The Suitability of Snow and Meteorological Conditions of South-Central Slovakia for Ski Slope Operation at Low Elevation—A Case Study of the Koš ú tka Ski Centre

: In this study, the snow conditions of South-Central Slovakia (Inner Western Carpathians; temperate zone) were analyzed to assess the suitability for ski slope operations without snow production under 1000 m a.s.l. For the study site of the Koš ú tka Ski Centre, meteorological conditions for snowmaking, snowpack characteristics, and snow water equivalent (SWE) compared with seasonal precipitation were identiﬁed. To identify the months suitable for snowmaking, the number of potential snowmaking days (PSD) and the required number of snowmaking days (RNSD) were calculated for six winter seasons from 2010–2011 to 2015–2016. The results showed that the conditions of natural snow cover were not appropriate for ski slope operation because of a low natural snow depth. For the Koš ú tka Ski Centre, it was concluded that the essential base layer snowmaking for ski slope operation is possible only for a few days in the winter season because of the increasing mean value of the mean average daily temperature and the consequently higher occurrence of liquid precipitation in the winter season. Essential high snow production results in the heterogeneous distribution of snow on the ski slope, and in high snow depth, density, and SWE of the ski slope snowpack, and in prolonged melting.


Introduction
Snowfall and snow cover have a significant impact on different human activities and, therefore, on the entire society and environment. This impact is obvious mainly in mountainous regions of the world with an abundance of natural snow [1]. In Central Europe (above 500 m a.s.l., but mainly above 1000 m a.s.l.), such regions are, particularly, the Alpine and Carpathian regions. In these parts of Europe, the amount of snow and the duration of snow cover are of great social and economic importance. The amount of snow mainly affects winter tourism and water supplies for hydroelectric power stations [2]. Winter tourism is one of the most important economic sectors of mountainous regions in the world [3]. However, winter tourism depends on good snow conditions and is very -to assess the suitability of meteorological and snow conditions for the ski slope operation without snow production; -to assess the suitability of meteorological conditions for snow production and for base layer snowmaking at the beginning of winter season; -to identify the seasonal variability of the ski slope snowpack characteristics (snow depth, as well as spatial variability, snow density, and snow water equivalent "SWE") and to compare the ski slope snowpack SWE with seasonal precipitation.

Study Site
The study was conducted at the Košútka Ski Centre (48.559 • N, 19.535 • E; Figure 1a) in the Inner Western Carpathians (Slovenské Rudohorie Mts., Veporic Unit, Slovakia; Figure 2). The study site belongs to the moderately warm climatic region with cool to cold winters [24], a mean annual temperature of 5.5 • C [25], a mean annual precipitation total of 850 mm [26,27], 90 days with snow cover, and a mean snow depth of 36.7 cm [28] (data from 1961 to 1990). The length of the ski slope is 950 m with an altitudinal difference of 220 m (500-720 m a.s.l.), western to northern aspect, and slope from 7 • to 25 • (Figure 2). Since the establishment of the ski center in 2007, the ski slope has been covered by artificial snow (Figure 1b). At the end of the winter season, a high volume of snow remains on the slope (Figure 1a). Fixed snow-making lances, which can reach a distance from 5 to 30 m and operate at a maximum working temperature of −4 • C, have been used to produce snow. The stream "Slanec" running at the foot of the ski slope is used as the water source.
Water 2018, 10, x FOR PEER REVIEW  3 of 19 cover, and a mean snow depth of 36.7 cm [28] (data from 1961 to 1990). The length of the ski slope is 950 m with an altitudinal difference of 220 m (500-720 m a.s.l.), western to northern aspect, and slope from 7° to 25° (Figure 2). Since the establishment of the ski center in 2007, the ski slope has been covered by artificial snow (Figure 1b). At the end of the winter season, a high volume of snow remains on the slope (Figure 1a). Fixed snow-making lances, which can reach a distance from 5 to 30 m and operate at a maximum working temperature of −4 °C, have been used to produce snow. The stream "Slanec" running at the foot of the ski slope is used as the water source.    cover, and a mean snow depth of 36.7 cm [28] (data from 1961 to 1990). The length of the ski slope is 950 m with an altitudinal difference of 220 m (500-720 m a.s.l.), western to northern aspect, and slope from 7° to 25° (Figure 2). Since the establishment of the ski center in 2007, the ski slope has been covered by artificial snow (Figure 1b). At the end of the winter season, a high volume of snow remains on the slope (Figure 1a). Fixed snow-making lances, which can reach a distance from 5 to 30 m and operate at a maximum working temperature of −4 °C, have been used to produce snow. The stream "Slanec" running at the foot of the ski slope is used as the water source.  The Košútka Ski Centre is one of the 15 South-Central Slovakian ski centers in operation ( Figure 2). Other ski centers are out of service because of a lack of natural snow or because of economic problems. At the 96 points of the snow course (red dots), the snow depth and density of the snow were measured, while the occurrence of snow was regularly recorded on the "observed area". The Košútka Ski Centre is situated in South-Central Slovakia (Central Europe), where 15 ski centers (blue dots) and four investigated precipitation stations (pink dots) are located.
The Košútka Ski Centre is one of the 15 South-Central Slovakian ski centers in operation ( Figure 2). Other ski centers are out of service because of a lack of natural snow or because of economic problems . The average length of the winter season at the 15 ski centers is 87 days (start: 20 December;  end: 17 March; Table 1), while the average ski slope elevation in all the ski centers is lower than 1000 m a.s.l., except for Skalka arena (1105 m a.s.l.). Ski slopes of the South-Central Slovakian ski centers are 68% artificially covered on average (Table 1). At eight ski centers, ski slopes are fully artificially covered, while in three ski centers snowmaking is not used. The investigated precipitation stations of the Slovak Hydrometeorological Institute (SHMÚ) have comparable elevation (from 575 to 771 m a.s.l.) and location in the South-Central Slovakia to the 15 ski centers (Figure 2). The macro climate conditions of the localities where the precipitation stations are situated are comparable to the characteristics of the Košútka Ski Centre study site (see above). Table 1. Characteristics of the South-Central Slovakian ski centers. Probable start/end of the expected winter season 2018-2019, determined according to www.onthesnow.sk [29]. Artificially covered slopes = the percentage of the artificially covered ski slopes. Investigated ski center highlighted in bold. Four precipitation stations ( Figure 2) situated below 1000 m a.s.l. were selected to identify the mean monthly snow depth in South-Central Slovakia. A depth of natural snow cover of at least 30 cm on grassland is considered sufficient for skiing [30]. In this study, a month was considered as suitable for ski slope operation when the mean monthly snow depth of natural snow cover was higher than 30 cm. Steiger [31] defined the minimum depth of the artificially covered ski slope snowpack as 20 cm, because of the higher snow density compared to natural snow cover. The artificially covered ski slope snowpack at the Kośútka Ski Centre was considered as suitable for operation when its depth was at least 20 cm. To analyze the meteorological and snow conditions, the mean monthly values were calculated from six seasons (from 2010-2011 to 2015-2016) and mean seasonal values were calculated from six months (XI-IV; the period with snow occurrence) or from four months (XII-III; the period when the South-Central Slovakian ski centers are in operation). To distinguish the seasonal values, the proper months are shown in brackets).

