FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesEasy
On a cold, clear autumn morning with calm winds, a pilot performing a preflight walk-around near a lake observes thin wisps of fog rising directly off the surface of the water shortly after a cold air mass moved over the relatively warm lake. Which type of fog is the pilot observing?
- AAdvection fog
- BRadiation fog
- CUpslope fog
- DSteam fog
Show answer & explanationAnswer & explanation
Correct answer: D. Steam fog
Steam fog (evaporation fog) forms when cold air moves over much warmer water; rapid evaporation saturates the cold air, producing a smoke-like rising fog. It often creates low-level turbulence and icing hazards for low-flying aircraft.
Why the other options are wrong
- A. Advection fog forms when warm moist air moves over a colder surface, the opposite temperature relationship.
- B. Radiation fog forms over land from nighttime radiational cooling, not over open water.
- C. Upslope fog forms when moist air is forced to rise and cool along sloping terrain.
Steam Fog (Evaporation Fog)
Fog that forms when cold, dry air moves over much warmer water, causing rapid evaporation and saturation of the cold air layer.
- Also called 'sea smoke' or 'arctic sea smoke'
- Requires cold air over warm water, not the reverse
- Can cause low-level turbulence and light icing
Memory trick: Steam rises off a hot cup of coffee in a cold room — cold air over warm water makes steam fog.