Definition
A thin layer of ice crystals that forms on a surface when water vapor in the air comes into contact with that surface at a temperature below freezing, causing the vapor to deposit directly as ice without first becoming liquid. On aircraft, frost typically forms on wings, control surfaces, and windshields during cold, clear nights when the surface cools below the dew point and below 0 degrees Celsius.
Plain English
A rough, white coating of tiny ice crystals that forms on cold aircraft surfaces, much like the frost you see on a car windshield on a cold morning.
Context Anchor
Seen during preflight inspections, cold-weather maintenance, and any time an aircraft has been parked in freezing or near-freezing conditions.
Derivation
From Old English 'forst,' meaning 'a freezing.' The word has always referred to the visible result of freezing temperatures on exposed surfaces.
Why Pilots Care
Frost disrupts smooth airflow over wings, reducing lift and increasing stall risk even in small amounts.
Analogy
A frosted wing is no longer smooth to the air; it can act more like fine sandpaper than clean glass.
Grounding Statement
On a cold, humid morning, moisture in the air can turn straight into tiny ice crystals on a cold wing.
Intuition Check
Do not assume frost is harmless because it looks thin or powdery. In aviation, frost on wings, tail surfaces, or control surfaces can be serious because it changes the surface the air must flow over.
Example Sentence 1
During the preflight walkaround on a clear winter morning, the pilot found a thin layer of frost on the upper wing surface and called for the aircraft to be deiced before departure.
Example Sentence 2
Even light frost can prevent the airplane from reaching rotation speed on takeoff.