Name of the Ski Center
The daily values of precipitation totals, the depth of new snow, and the depth of natural snow cover were measured by the Slovak Hydrometeorological Institute (SHMÚ) at the Snohy precipitation station (Figure 2; 771 m a.s.l.; 48.628 • N, 19.550 • E), 7 km away from the ski center (air direct distance). The daily average temperature, wind speed, and wind direction was recorded by the meteorological station at the ski center. The analyzed data were from seasons 2010-2011 to 2015-2016, and, during each season, the months XI-IV were evaluated.
The number of potential snowmaking days (PSD = days with optimal snowmaking conditions) and required number of snowmaking days (RNSD = number of days required for balancing snow melt by snow production) was identified for each month from XI to IV [15]. The number of PSD was calculated as a sum of days that reached the threshold of −2 • C daily average temperature [15]. The RNSD value was calculated according to a formula described in Steiger and Mayer [15]. A degree day factor of 3 mm was used in the formula because of the ski center's low elevation. This factor describes the runoff (in mm) per degree day (the sum of all positive daily average temperatures). A month in which the PSD value is greater than the RNSD value is defined by Steiger and Mayer [15] as a month suitable for snowmaking. If a positive difference between PSD and RNSD was determined in several days greater or equal to five days in a month, the month was considered suitable for base layer snowmaking. This is because the average snowmaking systems can produce snow cover suitable for skiing (20 cm thick) in five days [31]. Base layer snowmaking is defined as "the first area-wide snowmaking of a ski season, mostly started between mid-November and beginning of December" [32].

Characteristics of the Ski Slope Snowpack
After the natural snow had melted away in the surroundings of ski slope (Figures 1a and 2), the snow depth and density of the snow were measured on the artificially covered ski slope. The measurements were performed for only one day in each of the winter seasons from 2010−2011 to 2015−2016, except for 2013−2014 (when the ski center was out of service). The depth of the snow was always measured at 96 points, and the snow density was measured at least five points of the snow course ( Figure 2). The points of the snow course were localized by a GPS receiver and the SKPOS service (Slovak real-time positioning service). A model VS-43 snow sampling tube was used to obtain gravimetric samples of snow to measure the depth-averaged snow density. The method of the manual snow depth and density measurement is more thoroughly described in Hríbik et al. [33]. Ninety-six values of snow water equivalent were calculated from 96 values of snow depth multiplied by the mean value of the snow density.
The ski slope snowpack was observed in the field and on ski center webcams to detect the date of its snowmelt. The duration of the examined snowpack melting, compared to off-piste sites with natural snow, represents the period between the date of our measurements and the date of its snowmelt. The snowpack was considered melted when less than 5% of the observed area (4.1 ha) was covered by snow ( Figure 2).
To assess the distribution of snow over the groomed ski slope, the relationship between the snow depth and the closest distance from the fixed snow-making lances was tested using the set of 96 points of the snow course ( Figure 2). The distance of the points from the snow-making lances was generated in a GIS environment using the command "Near". Only those lances which were in operation in a particular season were used in the analyses.

Statistical Analysis
The relationship between the dependent variables (mean seasonal value of daily air temperature; mean seasonal depth of the ski slope snowpack; melting period of the ski slope snowpack; mean daily average temperature during the melting period) and the independent variables (six winter seasons; snowmelt day of natural snow cover) was tested in STATGRAPHICS using a simple linear regression. The output of the analyses included the Pearson correlation coefficient (r), which quantifies the strength of the linear statistical relationship. The significance of the relationship was examined by testing the variance of the values around the linear regression at the 95% significance level. If the "p" value was equal to or greater than 0.05, the relationship was not significant. In the chapter results, the mean values are presented with standard deviations (mean ± SD). To analyze the relationship between the snow depth and the closest distance from the fixed snow-making lances, the statistical tests of the logarithmic relationships (ANOVA) were used. The strength of each relationship was determined with the correlation coefficient (r). For the five-season multiple comparison of the ski slope, snowpack characteristics (snow depth, snow density, and snow water equivalent) used the multiple comparison procedure in STATGRAPHICS to determine which means were significantly different from each other. The output shows the estimated difference and statistically significant differences between each pair of means.

