FAA Private Pilot Airplane (PAR)Weather Theory and ServicesMedium
A pilot flying over gently rising terrain in a stable, moist air mass observes visibility decreasing to near zero as the aircraft climbs along the slope, with fog forming as the air is lifted and cooled adiabatically to its saturation point. What type of fog is this?
- AAdvection fog
- BSteam fog
- CRadiation fog
- DUpslope fog
Show answer & explanationAnswer & explanation
Correct answer: D. 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 (nighttime surface cooling), advection fog (horizontal movement over a colder surface), and steam fog (cold air over warm water).
Why the other options are wrong
- A. Advection fog results from warm moist air moving horizontally over a cooler surface.
- B. Steam fog forms when cold air moves over relatively warm water, causing evaporation and condensation.
- C. Radiation fog forms from nighttime radiational cooling of the ground, not from air being lifted.
Upslope Fog
Fog that forms when moist, stable air is forced to rise over sloping terrain, cooling adiabatically until saturation occurs.
- Requires stable, moist air and sufficient wind to push air upslope
- Can form during day or night, unlike radiation fog
- Common in areas with gradually rising terrain, such as the high plains
Memory trick: 'Upslope Uplifts, then Cools to Fog' — rising terrain forces air up and cools it to saturation.