Speciﬁcity of Meteorological and Biometeorological Conditions in Central Europe in Centre of Urban Areas in June 2019 (Bydgoszcz, Poland)

: The work describes diurnal meteorological and biometeorological conditions in June 2019 in the urban areas of Central Europe. UTCI, STI, Oh_H, WL, and OV indices were calculated based on 24-h data from Bydgoszcz (Poland) for hot days. The degree of risk connected with heat stress of different intensities, risk of hyperthermia, body water loss, and decreased oxygen volume was determined. The studies showed that June 2019 was an example of an extreme situation with a heatwave that generated high stress for the inhabitants of urban areas. The conditions were burdensome mostly due to “very strong” and “strong” heat stress and periodic risk of dehydration, situations that could quickly lead to overheating of the body and a decreased oxygen volume leading to stress.


Introduction
Since the mid-19th century in Europe, the air temperature has been systematically increasing along with the frequency of extreme weather events (IPCC 2018). Here, hot days, in particular, should be mentioned as numerous studies reveal that they occur with an increasing frequency and severity [1][2][3][4][5]. Studies at the beginning of the 21st century showed that in various parts of Europe the years 2003, 2006, 2010 [6,7], 2015 [8], 2018 [9], and 2019 [10,11] were exceptionally hot.
On a global scale, 2019 was the second warmest year (after 2016) since 1958, and the month of June in that year was the hottest June recorded both in Europe and in the world [12]. Throughout the month, particular warming was observed in certain parts of Central and Eastern Europe [13,14]. South-west and central Europe experienced a shortlasting but intense heatwave at the end of June, with mean diurnal temperatures locally exceeding those for 1981-2010 by as much as 9.0 • C [12]. Throughout Poland, June 2019 was an extremely hot month with average temperature anomalies around 5.0 • C, and in the west exceeding 6.0 • C, in comparison with the 1981-2010 period [15]. The average diurnal maximum on the analysed days exceeded 30.0 • C in most of Poland ( Figure 1). Such temperatures resulted from persisting high atmospheric pressure, responsible for the influx of hot tropical air masses from the south and south-east [17], leading to cloudless hot weather but-together with low water vapour content-triggered intensive transfer of heat from the soil into the air [13,14,18].
The conditions on hot days in particular strongly affect the inhabitants of cities, where artificial surfaces, dense building development, and additional heat sources increase the intensity of extreme temperatures [19]. The air temperature over concrete-paved structures in the city centre, can exceed that noted in open space over grass surfaces by more than 3.0 °C [20,21]. In addition, although a rise in air temperature in the city-an urban heat island (UHI)-is felt the most at night in cloudless and windless weather [21][22][23], on hot sunny days the perceived climate conditions can cause more stress inside rather than outside the city, with biothermal contrasts in densely built-up space between sunny and shaded places reaching up to 20-25 °C [24]. Furthermore, according to Li and Bou-Zeid [25], heat waves increase differences between temperatures in rural and urban areas even more. Thus, additional heat stress in cities can exceed the sum of the urban heat island and the heat wave effect. This is due to the lack of surface moisture in urban areas and low wind speed [25]. Hot air, intensified by solar radiation on summer days, has a strong impact on subjective temperature, causing considerable stress to inhabitants of large conurbations, living and working in closed spaces, and poorly adapted to strong atmospheric stressors. Elderly people suffering from respiratory and cardiovascular diseases, children with an underdeveloped thermoregulatory system, and people working long hours outdoors are most prone to heat stress [26].
Due to the high risk, biothermal conditions in a built-up area during heat waves are a popular study subject for bioclimatologists. Many studies are devoted to an increase in mortality associated with extreme thermal conditions. They have been carried out in cities of Australia [27,28], Africa [29,30], Asia [31][32][33], South America [34,35], and southern stretches of North America [36,37]. These works indicate a significant increase in the intensity of heat stress in the urbanized area compared to the non-urban area. Moreover, some of these studies focus on the problem of the urban heat island, which, in the period of heat waves, affects the nuisance of biothermal conditions even more, which is particularly dangerous in cities located in low latitudes, and in areas with an additional high degree of sunshine. In Europe, the Mediterranean basin is particularly exposed to rising air temperatures. Many works focus on cities such as Athens [38,39], Barcelona [40], Rome [41,42], and Valencia [43]. These studies deal with the issues of the intensity and Such temperatures resulted from persisting high atmospheric pressure, responsible for the influx of hot tropical air masses from the south and south-east [17], leading to cloudless hot weather but-together with low water vapour content-triggered intensive transfer of heat from the soil into the air [13,14,18].
