The Bay Area has two separate kinds of fog, and they share little beyond the water droplets they are made of. Coastal fog, the marine layer that defines San Francisco summers, forms over the cold Pacific and moves inland on the afternoon wind. Inland fog covers valley floors in Napa, Livermore and Petaluma on winter mornings. It forms in place over land that cools on calm, clear nights. The two kinds arrive in different seasons, behave differently through the day and affect different parts of the region. That is why the Bay Area's fog varies so much from place to place and month to month.
Coastal Fog: The Marine Layer
Coastal fog in the Bay Area is advection fog. It forms when moist air flows over the cold water of the California Current and cools to its dew point. The moisture condenses into a low cloud bank over the sea. The fog bank waits offshore until the afternoon, when the pull of hot inland air draws it through the Golden Gate and along the coast. It is mainly a summer and early fall fog, because that is when the contrast between the cold ocean and the hot interior is greatest.
Coastal fog peaks from June through August, with May and September on the shoulders of the season. In those months, the Outer Sunset and Outer Richmond in San Francisco can spend about half their daylight hours in fog. The fog usually arrives with the wind in the afternoon or evening. It pulls back in the morning as the sun warms the ground and thins it from below. How far it retreats varies, and on strong marine push days it may never clear, while on weak days it pulls back to the coast by noon.
Coastal fog follows the shape of the land around the bay. The Golden Gate is its main entry point, and the neighborhoods directly in its path see the most fog. As the marine layer moves inland over warmer ground, it gradually thins. San Francisco's western neighborhoods are far foggier than the Mission District, and Oakland is sunnier than the Sunset. Walnut Creek and Livermore, sheltered by the East Bay hills, can be 30 degrees warmer on the same afternoon.
Inland Fog: Radiation Fog in Bay Area Valleys
Inland fog in the Bay Area is radiation fog, and it forms in a completely different way. On clear, calm nights, the ground loses heat to the sky, and the air touching it cools as well. When that air reaches its dew point, its water vapor condenses into fog. The fog collects in low places where cold air drains and pools, such as valley floors, creek corridors and flat basins.
Inland fog belongs to fall and winter, when long nights give the ground time to cool. It also needs clear skies so heat can escape and calm air so the cold layer near the ground stays put. In the Bay Area, the main inland fog season runs from October through February. Napa Valley, the Sonoma Valley, the Livermore Valley and the Petaluma lowlands are the most fog-prone inland places. All of them are low, flat areas where cold air pools and the wind is too light to stir it.
Inland fog is easier to predict than coastal fog, because only the local sun is needed to clear it. On most winter days, it is gone by 10 AM or noon, leaving sunny and fairly mild afternoons even in December. A cold, foggy morning followed by a sunny, mild afternoon is the classic Bay Area valley winter day.
Key Differences at a Glance
Season and place are the easiest way to tell the two apart. If it is June and foggy in San Francisco, you are looking at coastal fog. If it is December and foggy in Napa, it is inland radiation fog. Most of the time, the split by season and location is that clean.
Coastal fog arrives in the afternoon and evening on the wind, pushed by the pressure difference between coast and interior. Inland fog forms late at night and thickens toward dawn in calm air. Coastal fog can last all day when the onshore push is strong, while inland fog almost always clears by noon. Coastal fog is thickest along the coast and thins as it moves inland. Inland fog is thickest in the middle of a valley and clears first around the edges, where the ground is higher and the fog layer is thinner.
When Both Types Coexist
In some periods, especially October and November, both kinds of fog form at once in different parts of the region. The marine layer may push fog through the Golden Gate into western San Francisco while radiation fog fills the Napa and Livermore valleys overnight. On those mornings, few places are clear, with the coast gray from the marine layer and the valleys white with ground fog. The hills and ridges between them may be the only spots in sunshine.
In late fall, finding clear, warm weather means checking which kind of fog you are dealing with. When radiation fog fills Napa but the marine layer is weak, Napa is still a good bet. Once the fog burns off by mid-morning, the afternoon there can be perfect. When the marine layer is strong and the valleys are clear, inland towns such as Napa, Walnut Creek and Livermore are the best bets for sun. On the mornings when both kinds of fog are out, the East Bay hills and the Marin peaks are often the clearest places in the region.
