Ground frost refers to the various coverings ofice produced by the directdeposition ofwater vapor on objects and trees, whose surfaces have a temperature below thefreezing point of water (0 °C, 32 °F).[1]
The three main types of ground frost are radiation frost (hoar frost), advection frost (advection hoar frost) and evaporation frost. The latter is a rare type which occurs when surface moisture evaporates into drier air causing its temperature at the surface to fall at or under the freezing point of water.[1] Rime (bothsoft andhard) is technically not a type of ground frost.
Ground frost may also refer to the condition when the temperature of the upper layer of the soil falls below the freezing point of water.[1]
From 1906 to 1960 theMet Office practice was to base thenumber of days of ground frost on this criterion: a day with a minimum temperature reaching 30 °F (−1 °C), probably because 32 °F (0 °C) was not considered enough cold to cause damage to growing plants. Since 1961 the statistics have referred to thenumber of days with grass minimum temperature below 0°C. Occasionally, the term ground frost can still be seen, but it means simply a minimum temperature below 0 °C.[2]
Typical depth of frost in Finland is 0–200 cm in winter. If the frost does not melt completely during the summer, it is calledpermafrost.
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