The conditions on hot days in particular strongly affect the inhabitants of cities, where artificial surfaces, dense building development, and additional heat sources increase the intensity of extreme temperatures [19]. The air temperature over concrete-paved structures in the city centre, can exceed that noted in open space over grass surfaces by more than 3.0 • C [20,21]. In addition, although a rise in air temperature in the city-an urban heat island (UHI)-is felt the most at night in cloudless and windless weather [21][22][23], on hot sunny days the perceived climate conditions can cause more stress inside rather than outside the city, with biothermal contrasts in densely built-up space between sunny and shaded places reaching up to 20-25 • C [24]. Furthermore, according to Li and Bou-Zeid [25], heat waves increase differences between temperatures in rural and urban areas even more. Thus, additional heat stress in cities can exceed the sum of the urban heat island and the heat wave effect. This is due to the lack of surface moisture in urban areas and low wind speed [25]. Hot air, intensified by solar radiation on summer days, has a strong impact on subjective temperature, causing considerable stress to inhabitants of large conurbations, living and working in closed spaces, and poorly adapted to strong atmospheric stressors. Elderly people suffering from respiratory and cardiovascular diseases, children with an underdeveloped thermoregulatory system, and people working long hours outdoors are most prone to heat stress [26].
Due to the high risk, biothermal conditions in a built-up area during heat waves are a popular study subject for bioclimatologists. Many studies are devoted to an increase in mortality associated with extreme thermal conditions. They have been carried out in cities of Australia [27,28], Africa [29,30], Asia [31][32][33], South America [34,35], and southern stretches of North America [36,37]. These works indicate a significant increase in the intensity of heat stress in the urbanized area compared to the non-urban area. Moreover, some of these studies focus on the problem of the urban heat island, which, in the period of heat waves, affects the nuisance of biothermal conditions even more, which is particularly dangerous in cities located in low latitudes, and in areas with an additional high degree of sunshine. In Europe, the Mediterranean basin is particularly exposed to rising air temperatures. Many works focus on cities such as Athens [38,39], Barcelona [40], Rome [41,42], and Valencia [43]. These studies deal with the issues of the intensity and duration of heat waves, monitoring of the city's heat island, but also, as in the case of Barcelona [40], an attempt to simulate biothermal conditions under the influence of changes in shading and wind speed. Studies in other parts of Europe-in big conurbations where bioclimatic conditions in summer have a strong impact on the human body-are equally significant. They have analysed single cities [44][45][46][47][48][49]-and groups of several or several dozen cities-over long-term intervals, and exhibit a strong increasing trend in heat stress [2,[50][51][52]. Many works discuss the relationship between climate and deaths, indicating that the peak of deaths usually occurs during heat waves [53][54][55]. Many bioclimatic studies currently use the universal thermal climate index (UTCI) e.g., [4,[56][57][58], and some also take into account other indices, such as the STI discussed in this study [59,60], Oh_H [60] WL [44,60], or OV [61].
The purpose of this study was to investigate the daily changes in meteorological and biometeorological conditions in a middle-sized Central European city during the extreme thermal event of June 2019. In particular, the intention was to determine the exposure of the human body to heat stress of varying intensity, to examine the risk of hyperthermia, to estimate the amount of water loss from the body, and to investigate the possibility of risk associated with a decrease in the oxygen content in the air.
Knowing potential hazards due to extreme temperatures in cities will improve the inhabitants' preparation for future climate change. This is essential, as currently already more than half of the world's population live in cities [62], and in the European Union, this figure exceeds 75%. The urban population is expected to grow in the future, reaching 82% by 2050 in Europe [63].

Materials and Methods
The authors used 5-min meteorological data from 15 days of June 2019, with the maximum air temperature of 30.0 • C or above. Data was sourced from an automatic HOBO weather station in Bydgoszcz (the Kuyavian-Pomeranian Voivodeship: ϕ-53.13 N; λ-18.01 E) within the premises of the Institute of Information Technology, Kazimierz Wielki University ( Figure 2). The station was located in a car park, above a grass surface, densely surrounded by buildings, and the instruments were installed at 1.6 m above ground level (air temperature and humidity) and 2.0 above ground level (solar radiation and wind speed).
Atmosphere 2021, 12, x FOR PEER REVIEW 3 of 18 duration of heat waves, monitoring of the city's heat island, but also, as in the case of Barcelona [40], an attempt to simulate biothermal conditions under the influence of changes in shading and wind speed. Studies in other parts of Europe-in big conurbations where bioclimatic conditions in summer have a strong impact on the human body-are equally significant. They have analysed single cities [44][45][46][47][48][49]-and groups of several or several dozen cities-over long-term intervals, and exhibit a strong increasing trend in heat stress [2,[50][51][52]. Many works discuss the relationship between climate and deaths, indicating that the peak of deaths usually occurs during heat waves [53][54][55]. Many bioclimatic studies currently use the universal thermal climate index (UTCI) e.g., [4,[56][57][58], and some also take into account other indices, such as the STI discussed in this study [59,60], Oh_H [60] WL [44,60], or OV [61]. The purpose of this study was to investigate the daily changes in meteorological and biometeorological conditions in a middle-sized Central European city during the extreme thermal event of June 2019. In particular, the intention was to determine the exposure of the human body to heat stress of varying intensity, to examine the risk of hyperthermia, to estimate the amount of water loss from the body, and to investigate the possibility of risk associated with a decrease in the oxygen content in the air.
