Which type of fog forms when the ground cools after sunset, cooling the air near the surface?

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

Answer: (D)

Fog forms when air near the ground cools enough for water vapor to condense into tiny droplets. After sunset, the ground loses heat through radiation and the air just above the surface cools rapidly. If it cools to the dew point and there isn’t much wind to mix the air, a shallow layer of fog can form right at the surface. This is typical on clear, calm nights, and it often disappears after sunrise when the ground warms up again.

Other fog types come from different situations. Advection fog happens when warm, moist air moves over a cooler surface, bringing moisture to the air and forming a fog layer. Steam fog occurs when cold air moves over relatively warm water, causing vapor to condense as the air cools. Upslope fog forms as air is forced to rise up a slope, cooling as it ascends.