Which types of fog depend upon wind in order to exist?

• A. Advection fog and upslope fog
• B. Radiation fog and steam fog
• C. Upslope fog and steam fog
• D. Advection fog and radiation fog

Answer: (A)

Fog forms when air is cooled to its dew point and becomes saturated with moisture. When that cooling or moisture supply relies on moving air, wind is the key factor.

Advection fog happens when moist air slides horizontally over a surface that is cooler than the air itself. The horizontal transport brings in the moist air and carries it into the cooler layer, where it cools to its dew point and condenses. Without that wind-driven supply and movement, the fog isn’t established in the same way.

Upslope fog forms as moist air is pushed up and along terrain by the wind. As the air rises, it expands and cools adiabatically, reaching dew point and condensing into fog. The wind is essential to lift the air up the slope.

Radiation fog occurs on clear nights with little wind, when the ground radiates heat away and the air near the surface cools and reaches its dew point. No horizontal movement of air is required for this type.

Steam fog develops when cold air moves over a relatively warm water surface, and water vapor from the surface condenses into visible fog as it mixes with the colder air. While wind can influence it, this fog is driven primarily by the temperature difference and evaporation, not by a wind-driven transport process.

So the fog types that depend on wind to exist are advection fog and upslope fog.