Knowing potential hazards due to extreme temperatures in cities will improve the inhabitants' preparation for future climate change. This is essential, as currently already more than half of the world's population live in cities [62], and in the European Union, this figure exceeds 75%. The urban population is expected to grow in the future, reaching 82% by 2050 in Europe [63].

Materials and Methods
The authors used 5-min meteorological data from 15 days of June 2019, with the maximum air temperature of 30.0 °C or above. Data was sourced from an automatic HOBO weather station in Bydgoszcz (the Kuyavian-Pomeranian Voivodeship: φ-53.13 N; λ-18.01 E) within the premises of the Institute of Information Technology, Kazimierz Wielki University ( Figure 2). The station was located in a car park, above a grass surface, densely surrounded by buildings, and the instruments were installed at 1.6 m above ground level (air temperature and humidity) and 2.0 above ground level (solar radiation and wind speed). The data included: air temperature (t, °C), relative humidity (f, %), wind speed (v, m•s −1 ), global solar irradiance (Kglob, W•m−2), and atmospheric pressure (p, hPa). The data was processed with the BioKlima ver. 2.6 software [64], to compute the following biometeorological and thermophysiological indices: The Universal Thermal Climate Index (UTCI, °C)-the equivalent air temperature at which, in reference conditions, human vital physiological parameters are identical to real environment values. It is based on the analysis of the human heat balance using the Fiala The data included: air temperature (t, • C), relative humidity (f, %), wind speed (v, m·s −1 ), global solar irradiance (Kglob, W·m −2 ), and atmospheric pressure (p, hPa). The data was processed with the BioKlima ver. 2.6 software [64], to compute the following biometeorological and thermophysiological indices: The Universal Thermal Climate Index (UTCI, • C)-the equivalent air temperature at which, in reference conditions, human vital physiological parameters are identical to real environment values. It is based on the analysis of the human heat balance using the Fiala multi-node heat transfer model [65], measured by objective changes in human physiological parameters under the influence of atmospheric conditions (Table 1).  The Subjective Temperature Index (STI, • C)-based on the Man-ENvironment heat EXchange model (MENEX_2005). The underlying value is the mean radiant temperature and the heat balance determined due to the body's adaptation to the environment [59].
Respective STI values refer to the specific thermal experience (Table 2). Table 2. Thermal Sensations based on the Subjective Temperature Index [66].

STI ( • C)
Subjective Thermal Sensation Hyperthermia Risk (Oh_H, min.)-at an internal temperature of around 40 • C, thermoregulation is disturbed, and at 43 • C there is a risk of heatstroke and hyperthermia. This index expresses the time after which too much heat accumulates in the body leading to potential hyperthermia, calculated based on the input and output heat transfer value [67].
Water Loss (WL, g·h −1 )-indicates how much water should be supplied to prevent dehydration. According to ISO/DIS 7933, the risk of dehydration occurs at two levels: alarm and danger. For non-acclimatised persons, the alarm level occurs when 520 < WL < 650 g·h -1 , and the danger level is a WL exceeding 650 g·h -1 [67]. The index is especially useful for active recreation and hiking at high temperatures.
Oxygen Volume (OV, g·m −3 )-ratio determining the absolute level of oxygen in air volume; the unit is calculated based on air temperature, water vapour pressure, and atmospheric pressure. Oxygen deficiency is a significant factor posing a risk of hypoxia, especially to people with impaired adaptation mechanisms suffering from respiratory and cardiovascular diseases [68,69] (Table 3). The oxygen content in the air is different in different air masses, therefore it is one of the evaluation elements in the objective biometeorological classification of weather. Table 3. Inter-day change of oxygen volume in the air and the intensity of the biothermal stimulus [70]. Based on the 5-min data, processed with Statistica ver. 13.3., diurnal mean values were calculated for each hot day, median, and maximum and minimum values were estimated for meteorological components and biometeorological and thermophysiological indices. The standard deviation was computed along with the lower and upper quartiles for all the parameters and indices. To assess the duration of hot weather, the number of minutes with a maximum temperature above or equal to 30.0 • C was calculated for each day, recording the start and end time above the limit of 30.0 • C. Moreover, for each day the time of extreme air temperature was determined. The analyses were supplemented by the averaged diurnal indices (all analysed days and the hottest day), and the 24 h prevalence of stressors for the human body. Data from all days was used to determine the percentage of hazards hour by hour.

