Frost is the solid deposition of water vapor from humid air.

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It is formed when the temperature of a solid surface is below the freezing point of water and also below the frost point. The size of frost crystals varies depending on the time they have been building up and the amount of water vapour available. Frost crystals are translucent, but scatter light in many directions, so that a coating of frost appears white. There are many types of frost, such as radiation and window frost. Frost may damage crops or reduce future crop yields, hence farmers may take measures to prevent it forming.

If a solid surface is chilled below the dew point of the surrounding air and the surface itself is colder than freezing, frost will form on the surface. Frost consists of spicules of ice which grow out from the solid surface. The size of the crystals depends on time, temperature, and the amount of water vapor available. Based on wind direction, «frost arrows» might form.

In general, for frost to form the deposition surface must be colder than the surrounding air. For instance frost may be observed around cracks in cold wooden sidewalks when moist air escapes from the ground below. Other objects on which frost tends to form are those with low specific heat or high thermal emissivity, such as blackened metals; hence the accumulation of frost on the heads of rusty nails. The apparently erratic occurrence of frost in adjacent localities is due partly to differences of elevation, the lower areas becoming colder on calm nights. It is also affected by differences in absorptivity and specific heat of the ground, which in the absence of wind greatly influences the temperature attained by the superincumbent air.

The formation of frost is an example of meteorological deposition.

Hoar frost (also called radiation frost or hoarfrost or pruina) refers to the white ice crystals, loosely deposited on the ground or exposed objects, that form on cold clear nig.