Hoar frost (or hoarfrost, sometimes simply frost) is the solid deposition of ice on the ground and on objects. It forms by direct deposition of water vapour into ice crystals. It should not be confounded with white dew which forms through condensation of vapour on cold objects and then freezing.

Hoar frost forms directly when the dew point is below 0 °C. In this case, the water vapour pass immediately into hoar frost.

Types of hoar frost

  • Radiation hoar frost is the most common type of hoar frost. It forms generally on the ground and on objects near the ground or evan on horizontal high places, such as roofs, chilled by radiation losses. It occurs commonly in winter (but also in autumn and early spring, depending on latitude) when the sky is clear and without fog, mist or wind; it may occur also by deposition of vapour over a layer of white dew.
  • Advection hoar frost occurs when a cold wind hits the branches of trees and other objects. Unlike advection frost, this type of hoar frost is more frequent on objects above the ground.
  • Surface hoar frost is frost forming on snow, often with a shape resembling ferns or flowers.
  • White frost is a special type of hoar frost, with big white crystals which occurs when the temperature is very cold (generally under -8 °C) and there's a high air humidity level; it is very similar to soft rime but occurs without freezing fog.
  • Window frost forms when a glass pane is exposed to very cold air on the outside and moist air on the inside. If the pane is not well insulated, water vapour passes directly into ice, forming fern-like patterns on the inner side of the pane.

See also

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