period. Their geographical extent varied both in terms of location and size. EHSs were observed at 57 out of the total of 59 weather stations in the study (the exceptions were Pecora and Cluj). The average air temperature of EHSs tended to exceed the relevant long-term average by 2-4 ^{\circ}C. The summer of 2010 was among the hottest (temperature anomaly 5.5-6 ^{\circ}C) and spatially largest

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

W pracy scharakteryzowano obciążenia cieplne człowieka w trzech najgorętszych miesiącach, które wystąpiły w Krakowie na początku XXI w., a mianowicie w lipcu 2006 r., sierpniu 2015 r. i czerwcu 2019 r. Podstawę badań stanowiły wartości wskaźnika UTCI obliczone na bazie czterech elementów meteorologicznych z trzech terminów obserwacji w ciągu doby (6, 12 i 18 UTC). Jako okres referencyjny przyjęto ostatnie trzydziestolecie 1991‑2020. W opracowaniu wykorzystano także kalendarz typów cyrkulacji i mas powietrznych nad południową Polską (Niedźwiedź, 1981, 2023). Na postawie średnich wartości charakterystyk termicznych, jak i wskaźnika UTCI, wykazano, że warunki odczuwalne w 3 niezwykle ciepłych miesiącach były podobne. Ogólnie dni ze stresem ciepła (o 12 UTC) było rekordowo dużo: od 70% w sierpniu 2015 r. do 84% w czerwcu 2019 r. Przebieg obciążeń z dnia na dzień był nieco odmienny. Potwierdziło się, zgodnie z dotychczasową wiedzą, że o nasileniu gorąca zadecydowały zarówno charakter cyrkulacji, jak i kierunek adwekcji i typ masy powietrznej. Podczas niezwykle ciepłych miesięcy w dniach z najsilniejszym stresem ciepła przeważała cyrkulacja antycyklonalna z sektora południowego (Sa i SWa), sprowadzająca gorące masy powietrza zwrotnikowego.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 ($t_{max}>30^{\circ}C$), but also and above all in the appearance of very hot days ($t_{max}>35^{\circ}C$), and tropical nights (${min}>20^{\circ}C$) - 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 $21^{st}$ 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 $25^{th}‑27^{th}$. 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

Exceptionally cold and warm spring months in Kraków against the background of atmospheric circulation (1874-2022)

In the changing climate, exceptionally warm (EWMs) and dry spring months are increasingly observed. At the same time, exceptionally cold months (ECMs) are less frequent, although their impact on a warming climate becomes significant. Due to the role that such climatic anomalies play in the environment and their effects on human activity, it is very important to explain the causes of their occurrence. For this reason, in this study, the authors have attempted to determine the circulation conditions favourable to the occurrence of extremely cold (ECM) and warm (EWM) spring months in Kraków in the years 1874-2022. The study used the average temperature of individual spring months (March-May), as well as types of atmospheric circulation and air masses from the daily Calendar of Atmospheric Circulation Types for southern Poland. A distinct increase in spring air temperature (0.181 $^{\circ}C$/10 years) and its individual months (0.162-0.191 $^{\circ}C$/10 years) was confirmed. It was accompanied by a significant increase in the occurrence of EWM and a decrease in ECM. It was also found that the direction of air advection and the related temperature characteristics of air masses have the greatest impact on the occurrence of exceptionally cold or warm months. A slight positive effect of zonal circulation on the temperature increase at the beginning of the spring season and the advection of air from the south in April and east in May was found. In the case of the coldest months (ECMs), low temperatures most often developed in the presence of advection from the NW-N-NE directions

The variability of maximum daily precipitation and the underlying circulation conditions in Kraków, southern Poland

This article studies the intra-annual and long-term variability in the maximum daily precipitation totals and their association with atmospheric circulation in Kraków. It investigates daily precipitation maxima by year and by month. The research is based on daily precipitation totals in the years 1863-2021 and draws on the calendar of atmospheric circulation types by Niedźwiedź. It examines the frequency of precipitation maxima in individual months and their variation from one year to another. No statistically significant trend of change in precipitation over the study period has been found. All annual maximum daily precipitation totals in Kraków fall into the category of heavy precipitation (>10 mm), and almost 99% qualify as very heavy (>20 mm). In the summer months, these are about 3-4 times higher than in winter. The share of the daily precipitation maximum in the monthly total exceeds 30% in all months. The maximum daily precipitation occurring on 5 August 2021 was the highest in the period that extends from the start of instrumental measurements. The study period saw 12 cases of maximum precipitation that belong to "flood-inducing" categories (over 70 mm/day). Such cases of the very heaviest precipitation occurred in cyclonic situations: Cc, Bc, Nc, NEc, Ec and SEc. Most spring and summer maxima were seen on days with a cyclonic circulation. The instances of high daily precipitation in the Kraków area led to the flooding of residential and historic buildings, as well as of municipal infrastructure

Variability of air-mass occurrence in southern Poland (1951-2010)

The paper discusses the frequency, persistence and succession sequence of six types of air mass during the period 1951-2010. The study, which relied on a calendar of air masses in southern Poland published by T. Niedźwiedź, concludes that there is no simple relationship between the persistence and the frequency of specific air masses in the region. The study found that there was a great variety in the persistence of specific types of air mass and that persistence depended more on the direction of air mass advection than on their frequency of occurrence. The study also failed to identify any strict overall rule of succession, as any air mass could follow after any other, but certain finer-grained patterns emerged. In winter and summer, arctic air (A) never followed directly after tropical air (T) or vice versa. Also, the most frequent succession sequence identified was from Polar maritime fresh air (mPf) to Polar maritime old and it accounted for the vast majority of successions from mPf into any other air mass (63 % annually and 76 % in summer)