Definition
A white, crystalline deposit of ice that forms directly on a surface when water vapor in the air comes into contact with that surface at a temperature below freezing. The vapor changes straight from gas to ice without first becoming liquid. On an aircraft, hoar frost typically appears as a rough, feathery coating on wings, tail, and fuselage after the airframe has been cold-soaked overnight or after descent through moist air.
Plain English
A thin layer of icy crystals that forms on a cold surface when moist air touches it. It looks like a fuzzy white coating, similar to the frost you see on a car windshield on a cold morning.
Context Anchor
Seen during preflight inspections, cold-weather operations, and weather discussions about frost forming on aircraft surfaces.
Derivation
From Old English 'har,' meaning 'gray' or 'aged' — the same root as 'hoary,' which describes something white with age. Hoar frost was named for the white, aged appearance it gives to surfaces, like the white hair of an elderly person.
Why Pilots Care
Even a thin layer of hoar frost on wings or control surfaces disrupts smooth airflow, reduces lift, and can cause dangerous handling problems on takeoff.
Grounding Statement
Picture a parked airplane on a clear, cold morning: the metal skin gets below freezing, and moisture from the air turns into tiny white ice crystals on the surface.
Intuition Check
Do not assume hoar frost is harmless because it looks light or fluffy. On an aircraft surface, any frost can be contamination that affects flight.
Example Sentence 1
During the early morning preflight, the pilot found hoar frost on the upper wing surfaces and delayed departure until the aircraft could be moved into a heated hangar.
Example Sentence 2
After a clear night with temperatures near freezing, the parked airplane was covered in hoar frost that required de-icing.