Results
The mean diurnal air temperature in the analysed period was 25.7 • C (Table 4), and three clear temperature rise periods were noted-on 11-12, on 26, and on 30 June, when the maximum diurnal temperature exceeded 36.0 • C, and the upper quartile exceeded 33.0 • C. The minimum temperature on six days did not fall below 18.0 • C, and on three days was not lower than 20.0 • C, leading to very warm and "tropical" nights. The hottest day was 26 June with the maximum diurnal temperature amounting to 38.3 • C, and the upper quartile equaled 35.0 • C (Figure 3). Table 4. Values and standard deviation: air temperature, relative humidity, wind speed, atmospheric pressure, and total diurnal solar irradiance, duration in minutes, and time of temperature rise above 30.0 • C measured in Bydgoszcz on hot days in June 2019. Air humidity mostly remained around the mean diurnal value, that is, around 55.6% (Table 4), although-on half of the days-it exceeded 70% over 25% of the time. The lowest diurnal humidity was recorded on 30 June (Table 4), when the lowest minimum value was 17.8%. The highest diurnal maximum humidity-90.9%-was recorded on 20 June (Figure 3). Despite the location of the measuring station 900 m from the river, the obtained relative humidity values on the tested days are indeed lower than the values characteristic for Bydgoszcz [69], which was probably due to the surroundings of the station with quite dense development.
The mean diurnal wind speed was on average 0.7 m·s −1 , and never exceeded 1.0 m·s −1 . Maximum diurnal values were equal to or slightly above 3.0 m·s −1 on seven days. On four days the wind blew with a speed of 1.5 m·s −1 for 1/4 of the day (Figure 3).
The surface pressure was variable. On average, atmospheric pressure was 1011.0 hPa (Table 4)-slightly lower at the beginning of the month with a short-term increase at the end of the first ten days. A clear increase was noted in the third ten days. In the analysed period, the diurnal extremes varied from the minimum value of 1003.3 hPa to the maximum pressure of 1022.8 hPa (Figure 3).
Total diurnal solar radiation ranged from 18.1 MJ to 25.8 MJ (Table 1), and was particularly intensified at the end of month. The maximum diurnal values for 19 and 20 June were close to 1000 W·m −2 (Figure 3).
In the analysed period, the number of minutes with an air temperature above 30.0 • C ranged from 5 to 780 min during 24 h, though not continuously. On 12 out of 15 days, this lasted longer than three h, and on six days continued for more than six hours. In total, 5535 min (about 92 h) with an air temperature above 30.0 • C were recorded. Normally, the rise above the limit started around noon and ended between 6:00 and 7:00 p.m. (Table 4).  The maximum diurnal air temperature was most frequently recorded around 4:00 p.m., and the minimum was recorded usually several minutes to 5:00 a.m. (Figure 4).
Mean diurnal values of the Universal Thermal Climate Index led to a conclusion that normally "moderate heat stress" occurred ( Table 5). The extreme values ranged from 11.6 °C to 42.8 °C, falling within categories from "no thermal stress" to "very strong heat stress". Maximum diurnal values pointed to at least "strong heat stress", and on five days suggested "very strong heat stress" (Figure 5). The maximum diurnal air temperature was most frequently recorded around 4:00 p.m., and the minimum was recorded usually several minutes to 5:00 a.m. (Figure 4).   The subjective temperature was on average around 28.7 °C, so it was "comfortable", although on some days the mean diurnal STIs were "warm" (Table 5). Extreme temperatures ranged from "cool" to "very hot". Subjective "hot" temperatures were particularly intense on 11, 25, and 26 June when the upper quartile exceeded 46 °C ( Figure 5).
The mean time after which hyperthermia could occur (excluding "no limit" scenarios) was about 1330.0 min, corresponding to 22 h (Table 5). Therefore, the possibility of fast, life-threatening overheating could be excluded. However, it was likely that hyperthermia could occur faster than on average, as suggested by the significant minimum values of the index informing that on eight days it was possible within less than 100 min -even after 72 min. The lower quartile showed that 25% was a safe time not exceeding 103.0-173.0 min, depending on the day ( Figure 5).
Given an average water loss slightly above 250.0 g•h −1 , on some days the mean values exceeded 300.0 g•h −1 and were diversified over 24 h (  Mean diurnal values of the Universal Thermal Climate Index led to a conclusion that normally "moderate heat stress" occurred ( Table 5). The extreme values ranged from 11.6 • C to 42.8 • C, falling within categories from "no thermal stress" to "very strong heat stress". Maximum diurnal values pointed to at least "strong heat stress", and on five days suggested "very strong heat stress" (Figure 5). formed of possible dehydration at 344.0-775.0 g•h −1 . On some days-for 25% of the time-water loss exceeding 500.0 g•h −1 was possible ( Figure 5). The mean diurnal oxygen volume was 267.8 g•m −3 at all times (Table 1). It considerably decreased on six days with minimum diurnal values below 260.0 g•m −3 ( Figure 5).  The subjective temperature was on average around 28.7 • C, so it was "comfortable", although on some days the mean diurnal STIs were "warm" (Table 5). Extreme temperatures ranged from "cool" to "very hot". Subjective "hot" temperatures were particularly intense on 11, 25, and 26 June when the upper quartile exceeded 46 • C ( Figure 5).
