FAA Instrument Rating Airplane (IRA)Weather and Weather ServicesEasy
A pilot is planning a departure from an airport situated at the base of a mountain range. Moist, stable air is forecast to flow from the valley up the mountain slopes overnight. Which type of fog is most likely to form under these conditions?
- AAdvection fog
- BUpslope fog
- CSteam fog
- DRadiation fog
Show answer & explanationAnswer & explanation
Correct answer: B. Upslope fog
Upslope fog forms when moist, stable air is forced to rise along sloping terrain, cooling adiabatically until it reaches saturation. This differs from radiation fog (clear, calm nights with radiational cooling) and advection fog (warm moist air moving over a cooler surface).
Why the other options are wrong
- A. Advection fog results from horizontal movement of warm moist air over a cold surface, not vertical lifting.
- C. Steam fog forms when cold air moves over relatively warm water, causing evaporation and saturation.
- D. Radiation fog forms from nighttime radiational cooling over flat terrain, not from air being lifted by terrain.
Upslope Fog
Fog that forms when moist, stable air is mechanically lifted up sloping terrain, cooling adiabatically to the dew point.
- Requires moist, stable air and sloping terrain
- Cooling occurs adiabatically, not radiationally
- Can occur with wind, unlike radiation fog
Memory trick: Up the slope, air cools — upslope fog forms.