Foehn wind is a non-periodic phenomenon occurring in most mountains worldwide. Its origin depends on the synoptic situation and terrain relief, and it appears on the lee side as a result of synoptic-scale, cross-barrier flow over the mountain range (Barry, Chorley 2003; American Meteorological Society 2023). Foehn is a very complex atmospheric phenomenon (cf. Hoinka 2007) and the first correct classical theory of its formation was put forward by Julius Hann, an Austrian climatologist (1866, as in Barry 2008). Hann’s theory assumes that foehn is a thermodynamic, descending (katabatic), warm, dry and gusty wind blowing along the leeward mountain slopes. Jansing et al. (2022) refer to the phenomenon as “winds and storms on mountain slopes”. There are several theories of the air descending on the leeward side (Steinacker 2006; Seibert 2005, 2012), mostly developed in the first half of the 20th century. Richner and Hächler (2013) postulate that the behaviour of air masses after crossing a mountain ridge is among the least understood mechanisms in flow dynamics. Hann (1866, as in Barry 2008) has postulated that conditions conducive to the formation of a foehn include a high pressure system on one side of a mountain chain, and a low pressure centre on the other side. The pressure gradient on either side of the mountain chain forces air masses over the mountains to flow from high pressure to low pressure, taking into account the Coriolis force. At the peaks of a mountain chain, wind speed increases which is related to the narrowing of the airflow section (Kożuchowski 2012). On the leeward side, the air flows katabatically, according to the slope of the terrain, and it warms according to the dry adiabatic gradient; the spontaneous heating and drying of air masses is referred to as the foehn effect (Kaczorowska 1977; Trepińska 2002; Barry, Chorley 2003). According to Hoinka (1985), an increase in air temperature can be recorded even 100 km from the mountain barrier. As the distance from the barrier grows, foehn wind loses its speed and gustiness and can only be identified based on the so-called foehn effect: an increase in temperature and a decrease in air humidity (Hoinka 1985). Foehn occurrence in the Polish Carpathians contributes to an increase in the mean annual temperature by approximately 1°C and a decrease in relative humidity by approximately 10% (Ustrnul 1992a). Sometimes, the air temperature can grow significantly, with increases by 30°C reported in the literature on the subject (Seibert 1990; Trepińska 2002). Descending dry air masses can cause the dispersion of clouds and create a rain shadow (Barry, Chorley 2003). As far as the warming mechanisms on the leeward side are concerned, the dynamic and thermodynamic warming mechanisms are among the best known (Barry 2008; Richner, Hächler 2013). Elvidge and Renfrew (2016) have been the first to quantify the contribution of the most important mechanisms leading to foehn warming, such as the isentropic adiabatic process during descending air motion, the latent heat exchange mechanism, and the mechanism of turbulent mixing of air, to name a few. According to them, each of the above-mentioned processes leading to an increase in temperature on the leeward side can be dominant, depending on the airflow dynamics.
The occurrence of foehn winds is of biometeorological importance and can influence changes in air pollutant concentrations on the leeward side of a slope (cf. Weber, Prévôt 2002; Solomos et al. 2018). There are papers in literature on the subject that document both decreases (Furger et al. 2005; Druet, Wiśniewski 2021) and increases in pollutant concentrations and deterioration of air quality during foehn winds (Nkemdirim, Leggat 1978; Natale et al. 1999; Seibert et al. 2000; Mintz et al. 2003; Gunia et al. 2008; Li et al. 2015; Kishcha et al. 2018; Álvarez, Carbajal 2019; Ning et al. 2019). Druet and Wiśniewski (2021) have reported a decrease in particulate matter (PM) concentrations in Zakopane at the foothills of the Tatra Mountains at halny speeds on Kasprowy Wierch ≥ 20 m/s. Sekuła et al. (2021) have determined the influence of foehn on PM10 concentrations, air pollutant dispersion conditions and the urban boundary layer in the area of Krakow located about 100 km north of the Tatra Mountains. They have found that the spatial-temporal distribution of PM10 depends on the way foehn winds move through the city. Abrupt weather changes during foehn winds on the leeward side can cause health problems, especially in individuals suffering from weather pains (Murínová 1991; Niedźwiedź 1992), and they can even increase the risk of suicide (Schiffer 1986; Trepińska et al. 2005; Koszewska et al. 2019).