The mean time after which hyperthermia could occur (excluding "no limit" scenarios) was about 1330.0 min, corresponding to 22 h (Table 5). Therefore, the possibility of fast, life-threatening overheating could be excluded. However, it was likely that hyperthermia could occur faster than on average, as suggested by the significant minimum values of the index informing that on eight days it was possible within less than 100 min-even after 72 min. The lower quartile showed that 25% was a safe time not exceeding 103.0-173.0 min, depending on the day ( Figure 5).
Given an average water loss slightly above 250.0 g·h −1 , on some days the mean values exceeded 300.0 g·h −1 and were diversified over 24 h (Table 5). Maximum values informed of possible dehydration at 344.0-775.0 g·h −1 . On some days-for 25% of the time-water loss exceeding 500.0 g·h −1 was possible ( Figure 5).
The mean diurnal oxygen volume was 267.8 g·m −3 at all times (Table 1). It considerably decreased on six days with minimum diurnal values below 260.0 g·m −3 ( Figure 5).
The averaged diurnal UTCI for all hot days shows that three types of heat stress occur over 24 h: "no thermal stress" from 8:50 p.m. to 8:10 a.m., "moderate heat stress" from 8:15 a.m. to 11:25 a.m. and from 6:30 p.m. to 8:45 p.m., and "strong heat stress" from 11:30 a.m. to 6:25 p.m. Compared with the average conditions, on the hottest day (26 June) "moderate" and "strong heat stress" started earlier and ended later than on average, and from 1:10 p.m. to 6:30 p.m. "very strong heat stress" occurred intermittently. The diurnal STI suggested no thermal stress on average between 7:05 a.m. and 8:20 a.m. and between 6:35 p.m. and 8:50 p.m.-on the hottest day between 6:25 a.m. and 7:30 a.m. and between 7:55 p.m. and 10:45 p.m.-and at night time STI was "cool". The subjective "hot" temperature occurred intermittently from 11:40 a.m. to 4:40 p.m., and on the hottest day-from 10:05 a.m. to 6:30 p.m. In addition, on 26 June in the afternoon, occasional "very hot" temperatures were recorded. The diurnal WL on average did not imply dangerous dehydration over 24 h, but on the hottest day such risk did occur between noon and 6:05 p.m. and from 3:40 p.m. and 4:50 p.m. reached a level dangerous for non-acclimatised people. The oxygen volume decreased during the day and rose at night. On the hottest day, the mean OV over 24 h was about 3.0 g·m −3 -and at the peak, the air temperature was greater than 5.0 g·m −3 -lower than on average. It is also significant that on average the OV at the hottest hour was nearly 3.0% lower than at night and on the hottest day-6.5% lower ( Figure 6). The averaged diurnal UTCI for all hot days shows that three types of heat stress occur over 24 h: "no thermal stress" from 8:50 p.m. to 8:10 a.m., "moderate heat stress" from 8:15 a.m. to 11:25 a.m. and from 6:30 p.m. to 8:45 p.m., and "strong heat stress" from 11:30 a.m. to 6:25 p.m. Compared with the average conditions, on the hottest day (26 June) "moderate" and "strong heat stress" started earlier and ended later than on average, and from 1:10 p.m. to 6:30 p.m. "very strong heat stress" occurred intermittently. The diurnal STI suggested no thermal stress on average between 7:05 a.m. and 8:20 a.m. and between 6:35 p.m. and 8:50 p.m.-on the hottest day between 6:25 a.m. and 7:30 a.m. and between 7:55 p.m. and 10:45 p.m.-and at night time STI was "cool". The subjective "hot" temperature occurred intermittently from 11:40 a.m. to 4:40 p.m., and on the hottest day-from 10:05 a.m. to 6:30 p.m. In addition, on 26 June in the afternoon, occasional "very hot" temperatures were recorded. The diurnal WL on average did not imply dangerous dehydration over 24 h, but on the hottest day such risk did occur between noon and 6:05 p.m. and from 3:40 p.m. and 4:50 p.m. reached a level dangerous for non-acclimatised people. The oxygen volume decreased during the day and rose at night. On the hottest day, the mean OV over 24 h was about 3.0 g•m −3 -and at the peak, the air temperature was greater than 5.0 g•m −3 -lower than on average. It is also significant that on average the OV at the hottest hour was nearly 3.0% lower than at night and on the hottest day-6.5% lower ( Figure 6).   The percentage of heat stress implies that on average over half of the day no thermal stress was recorded, and the physiological thermoregulatory response was sufficient to maintain thermal balance. This was observed from midnight until 6:55 a.m. almost all the time. At different hours, some thermal discomfort appeared. Around 9:00 a.m. and 8:00 p.m. heat stress was "moderate", before 2:00 p.m. and around 4:00 p.m. "strong" and from 3:40 p.m. to 4:40 p.m. "very strong". STI was rarely "comfortable" and was "cool" at night time. The most extreme, "very hot" temperatures occurred over less than 3.5% of the time. Mostly, on the analysed days in June, no risk of excessive water loss was noted. It appeared on four days, and on two-from 3:40 p.m. to 4:50 p.m.-posed a risk to health. On average, over 80% of the day staying outdoors did not pose a hyperthermia risk, and in 50% of cases, no time limits were necessary. Such friendly conditions occurred from 6:35 p.m. to 9:30 a.m. In about 10% of cases, safe time outdoors was 150 min. On some days, for more than 10% of the time it was necessary to reduce the time spent outdoors to two hours (around 2:30 p.m.), and occasionally to 90 min (Figure 7).
Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 18 maintain thermal balance. This was observed from midnight until 6:55 a.m. almost all the time. At different hours, some thermal discomfort appeared. Around 9:00 a.m. and 8:00 p.m. heat stress was "moderate", before 2:00 p.m. and around 4:00 p.m. "strong" and from 3:40 p.m. to 4:40 p.m. "very strong". STI was rarely "comfortable" and was "cool" at night time. The most extreme, "very hot" temperatures occurred over less than 3.5% of the time. Mostly, on the analysed days in June, no risk of excessive water loss was noted. It appeared on four days, and on two-from 3:40 p.m. to 4:50 p.m.-posed a risk to health. On average, over 80% of the day staying outdoors did not pose a hyperthermia risk, and in 50% of cases, no time limits were necessary. Such friendly conditions occurred from 6:35 p.m. to 9:30 a.m. In about 10% of cases, safe time outdoors was 150 min. On some days, for more than 10% of the time it was necessary to reduce the time spent outdoors to two hours (around 2:30 p.m.), and occasionally to 90 min (Figure 7).

Discussion
The researchers evaluated June 2019 as the hottest June in Central Europe since 1958, in terms of air temperature and the number of extremely hot days. Three heat waves were recorded in Europe on: 3-10, 11-12, and 23-28 June, each stronger than the previous one [14]. This long-lasting temperature rise affected large areas in Western and Central Europe [10], including Poland [18]. As reported by Cebulak and Limanówka [70], Koźmiński and Michalska [71], and Okoniewska [72], in Poland, hot days occur more rarely in June than in July and August and their number does not exceed 10 in a year. Several hot days in a single month imply an exceptional situation.
The rise in temperature in June was clearly observed by the Bydgoszcz city weather station where 15 hot days were recorded and special warming was noted on 11-12, 26, and 30 June-with the maximum diurnal temperature above 36.0 °C. The research conducted for Bydgoszcz shows that the city center is thermally privileged compared to the surrounding areas [73][74][75][76], which confirms the particular nuisance of conditions in the city. Więcław and Okoniewska [77] showed that in June 2005-2008 in Bydgoszcz, 16 hot days were recorded in total, while the city weather stations in Poznań reported that throughout summer in 2008-2015 their number ranged from about 11 to 35 [78]. The occurring heat wave was not one long wave, but rather it consisted of 3 short heat waves in the following system: 4 days + 3 days + 3 days separated by cooler days. The hottest day was 26 June 2019, with a maximum diurnal temperature of 38.3 °C-similar to the weather conditions in August 2015, when one of the city weather stations in Poznań measured a similar maximum temperature [78]. The authors of these studies emphasise that air temperature rises are particularly high in a strongly transformed city centre on industrial and commercial grounds.
Apart from temperature alone, it was significant how long it continued above 30.0 °C, which in the analysed period was from 5 min to 13 h. Overall, about 92 such hours were noted during the whole month. Other cities (Lublin, Włodawa in 1994-2018) recorded less than 50 h a year [79]. In June 2019, air temperature normally exceeded 30.0 °C before 1:00 p.m. and continued until the evening.
As demonstrated by the author's previous studies [80], in Poland, the maximum diurnal temperatures in summer are usually noted around 2:00 p.m. In this study, they were recorded around 4:00 p.m.-later than on average, and definitely later than when the sun is highest in the sky. Żarnowiecki [81] claims that due to dense building development, the temperature rise is the highest during the late afternoon and evening, while

Discussion
The researchers evaluated June 2019 as the hottest June in Central Europe since 1958, in terms of air temperature and the number of extremely hot days. Three heat waves were recorded in Europe on: 3-10, 11-12, and 23-28 June, each stronger than the previous one [14]. This long-lasting temperature rise affected large areas in Western and Central Europe [10], including Poland [18]. As reported by Cebulak and Limanówka [70], Koźmiński and Michalska [71], and Okoniewska [72], in Poland, hot days occur more rarely in June than in July and August and their number does not exceed 10 in a year. Several hot days in a single month imply an exceptional situation.