South-Central Slovakia
The depth of natural snow cover on grassland is considered to be sufficient for skiing (ski slope operation) when it is at least 30 cm [30]. In the six-season average, the mean monthly depth of natural snow cover in South-Central Slovakia was sufficient for ski slope operation for, maximally, two months (January, February; Figure 3b) in two of six seasons ( Table 2). The probability that, in the winter season, a month with sufficient monthly average snow depth will occur varied from 17% to 33%. High standard deviations displayed in Figure 3a express high inter-seasonal variability of the monthly average snow depth, which was the highest at all precipitation stations ("station") in February. The highest variability in February was detected at the Bane station, where the monthly average snow depth in February 2012 was 66 cm and in February 2014 was 1 cm. Seasons 2011-2012 and 2012-2013 were snow-rich because the monthly average snow depth in all precipitation stations was higher than 30 cm for at least one month ( Table 2). Two month-long periods, when the monthly average snow depth was higher than the threshold of 30 Table 2). different from each other. The output shows the estimated difference and statistically significant differences between each pair of means.

Meteorological and Snow Conditions
The depth of natural snow cover on grassland is considered to be sufficient for skiing (ski slope operation) when it is at least 30 cm [30]. In the six-season average, the mean monthly depth of natural snow cover in South-Central Slovakia was sufficient for ski slope operation for, maximally, two months (January, February; Figure 3b) in two of six seasons ( Table 2). The probability that, in the winter season, a month with sufficient monthly average snow depth will occur varied from 17% to 33%. High standard deviations displayed in Figure 3a express high inter-seasonal variability of the monthly average snow depth, which was the highest at all precipitation stations ("station") in  Table 2).     The mean monthly values calculated from six seasons (from 2010-2011 to 2015-2016) and the mean seasonal values calculated from six months (XI-IV) for the Košútka Ski Centre showed the following patterns in precipitation, snow, and temperature.
Solid precipitation (snow) in the study site occurred from November to April (Figure 4a,c). Snow represented the highest percentage of the total monthly precipitation in January (61%), December (59%), and February (39%) (Figure 4c). Mean monthly precipitation totals (mean ± SD) higher than 55 ± 13 mm (value of XI-IV, average) were determined in February (69 ± 60 mm), November (65 ± 44 mm), and January (62 ± 27 mm) (Figure 4a), while in December the lowest mean monthly precipitation total was identified (39 ± 3 mm). A high inter-seasonal variability of the monthly precipitation totals was identified in November, January, February, and March (53 ± 36 mm) because of the high standard deviation of the mean monthly precipitation totals ( Figure 4a The mean monthly values calculated from six seasons (from 2010-2011 to 2015-2016) and the mean seasonal values calculated from six months (XI-IV) for the Košútka Ski Centre showed the following patterns in precipitation, snow, and temperature.
Solid precipitation (snow) in the study site occurred from November to April (Figure 4a,c). Snow represented the highest percentage of the total monthly precipitation in January (61%), December (59%), and February (39%) (Figure 4c). Mean monthly precipitation totals (mean ± SD) higher than 55 ± 13 mm (value of XI-IV, average) were determined in February (69 ± 60 mm), November (65 ± 44 mm), and January (62 ± 27 mm) (Figure 4a), while in December the lowest mean monthly precipitation total was identified (39 ± 3 mm). A high inter-seasonal variability of the monthly precipitation totals was identified in November, January, February, and March (53 ± 36 mm) because of the high standard deviation of the mean monthly precipitation totals ( Figure 4a  In the six-season average, the mean monthly snow depth of natural snow cover was not sufficient for skiing in all months with snow occurrence (XI-IV; Figure 5a). The mean monthly snow depth was lower than the sufficient snow depth for ski slope operation without snow production (30 cm). The highest mean monthly snow depth (±SD) was determined in February (20.0 ± 21.2 cm) and January (14.9 ± 16.8 cm). The winter season at the study site starts at the end of December and ends in the middle of March (Table 1). Therefore, the mean seasonal value of snow depth for natural snow cover was calculated from these four months (XII-III). The mean seasonal depth of natural snow  In the six-season average, the mean monthly snow depth of natural snow cover was not sufficient for skiing in all months with snow occurrence (XI-IV; Figure 5a). The mean monthly snow depth was lower than the sufficient snow depth for ski slope operation without snow production (30 cm). The highest mean monthly snow depth (±SD) was determined in February (20.0 ± 21.2 cm) and January (14.9 ± 16.8 cm). The winter season at the study site starts at the end of December and ends in the middle of March (Table 1). Therefore, the mean seasonal value of snow depth for natural snow cover was calculated from these four months (XII-III). The mean seasonal depth of natural snow cover (XII-III) was lower than the sufficient snow depth for ski slope operation (30 cm) in all the six analyzed winter seasons (Figure 5c). The six-season average mean seasonal snow depth was 11.4 ± 11.3 cm (XII-III mean ± SD from seasons 2010-2016). The results showed high inter-seasonal variability in the mean seasonal depth of natural snow (Figure 5c). The highest mean seasonal values (mean ± SD) were identified in seasons 2012-2013 (27.7 ± 16.0 cm) and 2011-2012 (23.3 ± 20.1 cm). On the contrary, the lowest mean seasonal depths of natural snow were identified in seasons 2015-2016 (1.7 ± 2.2 cm) and 2013-2014 (1.9 ± 1.9 cm). The values of the mean seasonal and monthly snowfall totals showed the same pattern (Figure 5b,d). The highest mean monthly snowfall totals (mean ± SD) were determined for the following months: January (47.5 ± 32.2 cm), February (29.5 ± 29.1 cm), and December (26.5 ± 20.0 cm). No trend in the seasonal snow depth or snowfall totals was determined.
The mean seasonal value (XII-III) of daily air temperature has shown an increasing tendency (y = 0.5979x − 1.5342; Figure 5c

Snowmaking Conditions
The main meteorological conditions affecting snowmaking are (in order of importance): (i) temperature; (ii) relative humidity; (iii) wind speed; and (iv) wind direction.
According to Steiger and Mayer [15], the days with T lower than −2 °C daily average temperature are defined as potential snowmaking days with optimal snowmaking conditions. At the Košútka Ski Centre, potential snowmaking days occurred from November to March, with a peak in January (Figure 6a). The optimal conditions for snowmaking (Table 3) were, therefore, mainly in January (13 days), December (10 days), and February (9 days).

