Ewa Bednorz

The study concerns the determination of the characteristics of bioclimatic conditions, as well as the synoptic situations related to the occurrence of thermal stress conditions, in Poland. The study was based on daily data obtained from the Institute of Meteorology and Water Management – National Research Institute from the period 1966–2020 for 37 synoptic stations in Poland. Based on the obtained data, values of the Universal Thermal Climate Index (UTCI) were calculated. The occurrence of heat stress increases from the north to the south, corresponding with the variability of influx of solar radiation, and is modified by factors at a smaller spatial scale. The results of this paper evidently point to the cooling effect of the waters of the Baltic Sea. In circulation conditions favouring strong and very strong heat stress, e.g. in two of the designated circulation types (T1 and T2), the occurrence of an expansive high-pres­sure ridge in the Atlantic-European area is typical, stretching from the region of the Azores High towards the north-east, with a secondary high developed within its boundaries. In the third of the designated circulation types (T3), the high-pressure area extends from the Azores eastwards, reaching the Black Sea. Each of the three circulation patterns associated with the unfavourable biometeorological conditions of very strong and extreme cold stress in Poland is characterised by strong pressure centres formed in the Euroatlantic region, triggering the airflow from the northern (T4) or eastern (T5, T6) sector. The study revealed high variability of bioclimatic conditions in Poland, both in temporal and spatial terms. The lowest UTCI was recorded in the north-east of Poland and at the east coast of the Baltic Sea. The highest index values were observed in south-western and western regions of the country. High spatial variability of UTCI related to regional variability of climatic conditions in Poland permitted the designation of bioclimatic regions characterised by the different occurrence of heat stress, particularly in the cool season of the year. Regions in the south-west and west of Poland proved the most favourable in bioclimatic terms, with the highest number of days with no thermal stress. In these regions, the highest UTCI values were observed, while the lowest were recorded in the northeast of Poland and at the east coast of the Baltic Sea. Among unfavourable biometeorological conditions, the ones causing hypothermia have so far occurred more frequently than the ones causing overheating of the human organism.

The spatiotemporal variability of atmospheric blockings over the Euro-Atlantic region and their influence on the occurrence of the persisting extreme temperature conditions in Poland namely heat waves and cold spells during the period 1978-2023 was analysed. Blockings were identified at 500 hPa geopotential level, using the meridional geopotential gradient method, supplemented with the quantile filter and persistence filter. Heat waves (HWs) and cold spells (CSs) were defined as sequences of at least 3 days with the maximum air temperature above 30°C or below -10°C, respectively. The climatology of Euro-Atlantic blocking occurrence in the zonal belt between 45 and 75 degrees in the northern hemisphere exhibits high spatiotemporal variability. Blocking structures are most frequent in the spring (MAM) occurring with a 10% frequency in the arched belt extending from the west of the British Islands through the North Sea, south Scandinavia, to the east of the Baltic Sea. In summer (JJA) they are most frequent in the area spanning from south Finland to west Russia, as well as over Greenland. During winter (DJF) blockings are most commonly located over the southeast Atlantic (west of France) and extending northeastward up to southern Scandinavia with their occurrence being least frequent in autumn (SON). The mean annual number of HW days ranges from one day in the southern mountainous regions and in northern Poland to more than four days in the central and western parts. However, during the extremely hot summer of 2015, the number of HW days exceeded 20 in central and southern Poland. CSs have become rarer, with the average seasonal number of CS days amounting to 1-2 days in recent decades, and only in singular seasons such as 1986/1987, more than 10 days fulfilled the criteria of CS in northeastern Poland. The occurrence of HWs in Poland is constantly accompanied by blocking situations, most often located northeast of Poland, while the winter CSs are associated with the blockings located over the North Atlantic and northern Scandinavia.

Foehn wind occurrence has generated great interest among researchers because of the destructive power and impact on the local climate. Based on anemometric data provided by a high-mountain station on Kasprowy Wierch in the Polish Tatra Mountains, the characteristics of the potential occurrence of foehn wind (referred to as halny in the Polish Tatras) are presented, including its speed and duration, as well as the frequency of occurrence on a multiannual, annual and daily basis. Halny winds occur most frequently in the cold period of the year (Oct–Feb), with the frequency peaking in November, and sporadically in the summer. The occurrence of foehn winds is strongly dependent on the synoptic situation. Therefore, the main aim of the study was to identify the circulation conditions conducive to their occurrence on the Polish side of the Tatra Mountains. Circulation conditions responsible for foehn formation were analysed using gridded sea level pressure (SLP) data from the NCEP-DOE (National Centers for Environmental Prediction-Department of Energy) reanalyses. The occurrence of foehn wind in the Tatra Mountains is associated with a low pressure system over north-western Europe, and above normal pressure over south-eastern Europe. The location and intensity of the centres of atmospheric influence on foehn days can vary, as indicated by the three types of pressure systems favouring the occurrence of the phenomenon, distinguished by the hierarchical grouping method. In type 1, the cyclonic centre spreads over northern Europe, in type 2 over western Europe and in type 3 over north-western Europe. In types 1 and 3, the air masses come from the south-west, and in type 2 more from the south. Type 3 is characterised by the greatest horizontal pressure gradients among the three circulation types and with the greatest SLP anomalies.

