| Term: | Hoarfrost |
| Definition: |
A deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires, poles, etc. The deposition of hoar frost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing. It forms when air with a dew point below freezing is brought to saturation by cooling. NOAA-NWS
A deposit of interlocking ice crystals (hoar crystals) formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc.; the surfaces of these objects are sufficiently cooled, mostly by nocturnal radiation, to cause the direct sublimation of the water vapor contained in the ambient air. NSIDCCryosphere Deposit of ice crystals forming on objects whose surface is sufficiently cooled to cause the sublimation of the water vapour contained in the ambient air. WMOHydrology A deposit of interlocking ice crystals (hoar crystals) formed by direct deposition on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc. Also, frost may form on the skin of an aircraft when a cold aircraft flies into air that is warm and moist or when it passes through air that is supersaturated with water vapor. The deposition of hoarfrost is similar to the process by which dew is formed, except that the temperature of the befrosted object must be below freezing. It forms when air with a dewpoint below freezing is brought to saturation by cooling. In addition to its formation on freely exposed objects (air hoar), hoarfrost also forms inside unheated buildings and vehicles, in caves, in crevasses (crevasse hoar), on snow surfaces (surface hoar), and in air spaces within snow, especially below a snow crust (depth hoar). Hoarfrost is more fluffy and feathery than rime, which in turn is lighter than glaze. Observationally, hoarfrost is designated light or heavy (frost) depending upon the amount and uniformity of deposition. AMSglossary A deposit of ice having a crystalline appearance, generally assuming the form of scales, needles, feathers or fans; produced in a manner similar to dew (i.e. by condensation of water vapour from the air), but at a temperature below 0 C. SPRI A deposit of ice crystals (hoar crystals) formed by direct deposition on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc. It forms when air with a dew point below freezing is brought to saturation by cooling, i.e., usually radiative cooling. Hoarfrost also forms on snow surfaces (SH), within the snowpack (FC, DH), as well as inside unheated buildings and vehicles, in caves, and in crevasses (shcv). IACSSnow GCW |