Snowmaking Conditions
The main meteorological conditions affecting snowmaking are (in order of importance): (i) temperature; (ii) relative humidity; (iii) wind speed; and (iv) wind direction.
According to Steiger and Mayer [15], the days with T lower than −2 • C daily average temperature are defined as potential snowmaking days with optimal snowmaking conditions. At the Košútka Ski Centre, potential snowmaking days occurred from November to March, with a peak in January ( Figure 6a). The optimal conditions for snowmaking (Table 3) were, therefore, mainly in January (13 days), December (10 days), and February (9 days).   On average, the months of December, January, and February were identified as suitable for snowmaking. In these months, the number of potential snowmaking days (PSD = the days with optimal snowmaking conditions) was greater than the required number of snowmaking days (RNSD = number of days required for balancing snow melt with snow production). The difference between PSD and RNSD identifies the number of days suitable for base layer snowmaking (not for the restoration of melted snow). At the study site, four such days were identified in December, eight days in January, and three days in February (six-season average; Table 3). The average snowmaking systems can produce sufficient snow cover for skiing (20 cm thick) in five days [26]. Thus, at the Košútka Ski Centre, there were enough snowmaking days (for the 20-cm-thick base layer snowmaking) only in January (eight days; Table 3) on average. According to the ski center owner, base layer snowmaking is necessary at the beginning (XII-I) of each season. In December, it was possible during two seasons and in January during four seasons (six-season average; Figure 6b). In two seasons (2013-2014 and 2014-2015), less than a 20-cm-thick snow cover could be created (insufficient for skiing). Generally, at the beginning of the winter season (XII-I) there were 11 days with optimal snowmaking conditions (PSD), but only six days left for base layer snowmaking on average. The remaining five days were necessary for the restoration of melted snow. The number of PSD and RNSD is dependent on the daily average temperature. Figure 6b     On average, the months of December, January, and February were identified as suitable for snowmaking. In these months, the number of potential snowmaking days (PSD = the days with optimal snowmaking conditions) was greater than the required number of snowmaking days (RNSD = number of days required for balancing snow melt with snow production). The difference between PSD and RNSD identifies the number of days suitable for base layer snowmaking (not for the restoration of melted snow). At the study site, four such days were identified in December, eight days in January, and three days in February (six-season average; Table 3). The average snowmaking systems can produce sufficient snow cover for skiing (20 cm thick) in five days [26]. Thus, at the Košútka Ski Centre, there were enough snowmaking days (for the 20-cm-thick base layer snowmaking) only in January (eight days; Table 3) on average. According to the ski center owner, base layer snowmaking is necessary at the beginning (XII-I) of each season. In December, it was possible during two seasons and in January during four seasons (six-season average; Figure 6b). In two seasons (2013-2014 and 2014-2015), less than a 20-cm-thick snow cover could be created (insufficient for skiing). Generally, at the beginning of the winter season (XII-I) there were 11 days with optimal snowmaking conditions (PSD), but only six days left for base layer snowmaking on average. The remaining five days were necessary for the restoration of melted snow. The number of PSD and RNSD is dependent on the daily average temperature. Figure 6b clarifies that, in the months with lower mean daily average temperatures, the number of PSD increased and the number of RNSD decreased. Because of low mean daily average temperatures in December and January in two of the six seasons (seasons 2010-2011 and 2012-2013), a high number of PSD in these seasons was identified (Figure 6b). Wind speed and wind direction are not crucial meteorological conditions for snowmaking. Nevertheless, wind significantly affects the accumulation of artificial snow during its production. In three-season averages, during the potential snowmaking days in December and January, wind with a maximum speed of 1 m/s occurred 73% of the time, while it blew 65% of the time from the SW-SE side (Figure 7a,b). The mean difference in the wind speed categories between December and January Wind speed and wind direction are not crucial meteorological conditions for snowmaking. Nevertheless, wind significantly affects the accumulation of artificial snow during its production. In three-season averages, during the potential snowmaking days in December and January, wind with a maximum speed of 1 m/s occurred 73% of the time, while it blew 65% of the time from the SW-SE side (Figure 7a,b). The mean difference in the wind speed categories between December and January was 14 h (2% of occurrence; Figure 7a). During the days with optimal snowmaking conditions (PSD) at the beginning of the winter season (XII-I), the wind speed was proper for the snow production most of the time. At the beginning of the winter season, at the study site, 94% of the time the wind was calm or blew only lightly with a maximum speed of 2 m/s from the SW-SE side (Figure 8).
Water 2018, 10, x FOR PEER REVIEW 10 of 19 was 14 h (2% of occurrence; Figure 7a). During the days with optimal snowmaking conditions (PSD) at the beginning of the winter season (XII-I), the wind speed was proper for the snow production most of the time. At the beginning of the winter season, at the study site, 94% of the time the wind was calm or blew only lightly with a maximum speed of 2 m/s from the SW-SE side (Figure 8).

