FAA Flight Instructor Airplane (FIA)Weather and Weather ServicesEasy
On a clear, calm night with light winds and a surface temperature expected to drop to near the dew point, which condition is most conducive to the formation of frost on a parked aircraft?
- AA moist, unstable air mass with rising temperatures
- BStrong winds mixing the surface air layer
- CHigh cloud cover trapping outgoing radiation
- DClear skies and calm winds allowing rapid radiational cooling
Show answer & explanationAnswer & explanation
Correct answer: D. Clear skies and calm winds allowing rapid radiational cooling
Frost forms when clear skies allow strong radiational cooling of the surface and calm winds prevent mixing, letting the surface (and aircraft skin) cool below the dew point (and below freezing) while the air remains relatively undisturbed. Cloud cover or wind would inhibit this process.
Why the other options are wrong
- A. Rising temperatures and instability work against the cooling needed for frost.
- B. Wind mixes air and prevents the strong surface cooling needed for frost.
- C. Cloud cover reduces radiational cooling, making frost less likely.
Frost Formation
Frost forms on cold surfaces when clear skies and calm winds allow radiational cooling to drop surface temperature below the frost point, causing water vapor to deposit directly as ice.
- Requires clear skies for radiational cooling
- Calm winds prevent mixing that would slow cooling
- Surface temperature must fall to or below the frost point
Memory trick: Clear and calm, frost forms like a charm