Atmospheric icing
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Atmospheric icing occurs when water droplets in the air freeze on objects they come in contact with. This is very dangerous on aircraft, as the built up ice changes the aerodynamics of the flight surfaces, and can cause loss of lift, with a subsequent crash.
Not all water freezes at 0° C or 32° F. This is called supercooled liquid water. These supercooled droplets are what cause icing problems on aircraft. Usually, icing is not a problem in clouds if the temperature in the cloud is -20°C or colder. This is because at temperatures below -20°C clouds rarely consist of supercooled water droplets, but rather ice particles.
Icing also occurs on towers, wind turbines, boats, oil rigs, trees and other objects exposed to low temperatures and water droplets.
References
[Advisory Circular 20-113]: Pilot Precautions and Procedures to be taken in Preventing Aircraft Reciprocating Engine Induction System and Fuel System Icing Problems[Advisory Circular 20-117]: Hazards Following Ground Deicing and Ground Operations in Conditions Conducive to Aircraft Icing
[Advisory Circular 20-147]: Turbojet, Turboprop, and Turbofan Engine Induction System Icing and Ice Ingestion
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