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Droplets of supercooled water often exist in stratus and cumulus clouds. An aircraft flying through such a cloud sees an abrupt crystallization of these droplets, which can result in the formation of ice on the aircraft's wings or blockage of its instruments and probes, unless the aircraft is equipped with an appropriate ice protection system.
Icing conditions exist when the air contains droplets of supercooled water. They freeze on contact with a potential nucleation site, which in this case is the parts of the aircraft, causing icing. Icing conditions are characterized quantitatively by the average droplet size, the liquid water content and the air temperature.
The Wegener–Bergeron–Findeisen process (after Alfred Wegener, Tor Bergeron and Walter Findeisen []), (or "cold-rain process") is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) in regions where the ambient vapor pressure falls between the saturation vapor pressure over water and the lower saturation vapor pressure over ice.
These droplets, which have a diameter of about 10 μm, can exist in an unfrozen state down to temperatures near −40 °C (−40 °F). Contact between the snow crystal and the supercooled droplets results in the freezing of the liquid droplets onto the surface of the crystals. This process of crystal growth is known as accretion. Crystals that ...
It is composed of ice crystals or supercooled water droplets appearing as small unshaded round masses or flakes in groups or lines with ripples like sand on a beach. [45] [46] Cirrocumulus occasionally forms alongside cirrus and may be accompanied or replaced by cirrostratus clouds near the leading edge of an active weather system. This genus ...
Water does not always freeze at 0 °C (32 °F). Water that persists in liquid state below this temperature is said to be supercooled, and supercooled water droplets cause icing on aircraft. Below −20 °C (−4 °F), icing is rare because clouds at these temperatures usually consist of ice particles rather than supercooled water droplets.
Altostratus can be composed of water droplets, supercooled water droplets, and ice crystals, [4] but ice crystals make up the vast majority. [21] In some altostratus clouds made of ice crystals, very thin horizontal sheets of water droplets can appear seemingly at random, but they quickly disappear. [22]
Nitric acid and water polar stratospheric Sometimes known as type 1, a thin sheet-like cloud resembling cirrostratus or haze. Contains supercooled nitric acid and water droplets; sometimes also contains supercooled sulfuric acid in ternary solution.