Frost

Formation

If solid surfaces in contact with the air are chilled below the deposition point (see water vapor available.

Frost is often observed around cracks in wooden sidewalks due to the moist air escaping from the ground below. Other objects on which frost develops are those with low absorptivity and specific heat of the ground which in the absence of wind greatly influences the temperature attained by the superincumbent air.

Because cold air is more dense than warm air, in calm weather cold air pools at ground level. This is known as surface temperature inversion. It explains why frost is more common and extensive in low-lying areas. Areas where frost forms due to cold air trapped against the ground or against a solid barrier such as a wall are known as "frost pockets".

Frost is similar in appearance to meteorological deposition.

Types of frost

Radiation frost (also called hoar frost or, sometimes, hoarfrost) refers to the white ice crystals, loosely deposited on the ground or exposed objects, that form when the air is moist, the wind is weak or absent and surfaces are cold. It is often seen on clear winter nights, especially in valleys and hollows. Hoar frost can form in these areas even when the reported temperature is above the freezing point of water.

One can distinguish between some types of hoar frost, depending on where it forms. For example, air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, wires; surface hoar is formed by fernlike ice crystals directly deposited on snow, ice or already frozen surfaces; crevasse hoar consists in crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions; depth hoar refers to cup shaped, faceted crystals formed within dry snow, beneath the surface.

Depth hoar is a common cause of avalanches when it forms in air spaces within snow, especially below a snow crust, and subsequent layers of snow fall on top of it. The layer of depth hoar consists of angular crystals that do not bond well to each other or other layers of snow, causing upper layers to slide off under the right conditions, especially when upper layers are well bonded within themselves, as is the case in a slab avalanche.

Hoar frost does not occur exclusively in nature. Hoar frost is also found in and around freezers particularly in industrial cold storage facilities. It occurs in adjacent rooms that are not well insulated against the cold and around entry locations where humidity and moisture will enter and temperature.

White frost is a relatively heavy coating of hoar frost with big and interlocking crystals. It forms when the air has a relative humidity above 90% and a temperature below -8 °C (18 °F).

Advection frost (also called wind frost) refers to tiny ice spikes forming when there is a very cold wind blowing over branches of trees, poles and other surfaces. It looks like rimming the edge of flowers and leaves and usually it forms against the direction of the wind. It can occur at any hour of day and night.

Window frost (also called fern frost) forms when a glass pane is exposed to very cold air on the outside and moderately moist air on the inside. If the pane is not a good insulator (such as a single pane window), water vapour condenses on the glass forming patterns. The glass surface influences the shape of crystals, so imperfections, scratches or dust can modify the way ice nucleates. If, otherwise, indoor is very clear ice.

capillary action, goes out from the cracks and freezes on contact with the air.