Characteristics of the Ski Slope Snowpack
The ski slope snowpack melts several weeks (melting period "mp") after the natural snow cover in the surroundings melts away (Figure 9b). The melting period of the ski slope snowpack was the shortest in the 2012-2013 season (25 days) and the longest in the 2015-2016 season (47 days). In the six-season average, the snowpack of the artificially covered ski slope persisted on the slope 29 days (median) longer compared to off-piste sites with natural snow. The natural snow cover on the offpiste sites was melted away each season on a different date (Figure 9a). The highest difference of 51 days was identified between the seasons 2015-2016 (18 February) and 2012-2013 (10 April). A significant, linear relationship was proved between the date in the season (independent variable) when the natural snow cover on the off-piste sites was melted away (date of snowmelt) and the duration of the ski slope snowpack melting period (dependent variable 1) or the mean daily average temperature during the melting period (dependent variable 2) (Figure 9b). With an earlier date of snowmelt, the melting period of the ski slope snowpack was longer, and the daily average temperature during this melting period was lower. The remaining ski slope snowpack that was a mixture of artificial snow and natural precipitation had the following characteristics (five-season mean values ± SD; Figure 9a At the Košútka Ski Centre, the depth of the ski slope snowpack at the end of the each of the five seasons was always higher than the sufficient snow depth for skiing (20 cm; Figure 9c). The mean was 14 h (2% of occurrence; Figure 7a). During the days with optimal snowmaking conditions (PSD) at the beginning of the winter season (XII-I), the wind speed was proper for the snow production most of the time. At the beginning of the winter season, at the study site, 94% of the time the wind was calm or blew only lightly with a maximum speed of 2 m/s from the SW-SE side (Figure 8).

Characteristics of the Ski Slope Snowpack
The ski slope snowpack melts several weeks (melting period "mp") after the natural snow cover in the surroundings melts away (Figure 9b). The melting period of the ski slope snowpack was the shortest in the 2012-2013 season (25 days) and the longest in the 2015-2016 season (47 days). In the six-season average, the snowpack of the artificially covered ski slope persisted on the slope 29 days (median) longer compared to off-piste sites with natural snow. The natural snow cover on the offpiste sites was melted away each season on a different date (Figure 9a). The highest difference of 51 days was identified between the seasons 2015-2016 (18 February) and 2012-2013 (10 April). A significant, linear relationship was proved between the date in the season (independent variable) when the natural snow cover on the off-piste sites was melted away (date of snowmelt) and the duration of the ski slope snowpack melting period (dependent variable 1) or the mean daily average temperature during the melting period (dependent variable 2) (Figure 9b). With an earlier date of snowmelt, the melting period of the ski slope snowpack was longer, and the daily average temperature during this melting period was lower. The remaining ski slope snowpack that was a mixture of artificial snow and natural precipitation had the following characteristics (five-season mean values ± SD; Figure 9a At the Košútka Ski Centre, the depth of the ski slope snowpack at the end of the each of the five seasons was always higher than the sufficient snow depth for skiing (20 cm; Figure 9c). The mean

