The advisory will remain in effect until 11 a.m. and applies to San Francisco as well as its surrounding communities, including parts of Alameda, Contra Costa, Marin, Monterey, San Mateo, Santa Clara and Napa counties.
"It is fairly widespread making it really quite an extensive fog event," Charles Bell, a National Weather Service meteorologist, said.
Motorists are advised to plan for a longer commute and to leave plenty of distance between cars in front of them, Bell said.
