Abstract. Hoarfrost is a hydrometeor which is produced only in the presence of frost and it represents a major risk factor to agriculture especially when it appears early in autumn or late in spring. It is formed by direct condensation of water vapor to ice at temperatures below freezing and occurs when air is brought to its frost point. Hoarfrost produces significant damage in agriculture when overlap with the growing periods of the plants (after 20 March) or when vegetative rest is not installed yet (until 15 October). This paper shows an analysis of the parameters which define the hoarfrost phenomenon in order to highlight the vulnerability of the analyzed area regarding this phenomenon. In a first phase, in order to identify the periods of high vulnerability, we analyzed the frequency of the days with minimum temperature ≤ 0 o C. But the occurrence of temperatures below 0° C is not enough to guarantee the formation of hoarfrost. Additionally, the air must be initially damp enough so that when cooled it reaches saturation, and any additional cooling will cause condensation to occur. We also analyzed the medium annual number of days with hoarfrost and their multiannual variation, the monthly and season variation of the days with hoarfrost, and the medium and extreme dates of hoarfrost production. The analysis of the parameters which define the hoarfrost phenomenon was realized on the basis of the information recorded at the meteorological stations present in the Western Plain. Finally we analyzed some cases in which there was hoarfrost and we described synoptic conditions that favored this phenomenon. The optimal conditions for the occurrence of hoarfrost are anticyclone systems with clear sky and slight wind, high relative humidity and negative air temperatures. When intense weather cooling is predicted, farmers can take various measures against the risks associated with this cooling, respective to early hoarfrost phenomenon in autumn or late hoarfrost in spring.

Key words: frost, hoarfrost, frequency of hoarfrost days, vulnerability, risk

Presentation: oral