When and Where Does Frost Form?
Frost comes from the
atmosphere when the temperature falls below freezing, and invisible water vapor
in the air turns into white ice crystals, without first becoming a liquid. It
usually occurs when the skies are clear, when there is no wind and when a mass
of cold air descends on the land. This often happens during the night in the
spring and autumn of areas with temperate climates.
In the morning the fields and roofs are white with what would be dew if the temperature had been above freezing point. It is the most common type of frost and is often called hoar frost. Sometimes only the leaves of plants are fringed with white rime. This is formed when very small droplets of the moisture from fog have frozen on coming into contact with a cold object.
There is also black frost. This occurs when water vapor turns first into liquid and then freezes into a thin layer of ice instead of white crystals. As it is invisible, it is particularly dangerous when it forms on roads. The beautiful patterns, looking like trees, ferns or feathers, which are sometimes seen on windows, are made when the water vapor in a cold room condenses.
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 also may 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 from the fact that the crystalline spicules grow directly from desublimation of water vapour from air, and desublimation is not a factor in icing of freezing surfaces. For desublimation to precede the surface must be below the frost point of the air, meaning that it is sufficiently cold for ice to form without passing through the 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 absorptivity and specific heat of the ground strongly influence the temperature that the trapped air attains.