The rise in temperature in June was clearly observed by the Bydgoszcz city weather station where 15 hot days were recorded and special warming was noted on 11-12, 26, and 30 June-with the maximum diurnal temperature above 36.0 • C. The research conducted for Bydgoszcz shows that the city center is thermally privileged compared to the surrounding areas [73][74][75][76], which confirms the particular nuisance of conditions in the city. Więcław and Okoniewska [77] showed that in June 2005-2008 in Bydgoszcz, 16 hot days were recorded in total, while the city weather stations in Poznań reported that throughout summer in 2008-2015 their number ranged from about 11 to 35 [78]. The occurring heat wave was not one long wave, but rather it consisted of 3 short heat waves in the following system: 4 days + 3 days + 3 days separated by cooler days. The hottest day was 26 June 2019, with a maximum diurnal temperature of 38.3 • C-similar to the weather conditions in August 2015, when one of the city weather stations in Poznań measured a similar maximum temperature [78]. The authors of these studies emphasise that air temperature rises are particularly high in a strongly transformed city centre on industrial and commercial grounds.
Apart from temperature alone, it was significant how long it continued above 30.0 • C, which in the analysed period was from 5 min to 13 h. Overall, about 92 such hours were noted during the whole month. Other cities (Lublin, Włodawa in 1994-2018) recorded less than 50 h a year [79]. In June 2019, air temperature normally exceeded 30.0 • C before 1:00 p.m. and continued until the evening.
As demonstrated by the author's previous studies [80], in Poland, the maximum diurnal temperatures in summer are usually noted around 2:00 p.m. In this study, they were recorded around 4:00 p.m.-later than on average, and definitely later than when the sun is highest in the sky.Żarnowiecki [81] claims that due to dense building development, the temperature rise is the highest during the late afternoon and evening, while Błażejczyk [18] notes that the shadows cast by buildings or trees when the sun is highest in the sky can cause the maximum temperature to be lower than in an open space. The high air temperature was accompanied by average, although at times increased, air humidity and low wind speed. The burden of the perceived conditions was compounded by high solar irradiance with diurnal totals exceeding the mean total radiation for the month, amounting to 20.5 MJ·m −2 [82].
In the analysed period, the average heat stress load for humans was moderateranging from "no thermal stress" to "very strong heat stress", and the STI ranged from "cool" to "very hot". Extreme UTCI categories implied that it was necessary to replenish fluids in an amount exceeding 0.5 L·h −1 , limit physical activity, and avoid sunny places. Previous studies carried out in Bydgoszcz (however, outside the city centre) in 2005-2008, showed UTCI not exceeding 28.0 • C around noon, thus no "strong" and "very strong heat stress" was recorded, and the recommendations were limited to replenishing fluids in an amount of 0.25 L·h −1 [77]. In June 2019, it was noted that on each day the diurnal maximum UTCI corresponded to at least "strong heat stress", and the highest value of the index was 42.8 • C. The upper quartile on five analysed days pointed to "strong heat stress" occurring over 25% of the day. Similar biothermal conditions leading to heat stress in hot summer weather were already noted in studies for other cities, including Warsaw [19, 26,67,83], Toruń [84], Opole [85], Lublin [46], Kielce [81], and locations in the region of Kłodzko [86]. Krzyżewska et al. [85] demonstrated that during the heat wave in 2015 in Poland, "strong" and "very strong heat stress" continued for more than 10 h in less than 1/3 of the analysed weather stations in Poland, and the maximum UTCI amounting to 42.1 • C was recorded in Wrocław, which was only a little lower than during the discussed event. Miszuk [87], studying the Polish-Saxon frontier, found that in urban areas at very high air temperatures, low wind speed, and intensive radiation, at some points UTCI can exceed 46 • C. The impact of solar radiation on the conditions perceived in cities was also mentioned by Błażejczyk [24] and Błażejczyk and Kunert [88], who demonstrated that on a sunny summer afternoon the temperature felt in a sunny street canyon could be even 20-25 • C higher than in a shaded area.
Comparing the obtained UTCI values for the discussed area of Central Europe with the conditions in the south of the continent, it can be concluded that the perceptible conditions become slightly more similar to those in the Mediterranean region. Research conducted in Greece by Katavoutas and Founda [38], showed that UTCI values for Athens during hot days reach an average of 43.0 • C, and the time of exposure to very strong heat stress occurs most often between 10 a.m. and 5 p.m.
In Bydgoszcz, biothermal conditions causing most heat stress occurred around 4:00 p.m., which was later than in studies from the multi-annual period 2005-2008 [77]; however, the time of occurrence in Toruń was similar in the studies of Araźny et al. [84] from 2012.
