A deposit of interlocking ice 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.
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
). 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.
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