From SKYbrary Wiki
Hoar frost occurs when a sub-zero surface comes into contact with moist air. The water vapour in the air 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 may form on the upper or lower surface of the wings at ambient temperatures well above the freezing point due to the cold soaked fuel in the wing tanks. Larger aircraft types will avoid landing with wing tanks close to full when colder conditions exist aloft to avoid the cold fuel causing icing on the top surface of the wing after landing. Most airline operators have some guidance on the maximum amount of fuel that should be in the tanks upon landing when a shorter turn around is required to avoid an unnecessary de-icing procedure. Manufacturers often publish guidelines with respect to an acceptable amount of frost for takeoff when it is confined to lower wing surface within the fuel tank area.
Hoar frost is not as dangerous as clear ice but may affect the lift characteristics of the wings. Frost remaining on the wing upper surfaces is especially dangerous and should be removed prior to takeoff.