<P> The apparently erratic occurrence of frost in adjacent localities is due partly to differences of elevation, the lower areas becoming colder on calm nights . Where static air settles above an area of ground in the absence of wind, the absorptivity and specific heat of the ground strongly influence the temperature that the trapped air attains . </P> <P> Hoar frost (also hoarfrost, radiation frost, or pruina) refers to white ice crystals deposited on the ground or loosely attached to exposed objects, such as wires or leaves . They form on cold, clear nights when conditions are such that heat radiates out to the open air faster than it can be replaced from nearby sources, such as wind or warm objects . Under suitable circumstances, objects cool to below the frost point of the surrounding air, well below the freezing point of water . Such freezing may be promoted by effects such as flood frost or frost pocket . These occur when ground - level radiation loses cool air until it flows downhill and accumulates in pockets of very cold air in valleys and hollows . Hoar frost may freeze in such low - lying cold air even when the air temperature a few feet above ground is well above freezing . </P> <P> The word hoar comes from an Old English adjective that means "showing signs of old age". In this context, it refers to the frost that makes trees and bushes look like white hair . </P> <P> Hoar frost may have different names depending on where it forms: </P>

Where did the term hoar frost come from
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