Advection Fog

One of San Francisco’s signature landmarks is the dramatic low-rolling advection fog. This type of fog requires air that is advecting horizontally (or moving horizontally) from one place to another. When warm and moist air blows over a cold surface, the surface cools the air. Once the air temperature cools enough to equal the dew point temperature, condensation forms and creates a blanket-like thick fog formation. Not limited to the San Francisco Bay Area, advection fog occurs in inland areas when warm air moves over moist soil or dense snowpack. Whether it’s a warm front or southerly winds, as the warm air cools, the temperature will reach the dew point temperature, forcing the water vapor to condense into a cloud.