FAA Instrument Flight Instructor (FII)WeatherMedium
A pilot planning a flight into terrain rising gradually from the coast is briefed that moist, stable air is forecast to flow onshore and up the sloping terrain overnight. No front is present. What weather phenomenon should the pilot expect, and why does it form?
- AUpslope fog, from moist stable air cooling adiabatically as it rises over terrain
- BRadiation fog, from nighttime terrestrial cooling
- CGround fog, from temperature inversion trapping surface moisture
- DSteam fog, from cold air passing over warmer ground
Show answer & explanationAnswer & explanation
Correct answer: A. Upslope fog, from moist stable air cooling adiabatically as it rises over terrain
Upslope fog forms when moist, stable air is forced to rise over sloping terrain, cooling adiabatically until it reaches saturation. This is common on the eastern slopes of the Rocky Mountains and does not require a front, clear skies, or calm winds like radiation fog does.
Why the other options are wrong
- B. Radiation fog requires clear skies and calm winds with radiational cooling, not terrain lifting.
- C. Ground fog is a shallow radiation fog type, unrelated to terrain-forced lifting.
- D. Steam fog requires cold air over much warmer water, not applicable to terrain lifting.
Upslope Fog
Fog that forms when moist, stable air is mechanically lifted over rising terrain and cools adiabatically to the saturation point.
- Does not require clear skies or calm winds like radiation fog
- Common on the eastern slopes of the Rocky Mountains
- Can form with winds and overcast conditions, unlike radiation fog
Memory trick: Upslope fog: air climbs the hill and cools until it can't hold its water anymore.