Due to their impact on the local climate and bioclimate conditions, and due to their destructive power, foehn winds are of great interest and have been subjected to numerous comprehensive climate-related analyses for different mountain regions of the world. In Europe, the phenomenon has been most widely investigated in the Alps (e.g., Ambrosetti et al. 2005; Richner, Hächler 2013; Cetti et al. 2015), and worldwide, for example, in the Rocky Mountains (Brinkmann 1974; Whiteman, Whiteman 1974; Durran 1986), in the Andes (Antico et al. 2021), and in the Japanese Alps (Kusaka et al. 2021). Numerous studies in Polish climatology literature deal with the characteristics of foehn occurrence in the Sudetes and the Carpathians (e.g., Milata 1936; Malicki, Michna 1966; Stachlewski 1974; Gorączko 2005). There are a number of local names describing foehn winds (Brinkmann 1971). In the Tatra Mountains and the Podhale region, foehn is referred to as halny and it is a gusty wind that reaches high speeds (Trepińska 2002). Establishing the criteria for the occurrence of foehn events is complicated, with the most commonly adopted criteria being the anemometric and hydrothermal properties. Śliwińska and Ciaranek (2015) believe that these difficulties are due to the many transitional forms of foehn, i.e., anticyclonic, cyclonic, upper-level, slow, and potential. To some extent, the threshold values adopted to qualify wind phenomena are subjective and depend on the researcher (Cetti et al. 2015). Short-lived increases in wind speed are also a problem which can be caused by local factors, so the criterion of a phenomenon’s duration is often used in addition (e.g., Ustrnul 1992b). Stoev and Guerova (2020) believe that foehn occurrence is best observed during the morning and evening hours, when the increase in temperature and decrease in relative humidity deviate from the typical diurnal cycle. Ambrosetti et al. (2005) have also used synoptic and mesoscale criteria to identify foehn winds.
Atmospheric circulation largely determines the temporal and spatial variability of weather conditions, and is also a factor determining the formation of a foehn wind. The subject of the occurrence of foehn in conjunction with different types of atmospheric circulation has been addressed in different parts of the world, with a number of papers dealing with the weather conditions of selected events (e.g., Trepińska, Bąkowski 2000; Vüllers et al. 2018). Foehn most often occurs with strong synoptic-scale flow perpendicular to the mountain ridge (Drobinski et al. 2007) but synoptic weather patterns are diverse and various methods are used to classify them. Stoev and Guerova (2020) have distinguished the types of foehn in Sofia (Bulgaria) according to atmospheric circulation based on air pressure, geopotential height at 500 hPa, geopotential height and air temperature at 850 hPa, and relative humidity at 700 hPa. Stoev et al. (2022) have investigated whether atmospheric circulation affects the decrease in the occurrence of foehn in the Sofia Valley using two classifications: based on the Gross Wettertypen (GWT) circulation pattern catalogue and the Jenkinson-Collison weather type catalogue (JCT). The authors indicate a decrease in W and NW circulation types during the analysed period, which translates into a decrease in the number of days with foehn. Considering atmospheric patterns during foehns in Japan, Kusaka et al. (2021) have used subjective classifications and objective classification with self-organising maps (SOM) to find that in Japan, foehns are associated with extra-tropical cyclones in the Sea of Japan, anticyclones south of Japan, and typhoons around Japan. Shibata et al. (2010) have also pointed to the role of tropical and extra-tropical cyclones in amplifying the pressure gradient in the lower troposphere around Japan, which creates favourable conditions for the occurrence of foehn wind. An exceptionally prolonged (5-day) foehn in central Japan in the summer of 1999 was associated with an intensifying anticyclone east of the island in combination with a rapidly developing cyclone over south-eastern Siberia, and a typhoon moving north into the East China Sea. This situation contributes to the definite intensification of southerly geostrophic winds over central Japan which, in turn, stimulates the occurrence of a prolonged foehn (Inaba et al. 2002). Gaffin (2007) has identified the typical characteristics and synoptic conditions of a foehn that caused large temperature changes over the southern Appalachians. Typically, foehn on the western side of the Appalachians occurred during the south-easterly circulation and developed in the foreground of a low pressure system located over the Mississippi Valley. In contrast, foehns on the eastern side of the Appalachians were mainly north-westerly in direction, and associated with the passage of a shallow cold front. Speirs et al. (2010) have concluded that foehn occurrence in the McMurdo Dry Valleys in Antarctica are caused by strong pressure gradients over mountain ranges associated with synoptic-scale cyclones located off Marie Byrd Land.
Stachlewski (1974) has investigated the circulation origin of foehn occurrences in the Polish Tatra Mountains and their foreland on the basis of the Konček and Rein’s (1971) calendar of atmospheric circulation types in Central Europe. The following circulation types were particularly conducive to foehn occurrence: southern anti-cyclonic, western cyclonic, south-western cyclonic, low pressure trough over Central Europe and a central cyclone. In an analysis of two severe cases of the halny wind in the Tatra Mountains, Śliwińska and Ciaranek (2015) have pointed out that synoptic conditions were affected by a quasi-stationary high-pressure ridge that extended to the south-east of Poland and a low pressure are located over north-western Poland.
The aim of the study was to determine the climate-related regularities with regard to the potential occurrence of the halny wind on the Polish side of the Tatra Mountains, represented by the high-mountain station on Kasprowy Wierch, and to establish typical pressure field systems and synoptic conditions over Europe during days with halny.