Editing Frost (section)

==Formation==
[[File:Nevada-Apartadero-Merida-Venezuela.jpg|thumb|Frost in the [[list of highest towns by country|highest town in Venezuela]], [[Apartaderos]]: Because of its location in an [[alpine tundra]] [[ecosystem]] called ''[[páramo]]'', a daily freeze-and-thaw cycle, sometimes described as "summer every day and winter every night", exists.]]
If a solid surface is chilled below the [[dew point]] of the surrounding humid air, and the surface itself is colder than freezing, ice will form on it. If the water deposits as a liquid that then freezes, it forms a coating that may look glassy, opaque, or crystalline, depending on its type. Depending on context, that process may also be called [[atmospheric icing]]. The ice it produces differs in some ways from crystalline frost, which consists of spicules of ice that typically project from the solid surface on which they grow.

The main difference between the ice coatings and frost spicules arises because the crystalline spicules grow directly from [[deposition (phase transition)|desublimation]] of water vapour from air, and desublimation is not a factor in icing of freezing surfaces. For desublimation to proceed, the surface must be below the [[dew point#Frost point|frost point]] of the air, meaning that it is sufficiently cold for ice to form without passing through the [[liquid|liquid phase]]. The air must be humid, but not sufficiently humid to permit the condensation of liquid water, or icing will result instead of desublimation. The size of the crystals depends largely on the temperature, the amount of [[water vapor]] available, and how long they have been growing undisturbed.

As a rule, except in conditions where supercooled droplets are present in the air, frost will form only if the deposition surface is colder than the surrounding air. For instance, frost may be observed around cracks in cold wooden sidewalks when humid air escapes from the warmer ground beneath. Other objects on which frost commonly forms 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. Where static air settles above an area of ground in the absence of wind, the [[molar absorptivity|absorptivity]] and specific heat of the ground strongly influence the temperature that the trapped air attains.