Characteristics of the Ski Slope Snowpack
The ski slope snowpack melts several weeks (melting period "mp") after the natural snow cover in the surroundings melts away (Figure 9b). The melting period of the ski slope snowpack was the shortest in the 2012-2013 season (25 days) and the longest in the 2015-2016 season (47 days). In the six-season average, the snowpack of the artificially covered ski slope persisted on the slope 29 days (median) longer compared to off-piste sites with natural snow. The natural snow cover on the off-piste sites was melted away each season on a different date (Figure 9a). The highest difference of 51 days was identified between the seasons 2015-2016 (18 February) and 2012-2013 (10 April). A significant, linear relationship was proved between the date in the season (independent variable) when the natural snow cover on the off-piste sites was melted away (date of snowmelt) and the duration of the ski slope snowpack melting period (dependent variable 1) or the mean daily average temperature during the melting period (dependent variable 2) (Figure 9b). With an earlier date of snowmelt, the melting period of the ski slope snowpack was longer, and the daily average temperature during this melting period was lower. The remaining ski slope snowpack that was a mixture of artificial snow and natural precipitation had the following characteristics (five-season mean values ± SD; Figure 9a,c,d): snow depth: 44.5 ± 9.5 cm, density: 618.8 ± 63.1 kg/m 3 , snow water equivalent (snow water equivalent (SWE): 281.4 ± 46.2 mm.
At the Košútka Ski Centre, the depth of the ski slope snowpack at the end of the each of the five seasons was always higher than the sufficient snow depth for skiing (20 cm; Figure 9c). The mean depth of the ski slope snowpack measured at the end of the season showed a decreasing inter-seasonal trend (Figure 9c; y = −5.181x + 60.051, r = 0.864) and significant differences ( Table 4). The highest difference of 22.9 cm was identified between seasons 2011-2012 and 2015-2016. The mean depth of snow measured on the artificially covered ski slope at the end of the winter season was 2.3 times higher (31.2 cm difference) than the mean seasonal depth (XII-III) of natural snow in the surroundings (five-season average; Figure 9c). The highest difference between these two values was determined in the seasons with a shortage of natural snow (seasons 2010-2011, 2011-2015 and 2015-2016). Because of the high density of the ski slope snowpack, a high amount of water was still stored in it after the natural snow had melted away. In the five-season average, the mean SWE value of the remaining ski slope snowpack (281.4 ± 46.2 mm; mean ± SD) represented 120% of the seasonal precipitation totals (XII-III; 234.1 ± 94.7 mm; Figure 9d), and the water input from the melting ski slope snowpack (=SWE; 281.4 ± 46.2 mm; mean ± SD) was 1.3 times higher than the seasonal (XI-IV) solid precipitation totals (124.6 mm). A significantly different mean SWE of the ski slope snowpack, measured at the end of these seasons, was identified between five seasons ( Table 4). The highest difference (126.5 mm) was determined between the same seasons, as in the case of the mean snow depth (2011-2012 vs. 2015-2016). At the end of the season, the mean snow density of the ski slope snowpack showed significant inter-seasonal differences ( Table 4). The five-season average showed above-average mean snow density (higher than 618.8 kg/m 3  depth of the ski slope snowpack measured at the end of the season showed a decreasing interseasonal trend (Figure 9c; y = −5.181x + 60.051, r = 0.864) and significant differences ( Table 4). The highest difference of 22.9 cm was identified between seasons 2011-2012 and 2015-2016. The mean depth of snow measured on the artificially covered ski slope at the end of the winter season was 2.3 times higher (31.2 cm difference) than the mean seasonal depth (XII-III) of natural snow in the surroundings (five-season average; Figure 9c). The highest difference between these two values was determined in the seasons with a shortage of natural snow (seasons 2010-2011, 2011-2015 and 2015-2016). Because of the high density of the ski slope snowpack, a high amount of water was still stored in it after the natural snow had melted away. In the five-season average, the mean SWE value of the remaining ski slope snowpack (281.4 ± 46.2 mm; mean ± SD) represented 120% of the seasonal precipitation totals (XII-III; 234.1 ± 94.7 mm; Figure 9d), and the water input from the melting ski slope snowpack (=SWE; 281.4 ± 46.2 mm; mean ± SD) was 1.3 times higher than the seasonal (XI-IV) solid precipitation totals (124.6 mm). A significantly different mean SWE of the ski slope snowpack, measured at the end of these seasons, was identified between five seasons ( Table 4). The highest difference (126.5 mm) was determined between the same seasons, as in the case of the mean snow depth (2011-2012 vs. 2015-2016). At the end of the season, the mean snow density of the ski slope snowpack showed significant inter-seasonal differences ( Table 4). The five-season average showed above-average mean snow density (higher than 618.8 kg/m 3   The distribution of snow on the ski slope was highly heterogeneous (Figure 10). The high variability of the snow depth data expressing the standard deviation of the mean snow depth is shown in Figure 9c. The observed snow depth minima at the end of each season were close to 0 cm, while the maxima reached values from 100 to 200 cm. The maxima always occurred at a distance below 30 m from the lances, which is the maximum distance of the used snowmaking technique. The The distribution of snow on the ski slope was highly heterogeneous (Figure 10). The high variability of the snow depth data expressing the standard deviation of the mean snow depth is shown in Figure 9c. The observed snow depth minima at the end of each season were close to 0 cm, while the maxima reached values from 100 to 200 cm. The maxima always occurred at a distance below 30 m from the lances, which is the maximum distance of the used snowmaking technique. The maximum snow depths were close to the snow-making lances. Significant logarithmic relationships were proved between the snow depth and the distance from the fixed snow-making lances in all seasons (ANOVA: p < 0.05; Figure 10). The snow depth decreased as the distance from the lances increased. The correlation was the lowest in the winters of 2011-2012 and 2012-2013 (Figure 10), which were characterized by the highest mean seasonal snow depth of natural snow cover in the ski slope surroundings (Figure 5a). In contrast, in the remaining three seasons with low mean seasonal depth of natural snow cover, the need to produce a high volume of snow resulted in a more uneven distribution of snow on the ski slope and, therefore, a higher correlation of the analyzed relationships (higher correlation coefficient; Figure 10). Table 4. Multiple, inter-seasonal comparisons of mean snow depth, snow water equivalent (SWE), and snow density of the ski slope snowpack, measured at the end of the five winter seasons. Displayed are the estimated differences between each pair of means. maximum snow depths were close to the snow-making lances. Significant logarithmic relationships were proved between the snow depth and the distance from the fixed snow-making lances in all seasons (ANOVA: p < 0.05; Figure 10). The snow depth decreased as the distance from the lances increased. The correlation was the lowest in the winters of 2011-2012 and 2012-2013 ( Figure 10), which were characterized by the highest mean seasonal snow depth of natural snow cover in the ski slope surroundings (Figure 5a). In contrast, in the remaining three seasons with low mean seasonal depth of natural snow cover, the need to produce a high volume of snow resulted in a more uneven distribution of snow on the ski slope and, therefore, a higher correlation of the analyzed relationships (higher correlation coefficient; Figure 10). Table 4. Multiple, inter-seasonal comparisons of mean snow depth, snow water equivalent (SWE), and snow density of the ski slope snowpack, measured at the end of the five winter seasons. Displayed are the estimated differences between each pair of means.

South-Central Slovakia
If the operability of the South-Central Slovakian ski centers was dependent only on natural snow, the ski slopes could have been opened only in two of the six analyzed winter seasons. The reason for this is the low monthly average snow depth, which was highly variable between the seasons. The high inter-seasonal variability of the monthly average snow depth confirms a study by Hríbik et al. [33] which was conducted in South-Central Slovakia (660 m a.s.l.). Hríbik et al. [33] identified three of six winter seasons (snow-poor seasons 2006-2009) in which the monthly average snow depth was lower than 30 cm (snow depth > than 30 cm is sufficient for skiing). In the presented study, a monthly average snow depth higher than 30 cm occurred only in January or February, but most often in February. Therefore, continual seasonal operability of the South-Central Slovakian ski slopes could not be achieved from December to March without the help of artificial snowmaking. Hríbik et al. [33] confirmed that the peak snow depth most often occurs in February. Nevertheless, in the snow-rich season 2004-2005, March was identified by Hríbik et al. [33] as the month with the highest monthly average snow depth.