Recent climatic changes, most evident in air temperature, also significantly impact sensible conditions, particularly affecting the human body during the summer season in urban areas. This study utilized hourly values of air temperature (t, °C) and relative humidity (RH, %) for 2008–2022 obtained from nine measurement points located in the city of Poznań. The measurement network was set up and supported by the Department of Meteorology and Climatology of Adam Mickiewicz University in Poznań. Based on these data, thermal conditions were characterized using the Humidex (HD), as well as “hot days” (Tmax > 30 °C) and heat waves determined based on them. The conducted research revealed variability in thermal conditions across the city. The highest average Tmax (27.4 °C) and HD values (31.3 °C), as well as the greatest average number of hot days (15 days), were recorded in the compact, dense city center. In contrast, the lowest values (respectively: 25.9 °C, 27.5 °C, 8 days) were observed in areas with a significantly greater proportion of green spaces located further from the center. Furthermore, significant trends of change were observed: an annual average Tmax increase of 1.2–1.6 °C/decade and HD values showing an increase of 1.5–3.0 °C/decade. This study also highlighted the noteworthy impact of urbanization and the presence of urban greenery on the frequency and total duration of heat waves. Between 2008 and 2022, the area of compact development experienced 26 heat waves lasting a total of 115 days, whereas the area with a high proportion of greenery recorded 14 cases and 55 days, respectively. The longest recorded heat wave, spanning 9 days, occurred in July 2010. Individual heat waves identified based on Tmax (>30 °C) may exhibit significant variations in terms of perceived conditions, as indicated by HD.

Significant increasing trends in the air temperature were found both in the surface station of Svalbard Lufthavn and in the low-tropospheric temperature field over the Atlantic Arctic. The variability in temperature, as well as the multiannual trend, is at least three times bigger in the winter months than in summer. An attempt was made to explain the high day-to-day variability in the winter air temperature by the daily variability in the regional pressure field and circulation conditions. Six regional-scale circulation patterns were found by applying the principal component analysis to the mean daily sea level pressure (SLP) reanalysis data and their impact on the low-tropospheric air temperature variability was determined. A bipolar pattern, with a positive center over Greenland and a negative center over the White Sea, dominates in the region and strongly influences the air temperature field at 850 hPa geopotential height (correlation coefficients up to –0.65). The second pattern that impacts the temperature field in the Atlantic Arctic is the one with a center of action over Svalbard (mostly a low-pressure center in winter), strongly influencing the air temperature over the Barents Sea. The remaining circulation types, explaining only 5–8% of the total variance of the SLP field each, do not modify significantly the air temperature at 850 hPa geopotential level over the Atlantic Arctic, and none of the circulation types seems to influence the multiannual temperature trends.

The research identified patterns in the multiannual course of start and end dates, and length of growing sea- son (GS) in Central Europe since the end of the 19th century in selected cities of Central Europe in the period 1893–2020. GS start in the analysed stations was characterised by high year-to-year variability, particularly in those located more southwards, i.e. in Prague and Vienna. A smaller variability occurred in GS end dates. The GS was subject to prolon- gation, although these changes in particular cities were uneven and had different causes. In Toruń and Potsdam, its increase was caused by a greater shift of the end date, and in the remaining stations, it was determined by its earlier start date. Two subperiods were distinguished that differ in terms of intensity of changes of the start and end dates, as well as the length of the GS. The intensification was observed recently.

The present study concerns the determination of the characteristics of bioclimatic conditions, as well as the synoptic situations related to the occurrence of thermal stress conditions, in Poland. The study was based on daily data obtained from the Institute of Meteorology and Water Management – National Research Institute from the period 1966–2020 for 37 synoptic stations in Poland. Based on the obtained data, values of the Universal Thermal Climate Index (UTCI) were calculated. The occurrence of heat stress increases from the north to the south, corresponding with the variability of influx of solar radiation, and is modified by factors at a smaller spatial scale. The results of this paper evidently point to the cooling effect of the waters of the Baltic Sea. In circulation conditions favouring strong and very strong heat stress, e.g. in two of the designated circulation types (T1 and T2), the occurrence of an expansive high-pressure ridge in the Atlantic-European area is typical, stretching from the region of the Azores High towards the north- east, with a secondary high developed within its boundaries. In the third of the designated circulation types (T3), the high-pressure area extends from the Azores eastwards, reaching the Black Sea. Each of the three circulation patterns associated with the unfavourable biometeorological conditions of very strong and extreme cold stress in Poland is characterised by strong pressure centres formed in the Euroatlantic region, triggering the airflow from the northern (T4) or eastern (T5, T6) sector.