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Clear, or Glaze, ice is widely considered to be the most dangerous form of icing.
Clouds can support high concentrations of liquid water with relatively large drop sizes. When these water drops are carried above the freezing level, they become supercooled. Because of their large drop size they do not freeze instantly on contact with the aircraft surface but freeze gradually as they flow back across the surface, leaving a smooth, hard, glossy, and transparent covering of ice over the wing surface called clear ice. Clear icing is most significant in the range of 0c to -20c. Ice accretion can be very fast in the highest concentrations of supercooled water.
Clear ice is heavy and difficult to see and to remove with de-icing methods, hence the danger it poses to flight operations.
Occasionally, certain temperature and droplet size combinations can lead to the formation of a “double ram’s horn” shape forward of the wing leading edge with protrusions from both the upper and lower wing leading edge surfaces.