Košútka Ski Centre
A general high occurrence of liquid precipitation in the winter season was identified in the presented study, while this occurrence was highly variable between the individual seasons. A tendency of increasing mean seasonal value of the average daily temperature (seasonal = months XII-III) was discovered. These results are in good agreement with those of Pecho et al. [34], who have pointed out that winters after 1991 were not as cold as in the past, which resulted more often in the occurrence of mixed and liquid precipitation. From the six seasons (2010-2016) presented in this paper, the season 2012-2013 was identified as highly above average in precipitation totals (XI-IV) and in snowfall totals (XII-III). This season was described by Mikloš et al. [35] as snow-rich with high peak snow water equivalent in the mountainous watershed located only 10 km northwest of the Košútka Ski Centre. Pecho et al. [36] described this weather situation in more detail. According to Faško [37] and Falt'an et al. [38], precipitations occur currently with a higher intensity than in the past. This leads to the occurrence of months with extremely above normal and below normal levels of precipitation. The presented work confirms these conclusions because of the relatively high standard deviation of the mean monthly precipitation totals in November, January, February, and March. From a comparison of climate regions in Slovakia between the Landscape Atlas of the Slovak Republic published in 2002 [39] and the Climate Atlas of the Slovak Republic published in 2015 [40] there are obvious important changes in some regions and sub-regions [37]. In the areas located approximately 20 km south of the study site (Košútka Ski Centre), the warm, dry sub-region with cold winters (T3) changed to a warm, dry sub-region with mild winters (T2). The increasing seasonal values (XII-III) of the daily average temperature confirm these warming tendencies of South-Central Slovakia [13].
The meteorological and snow conditions of the study site are not suitable for the continuous operation of the ski slope from December to March without artificial snowmaking. The reason for this is the relatively high variability of the monthly average snow depth in the individual seasons. A deficit of natural snowfall due to the variability of meteorological and snow conditions was also identified by Durand et al. [41] in the European Alps. The vulnerability of the French ski resorts to the lack of natural snow was observed by Spandre et al. [42]. The altitude of the snow line with sufficient snow conditions for skiing has a rising trend in the European Alps [43,44]. If the ski slope operation in the investigated low-elevation ski center were dependent only on natural snow, then skiing would only be possible in some days or months during the season.

Snowmaking
This study clarifies that operability during every season of the South-Central Slovakian ski slopes is not possible without artificial snowmaking. Nevertheless, snowmaking is dependent on a low air temperature [3]. In South-Central Slovakia, creation of the base layer for skiing is essential at the beginning of the winter season (end of December-beginning of January) because of the high public demand (Christmas holidays). Thus, a high number of potential snowmaking days (PSD; days with optimal snowmaking conditions) is essential mainly in the seasons with a low depth of natural snow. In the Košútka Ski Centre, a low seasonal depth of natural snow cover was identified in four of six seasons (lower than 10 cm). Because of the low air temperature in December or January, the base layer snowmaking was possible in the snow-poor seasons of 2010-2011 and 2015-2016. In the relatively warm season of 2014-2015, the number of PSD was insufficient for base layer creation, but snowmaking was supplied by the above-average seasonal snowfall totals. In the 2013-2014 season, the ski slope of the Košútka Ski Centre was out of service because of warm winter temperatures in which base layer snowmaking was not possible and because of low seasonal snowfall totals. Additionally, Hopkins and Maclean [45] concluded that some regions in Scotland were not able to produce artificial snow due to inadequate meteorological conditions. Many authors claim that the use of artificial snow is a sufficient adaptation strategy against climate change [46,47], although low-altitude resorts remain negatively impacted by seasons with poor snow conditions [48][49][50]. Moreover, Vojtek et al. [10] and Škvarenina et al. [11] pointed out the general tendency of the decreasing duration of snow cover and solid precipitation occurrence at lower altitudes in Slovakia (Central Europe). In the current meteorological conditions of South-Central Slovakia, the operability of the investigated low-elevation ski slope is possible because of high snow production, which is possible only for a few days in the winter season.
In the investigated low-elevation ski center, the wind speed and wind direction are probably not limiting factors for snowmaking. The reason for this is the low occurrence of wind with a speed higher than 2 m/s and a relatively constant wind direction during the days with optimal snowmaking conditions in December and January. Spandre et al. [51] also found ideal wind speed (low) and wind direction (constant) conditions for snowmaking, but in the higher elevated alpine ski resort "Les 2 Alpes" (1680 m a.s.l.). Suitable wind conditions for snowmaking in the investigated ski center could be explained by the results of Spandre et al. [52]. They concluded that the topography and vegetation have significant impacts on the efficiency of snow production. Ideal wind conditions during the potential snowmaking days increase the efficiency of the snowmaking because of low water losses. According to Olefs et al. [53], the water losses during snowmaking (evaporation, sublimation, wind erosion) varied between 5% and 15% for fan guns and from 15% to 40% for snowmaking lances. The mostly constant wind direction is, therefore, beneficial, especially for the use of snow-making lances that have fixed positions, as in the investigated Košútka Ski Centre.

