Definition
A thin layer of ice crystals that forms on the top surface of a wing or other airfoil when the surface temperature is at or below freezing and moisture from the air sublimates directly onto it. Even when the layer is rough rather than thick, it disrupts the smooth airflow over the wing, reducing lift and increasing drag.
Plain English
A coating of frost on top of the wing. It looks harmless because it is so thin, but the rough texture spoils the airflow and the wing produces noticeably less lift.
Context Anchor
Seen in icing, lift, stall, and preflight inspection discussions, especially when checking whether a wing is clean before takeoff.
Derivation
“Frost” comes from an old word for freezing or cold. In aviation, it means frozen moisture crystals on an aircraft surface, not just a harmless white coating.
Why Pilots Care
Even a thin coating of upper surface frost reduces lift, increases drag, and can cause an early stall at lower-than-expected angles of attack.
Analogy
A wing is shaped to let air flow smoothly over it. Frost can act like fine sandpaper on that surface, making the airflow less smooth even though the frost layer is thin.
Grounding Statement
Picture a cold morning wing with a thin white layer on top; that small rough layer can change how the air flows over the wing.
Intuition Check
Do not assume upper surface frost is harmless because it is thin. On the top of a wing, roughness alone can reduce lift.
Example Sentence 1
During preflight on a clear, cold morning, the pilot found upper surface frost on both wings and delayed departure until it had been fully removed.
Example Sentence 2
Upper surface frost formed overnight on the parked aircraft in clear, cold, humid air and required de-icing before the instrument flight could begin.