frost or hoarfrost, ice formed by the condensation of atmospheric water vapor on a surface when the temperature of the surface is below 32°F (0°C). In the formation of frost, a gas (water vapor) is changed directly to a solid (see dew). Frost often appears as a light feathery deposit of ice, often of a curious and delicate pattern. The dates on which killing frosts (frost destructive to vegetation and staple agricultural products) occur vary considerably. Maps showing the growing season and the probable date of occurrence of frost may be obtained from the U.S. National Weather Service. The Weather Service stations issue warnings when frost is likely to occur; such warnings are broadcast by radio and are telegraphed or telephoned to farmers and fruitgrowers, who may protect their crops accordingly. Methods of protection vary: small flower beds and vegetable gardens are commonly protected by a screen or cloth that prevents excessive radiation from the earth and from the plants; in orchards, especially in California and Florida, simple oil-burning stoves or smudge pots placed at intervals throughout an orchard are used to heat and circulate the air sufficiently to prevent frost. Valleys are more subject to frosts than slopes, since cold air "slides" downhill and settles in depressions; orchards and citrus fruit groves are usually planted on slopes. Other factors in the occurrence of frost are altitude, latitude, proximity to large bodies of water, and other determinants of temperature. Frost, an element of climate, is an important agent of erosion. Frost heaving, an upthrust of ground caused by freezing, is a factor of consideration in engineering construction, especially in highway foundations. Frost is also a factor in the layer by layer mechanical weathering (exfoliation) of many types of rock masses. In England the word frost denotes freezing weather and degrees of frost means the number of degrees that the temperature falls below the freezing point.
See R. L. Berg and E. Wright Frost Action and Its Control (1984).