Hoarfrost

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 freeze instantly depending on the freezer temperature.