Characteristics of Ski Slope Snowpack
This study confirms that the snowpack of an artificially covered ski slope characterizes high snow depth and density of snow [22]. In contrast to previous surveys [22,33], the presented research was conducted in a low-elevation ski center under 1000 m a.s.l. Mossner et al. [54] reported that snow density on the ski slope ranging between 420 and 620 kg/m 3 with a mean value of 556 kg/m 3 (undefined ski slope in Switzerland). The results of our study show a high mean snow density of 619 kg/m 3 at the end of the winter season (five-season mean value). A high density of the ski slope snowpack at the end of the winter season was also found by Keller et al. [55]. They stated that, in the middle of December, the snow density on slopes with artificial snow is around 500 kg/m 3 . Such snow becomes increasingly compact over time until it becomes a mixture of hard snow with clear ice with a maximum density around 700 kg/m 3 . In our case, the mean snow density was the highest at the end of the 2014−2015 season, with a value of 737 kg/m 3 . Such high mean density was not recorded by the other authors, probably because of the low elevation of the study site and late date of measuring.
The high density could be the result of refreezing water or water saturation from the melting snow. The artificial snow significantly increases the depth of snow on the ski slopes [22]. Our results confirm these findings because, after the disappearance of natural snow on the off-piste sites, the depth of the snowpack with artificial snow was still 45 cm (five-season average). The mean depth of snow measured on the artificially covered ski slope at the end of the winter season was 2.3 times higher (70% difference) than at the off-piste sites with natural snow (five-season average). Stoeckli and Rixen [56] showed a 20% difference in the depth of the artificially covered ski slope snowpack compared to controlled plots with undisturbed natural snow (Swiss Alps; 1150-2515 m a.s.l.). The water input from the melting ski slope snowpack at the end of the season was 1.3 times higher than the seasonal (XI−IV) solid precipitation totals. Studies from the Alps confirmed that the water input from the melting ski slope snowpack usually reaches 0.7-2 times (up to five times) that from natural snow [56,57]. The depth of the ski slope snowpack at Košútka was significantly dependent on the distance from the snow-making lances. The maxima (over 2 m) were observed at a distance below 30 m from the lances. This is caused by the high snow production (base layer creation) at the beginning of the winter season. The influence of the wind during base layer snowmaking was probably not significant because of the low wind speed and constant wind direction. Spandre et al. [58] also showed that the highest snow depth occurred within 30 m from the snow gun while low wind speed conditions were determined. In the presented study, the ski slope snowpack (groomed) was examined at the end of the season, while Spandre et al. [58] described the characteristics of the artificial snow pile (undisturbed) from the beginning of the winter season. Therefore, for the managers of the Košútka ski slope, a more equal distribution of artificial snow would be beneficial.
The changed characteristics of the ski slope snowpack result in prolonged melting. Keller et al. [55] and Rixen et al. [22] state that artificial snow in the Alps (above 1000 m a.s.l.) melts 28 days and 17 days longer than natural snow, respectively. We observed a prolongation of about 29 days (610 m a.s.l.). The cause is probably the high snow production and the earlier snowmelt of natural snow in the low-elevation study site. In the 2015-2016 season, the natural snow cover melted away at the earliest date (18 February) in the inter-seasonal comparison. This was because of the low seasonal snowfall totals and, consequently, low mean seasonal snow depth.
The economic success during the winter season for ski centers is dependent on the income during specific periods (Christmas-end of December; spring academic holidays-February to the beginning of March) [59,60]. As deduced from the six-season average, the ski centers in South-Central Slovakia (under 1000 m a.s.l.) that do not use artificial snowmaking have no chance to expect that the mean monthly snow depth in December will be sufficient for skiing, but the probability that the mean monthly snow depth will be sufficient for skiing in February varied between 17% and 33%. Therefore, snowmaking is the only possible solution to increase the snow depth on South-Central Slovakian ski slopes. In the investigated Košútka Ski Centre, snowmaking has been effective, allowing the ski slope to be operational in five of six winter seasons.

Conclusions
The results from this study showed that the snow depth of natural snow cover in South-Central Slovakia under 1000 m a.s.l. is not sufficient for continuous ski slope operation from December to March. The probability that, in the winter season, there will be a month with sufficient snow depth for ski slope operation varies from 17% to 33%. At the Košútka Ski Centre (the study site in South-Central Slovakia) the seasonal snowfalls and mean seasonal snow depth are highly variable. Every season, ski slope operability in this ski center is not possible without artificial snowmaking. A high percentage of the winter precipitation consists of liquid precipitation at the study site. Because of the low snow depth of the natural snow cover in the study site, the base layer creation at the beginning of each season is crucial for ski slope operability. A sufficient number of days for base layer snowmaking was identified only in four of six winter seasons. Essential, large production of artificial snow on the ski slope results in unequally distributed snow with high density, depth, and snow water equivalent (SWE). An inter-seasonal comparison of the ski slope snowpack characteristics, measured at the end of the season, showed significant differences in the mean snow depth, SWE, and snow density. The mean snow depth showed a decreasing trend. The distribution of snow on the artificially covered ski slope of Košútka Ski Centre is highly heterogenous. The snow depth decreases as the distance from the snowmaking lances increases. To improve the heterogeneous distribution of snow, it is necessary to move accumulated artificial snow under the snow-making lances further away to the opposite edge of the ski slope. The snowpack of the artificially covered ski slope persists on the slope longer than the natural snow on the off-piste sites. With an earlier snowmelt date of natural snow cover, the melting period of the ski slope snowpack from this date is longer, and the daily average temperature during this period is lower. Because of the high depth and density of the ski slope snowpack, a high amount of water is still stored in it after the natural snow has melted away from the off-piste sites. On average, this amount of water is higher than the seasonal precipitation (period XII-III).
We can conclude that, in the current meteorological and snow conditions of South-Central Slovakia, the operability of the low-elevation ski slopes is possible only with high snow production. From the example of the Košútka Ski Centre, it was found that essential base layer snowmaking for ski slope operation is possible only for a few days at the beginning of the winter season. The increasing mean value of the daily average temperature and high occurrence of winter liquid precipitation can finally result in inadequate conditions for ski slope operation.
Author Contributions: M.M. did main part of the data collection, analyses and writing. M.J. wrote introduction and discussion; K.K. helped with data collection; J.Š. provided overall guidance; and J.Š. supervised the study.
Funding: This research received no external funding.