Analysis of hoarfrost and cold days over the period 1971-2024 in Slovenia

Frost occurs almost every year in Slovenia and usually affects smaller, exposed areas, but sometimes also larger areas. Hoarfrost is a hydrometeor in the form of ice crystals that form by direct deposition on objects such as tree branches, plant stems, leaves or branches. It forms when air with a dewpoint temperature below freezing is saturated by cooling and is a major risk factor for agriculture, especially when it occurs during sensitive phenophases, particularly in late spring or early autumn. Although a spring frost risk assessment was carried out for several apple, sweet cherry and grapevine varieties in relation to their phenophases, a broader analysis of hoarfrost-susceptible locations and conditions has not yet been done for Slovenia. To determine the periods and locations with higher frost susceptibility, we first analyzed the frequency of cold days (days with a minimum temperature < 0°C), as the occurrence of temperatures below 0°C is not sufficient to guarantee the formation of hoar frost.

For the occurrence of hoarfrost and cold days, we analyzed the average annual and monthly values and their long-term variation as well as the extreme data over a period of 55 years (1971-2024). To analyse the spatial variability, we selected meteorological stations of the Slovenian national weather network that represent the variability of the climate in Slovenia and belong to different climate regions: Submediterranean climate region, Moderate climate of the hilly region, Subcontinental climate region and Subalpine climate region, with altitude ranging from 55 m to 864 m. The average multi-year number of days with hoarfrost is about 50 days/year at most locations, and the number of cold days exceeds the number of days with hoarfrost at a single station by twenty percent to three times the value. It is therefore important to monitor both parameters in order to assess the frost risk. The probability of cold days and hoarfrost days varies greatly between years and locations. Very often cold days and hoarfrost occur late in May and in the autumn as early as September, especially at higher elevations, and in extreme cases such events have been recorded even in June and August.

Monitoring and analyzing long-term values of hoarfrost and cold days and their temporal and spatial variability can help in frost forecasting so that farmers can take various measures against frost risk to prevent damage. A comprehensive frost management strategy should consider the early and main season as part of the farm's annual agrometeorological planning.