Which fog forms when moist air is forced to rise up sloping terrain and can persist for extended periods?

• A. Radiation fog
• B. Advection fog
• C. Frontal fog
• D. Upslope fog

Answer: (D)

Fog forms when air cools to its dew point and moisture condenses into tiny droplets near the surface. Upslope fog happens when moist air is forced to rise as it moves up sloping terrain. The rising air expands and cools adiabatically; when its temperature drops to the dew point, condensation occurs and fog forms along the slope. It can persist for extended periods because the wind keeps pushing air upslope and a steady supply of moisture maintains the cooling and condensation. This is different from radiation fog, which forms from ground cooling overnight; advection fog, which comes from moist air moving horizontally over a cooler surface; and frontal fog, which arises from evaporation of precipitation within a front.