In June 2019, between 2:00 p.m. and 6:00 p.m., the heat stress was mostly "strong" and between 3:40 p.m. and 4:40 p.m.-"very strong", accompanied by an STI being "hot" and at times "very hot". During the daytime "very strong heat stress" could appear as early as before noon, and continue until the evening hours, so it was longer than during the heat wave in 2015 in Lublin when it was observed between 12:00 p.m. and 2:00 p.m. [46]. At night time, UTCI implied the predominance of no thermal stress, which is typical of that time of day, and STI indicated a "cool" feeling and sometimes "moderate heat stress" before midnight.
Based on the mean diurnal Oh_H, in most cases very fast, life-threatening overheating was excluded, although the risk varied in time. Sometimes (between 2:30 p.m. and 5:00 p.m.) hyperthermia exposure could occur in less than two hours-even within 70 min. According to Okoniewska [60,72], the risk of overheating increases during intensified physical activity, particularly around noon, when hyperthermia is possible after 30-40 min.
On hot days-as shown by Błażejczyk [44]-heat loss on evaporation is predominant and can reach up to 80 W·m −2 around noon, leading to excessive dehydration despite a high acclimatisation capacity. The average water loss was approximately 250 g·h −1 , although the WL at the level of 775 g·h −1 was also recorded. The most dangerous were the times between 3:40 p.m. and 4:50 p.m. It should be remembered that the values refer to moderate physical activity, so more active people were at higher risk of dehydration as physical activity increases the amount of water evaporated from skin surface up to over 1000 g·h −1 [60].
A significant problem in cities when the air temperature rises high is decreased oxygen volume. As indicated by Dubicki [89] and Majewski and Cichocka [61], oxygen deficit during the summer heat, especially in a densely built-up area-aggravated by the absence of green plants and consumption of oxygen by industry and transport-can trigger strong blood oxygen deficiency and affect the circulatory system. Studies by Janka [90] in Opole, show that oxygen volume at road intersections can be lower by up to 3.5% than in the background. Variations in the mean OV were observed ranging from 262.4 to 274. 9 g·m −3 . On six days, the minimum diurnal levels did not exceed 260 g·m −3 . Similar values were recorded in Warsaw in July 2009, when-within a densely built-up area-the OV decreased to 253.7 g·m −3 [61]. On average, the OV at the hottest time was nearly 3% lower than at night time and the lowest level was observed at 4:00 p.m., which coincides with studies by Majewski and Cichocka [61]. Variations were also observed in interdiurnal levels that were a significant biothermal stimulus on three days.
The hottest day was 26 June, when not only was the highest air temperature recorded but the UTCI did not fall below 36.6 • C for 1/4 of the day. This was higher than during a heat wave in August 2015, when the UTCI in the centre of Lublin was 40.7 • C, while the stress load for the human body was identical [46]. On 26 June 2019, between 1:10 p.m. and 6:30 p.m., "very strong heat stress" was noted, accompanied by a feeling of heatsometimes very strong-between 10:05 a.m. and 6:30 p.m. (intermittently). The burden caused by biothermal conditions on that day was corroborated by an Oh_H suggesting a risk of hyperthermia after 1.5 h outdoors. The WL could exceed up to 700 g·h −1 at that time, and the OV in the afternoon dropped to 255 g·m −3 .
The presented characteristics of the thermal anomaly in June 2019 imply that it was unprecedented, and further warming should be expected. The temperature rise is not only connected with the deterioration of human living environments or health impairments but also contributes to increasing mortality [50,53,91]. Therefore, it is necessary to implement relevant strategies to mitigate the adverse effects of heat waves, giving rise to numerous socio-economic challenges [5,92]. Adaptation to climate change, particularly in urban areas, should be a priority in decision-making both at a national and international level.

Conlusions
In the course of the research, the following conclusions were drawn regarding the nuisance of climatic and bioclimatic conditions, in the area of a medium-sized city in Central Europe: during a heat wave, the maximum air temperature in the city center may exceed 36.0 • C the temperature exceeds the threshold of 30.0 • C, indicating the presence of hot conditions, usually starting before 1 p.m. and lasting until the evening hours the daily maximum temperature, and at the same time the most onerous bioclimatic conditions in the city, usually occur around 4:00 p.m. -in the city, the heat load on the human body during hot days varies from no heat to a very strong heat stress, at the same time thermal sensations vary from cool to very hot there is a possibility of hyperthermia, which in extreme cases may appear after an hour -in terms of the possibility of dehydration, the most dangerous hours are 3:40 p.m.-4:50 p.m., when water losses with moderate activity can be as much as 775 g·hour −1 -during hot afternoon hours (around 4:00 p.m.), the risk of low oxygen content in the air is observed Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data from the automatic meteorological station belonging to the Kazimierz Wielki University were used for publication. In addition, public data from https://www. ogimet.com/gsynres.phtml.en was used (accessed on 14 April 2020).

Conflicts of Interest:
The authors declare no conflict of interest.