FAA Instrument Flight Instructor (FII)WeatherEasy
During a winter morning, very cold arctic air moves across a relatively warm, unfrozen lake. A CFII observes wisps of fog rising directly off the water surface. What type of fog is this, and how does it form?
- AAdvection fog, from warm moist air moving over a cold surface
- BUpslope fog, from moist air rising over terrain
- CSteam fog, from cold air moving over warmer water causing evaporation
- DRadiation fog, from nighttime surface cooling
Show answer & explanationAnswer & explanation
Correct answer: C. Steam fog, from cold air moving over warmer water causing evaporation
Steam fog (evaporation fog) forms when cold, dry air moves over relatively warm water. Rapid evaporation saturates the air near the surface, and the vapor condenses immediately upon mixing with the cold air, producing wisps commonly called 'arctic sea smoke.'
Why the other options are wrong
- A. Advection fog requires warm moist air over a colder surface, the opposite of this scenario.
- B. Upslope fog requires moist stable air rising over terrain, not applicable here.
- D. Radiation fog forms from ground cooling on clear, calm nights, not from air moving over warm water.
Steam Fog (Evaporation Fog)
Fog that forms when cold air moves over much warmer water, causing rapid evaporation and condensation just above the surface.
- Also called 'sea smoke' or 'arctic sea smoke'
- Common over lakes/oceans in cold weather
- Can cause significant low-level turbulence and icing
Memory trick: Steam rises when cold air steals warmth from water below.