supercooling"(Also called subcooling or undercooling; see note below.) The reduction of temperature of any liquid below the melting point of that substance's solid phase; that is, cooling beyond its nominal freezing point.
Factors that favor rime formation are small drop size, slow accretion, a high degree of supercooling, and rapid dissipation of latent heat of fusion. The opposite effects favor glaze formation.
With very pure water (i.e. free from pollutants), supercooling of liquid drops can occur down to around minus 40°C. Altocumulus clouds, for example, are usually composed of water droplets at temperatures well below zero.
However, since the cold layer is so shallow, the drops themselves do not freeze, a phenomena called supercooling (or forming "supercooled raindrops").
Rime ice is less dense than glaze ice ( 0.2 to 0.3 grams per cubic centimetre) and clings less tenaciously. Factors favouring rime formation include: small water drop size, slow accretion, high supercooling of water in the drop, ...
 See also: Temperature, Layer, Freezing, Atmosphere, Water
  
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