FAA Aircraft Dispatcher (ADX)MeteorologyMedium
A dispatcher notes that a light easterly wind is pushing moist, stable marine air up the windward slopes of coastal hills near an airport, and low stratus is forming as the air cools adiabatically during ascent. What type of fog is most likely forming?
- AUpslope fog
- BRadiation fog
- CSteam fog
- DAdvection fog
Show answer & explanationAnswer & explanation
Correct answer: A. Upslope fog
Upslope fog forms when moist, stable air is mechanically forced to rise along sloping terrain, cooling adiabatically until saturation is reached. Unlike radiation fog, it requires wind and terrain lift rather than nighttime radiational cooling, and unlike advection fog, its formation mechanism is orographic lift rather than horizontal movement over a cold surface.
Why the other options are wrong
- B. Radiation fog forms from nighttime radiational cooling under clear, calm conditions, not from forced ascent over terrain.
- C. Steam fog forms when cold air moves over much warmer water, causing evaporation and immediate condensation.
- D. Advection fog forms from horizontal movement of moist air over a colder surface, not primarily from vertical lift.
Upslope Fog
Fog that forms when moist, stable air is forced to rise along sloping terrain, cooling adiabatically to the point of saturation.
- Requires wind to push air up terrain (not calm like radiation fog)
- Can occur day or night, and even through cloud cover
- Common on the eastern slopes of the Rocky Mountains
Memory trick: Upslope fog: air climbs the hill and cools itself into a cloud — 'hill-chill fog'.