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Definition:
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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).
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