FAA Instrument Rating Airplane (IRA)Weather and Weather ServicesMedium
A pilot is briefed that a warm front is approaching the departure airport, with widespread stratus clouds and light rain preceding it. What type of fog is most likely to form ahead of the warm front, and why?
- ASteam fog, caused by cold air moving over a warm water surface
- BUpslope fog, caused by moist air being lifted over rising terrain
- CPrecipitation-induced (frontal) fog, caused by rain saturating cool air near the surface ahead of the front
- DRadiation fog, caused by nighttime cooling of the surface under clear skies
Show answer & explanationAnswer & explanation
Correct answer: C. Precipitation-induced (frontal) fog, caused by rain saturating cool air near the surface ahead of the front
Precipitation-induced fog forms when relatively warm rain falls through cooler air near the surface ahead of a warm front, evaporating and saturating the air to the point of fog formation. This is common in the cool sector ahead of an approaching warm front and can significantly reduce visibility.
Why the other options are wrong
- A. Steam fog requires cold air over warm water, not associated with warm front rain.
- B. Upslope fog requires moist air moving up sloping terrain, not related to frontal precipitation.
- D. Radiation fog requires clear skies and calm winds at night, inconsistent with an approaching frontal system with clouds and rain.
Precipitation-Induced (Frontal) Fog
Fog that forms when warm rain falls through cooler air near the surface, saturating it ahead of a warm front or stationary front.
- Common ahead of warm fronts
- Caused by evaporation of falling rain into cool air
- Can produce widespread low visibility over large areas
Memory trick: Rain falling into cold air breeds fog before the front arrives.