Hoar frost (sometimes hoarfrost) refers to the white ice crystals, deposited on the ground or exposed objects, that form when the air is moist and surface is cold. Hoar frost is often seen on cold, clear autumn nights.

Hoar frost is similar in appearance to rime ice, but the two are distinct. In formation of hoar frost, the water vapour condenses through deposition directly to solid ice whereas rime ice is formed following initial condensation into liquid droplets. Thus, hoar frost is formed when the dew point is warmer than surface but colder than the air temperature, and colder than freezing point. The formation of frost is an example of meteorological deposition.

Water ice

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