FAA Private Pilot Helicopter (PRH)WeatherMedium
A helicopter is parked outside overnight under clear skies with calm wind. The temperature drops to 2°C, matching the dew point. The next morning, the pilot finds a thin, white crystalline deposit covering the rotor blades. This deposit most likely formed because
- Aradiation fog condensed and then froze on contact with the blades
- Bfreezing rain fell and froze on contact with the cold rotor blades
- Cwater vapor sublimated directly onto surfaces that cooled to or below both freezing and the dew point
- Dsupercooled water droplets struck the blades and froze instantly
Show answer & explanationAnswer & explanation
Correct answer: C. water vapor sublimated directly onto surfaces that cooled to or below both freezing and the dew point
Frost forms through deposition (sublimation), where water vapor changes directly to ice on a surface that has cooled to or below both freezing and the dew point (frost point), typically under clear, calm, radiational cooling conditions. No precipitation or supercooled droplets are required.
Why the other options are wrong
- A. Frost is not fog that freezes; it forms directly from water vapor via deposition.
- B. No precipitation was mentioned; freezing rain requires falling liquid precipitation.
- D. This describes structural icing in flight from supercooled droplets, not surface frost.
Frost Formation
Frost forms when water vapor deposits directly as ice crystals on a surface that has cooled to or below freezing and at or below the dew point, usually on clear, calm nights.
- Requires clear skies for radiational cooling
- Requires calm or light wind
- Surface temperature must be ≤ freezing and ≤ dew point
- Frost disrupts smooth airflow and must be removed before flight
Memory trick: 'Clear, Calm, Cold, and Condensed'—the four C's of frost