Frost

frost, the ice crystals formed by freezing of water vapour on objects which have cooled below 0 °C/32 °F. Such frosts are known as white frosts, or hoar frosts. Black frosts cause freezing and extensive killing of plant tissue itself, without any necessary hoar formation. Frost is a major viticultural problem as it can damage and kill shoots and fruit, in spring and autumn. frost protection is expensive and not always effective; see also frost damage.

Frost frequencies are, in many studies, imputed arbitrarily from weather records. Temperatures as recorded in the standard Stevenson screen used by meteorologists, at 1.25 m/4.1 ft above the ground, are always higher than at ground level. A screen temperature of 2.2 °C/36 °F is normally assumed to indicate a light ground frost, and one of 0 °C a heavy ground frost. Temperatures at vine height of −1 °C or lower after budbreak in spring will usually cause serious injury to the young shoots. Even ‘light’ frosts can often cause damage somewhere in the vineyard, because their incidence tends to be patchy, depending on topography. Geiger covers this aspect in detail.