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|Tag(s)|| Weather Phenomena|
Hoar frost occurs when a sub-zero surface comes into contact with moist air. The water vapour turns directly into ice by sublimation forming a white crystalline ice coating which can normally be brushed off. Hoar frost will form in clear air when an aircraft has been parked overnight in sub-zero temperatures and will form when an aircraft flies from sub-zero temperature air into warm moist air, such as in descent or climbing through a temperature inversion layer. Hoar frost is not as dangerous as clear ice but may affect the lift characteristics of the wings. Frost remaining on the wings at take-off is especially dangerous.