What type of fog forms when warm, moist air moves over a colder surface?

Advection fog forms when warm, moist air moves over a colder surface and cools to its dew point.

The cooling may occur above cold ocean currents, snow-covered ground, or chilled land. As the air temperature falls, water vapor condenses around microscopic particles such as sea salt, producing a low cloud at the surface. Coastal California is a classic setting because moist marine air often passes over cold upwelled water.

Advection means horizontal movement, usually carried by wind. That distinguishes this fog from radiation fog, which develops when the ground loses heat overnight, and upslope fog, which forms as air rises and cools along terrain. Steam fog is the reverse situation: cold air moves over warmer water and picks up moisture.

Advection fog can be especially persistent when a temperature inversion traps the cooled, saturated air near the ground. It is a major visibility hazard for ships, aircraft, and road users, although moderate wind may either transport it inland or eventually mix it away.