Morning fog is the Bay Area's most familiar weather. You wake to gray skies, step out into cool, damp air and wonder whether it will burn off by noon. The answer depends on which kind of fog you have, because Bay Area morning fog comes from two different sources depending on the season. In summer, it is usually the marine layer that rolled in the afternoon before. In winter, morning fog in the inland valleys is usually radiation fog that formed overnight. The two look alike from a window, but they behave differently, clear at different times and affect different parts of the region.
If you want to know what is behind the fog outside right now, start with why it is so foggy today. That guide covers the common causes of a foggy day and links to the live Bay Area forecast.
Summer Morning Fog: The Tail of the Marine Layer
San Francisco's summer fog follows a daily cycle that runs almost like clockwork on most June, July and August days. The marine layer rolls in from the Pacific in the afternoon, pulled inland by the pressure difference between the cold ocean and the hot interior. It usually covers the western neighborhoods by 3 or 4 PM and thickens through the evening. By morning it is often at its most widespread, reaching neighborhoods that were sunny when it arrived.
The fog spreads overnight because cooling at night stops the mixing that thins it during the day. With no sun to stir the air, the marine layer has all night to fill in. Its base is usually lowest around 7 or 8 AM, often just a few hundred feet above the ground, and it hides the sky completely.
Burn-off begins after sunrise, as sunlight warms the top of the fog and filters through to heat the ground. Warmth from the ground stirs the cool, foggy air and mixes it with the warmer, drier air above the marine layer. The fog base lifts, the layer thins and patches of blue sky appear. In San Francisco's western neighborhoods, the fog usually clears between 11 AM and 2 PM. On days with a strong marine push, it may never clear at all.
Winter Morning Fog: Radiation Fog in the Valleys
In fall and winter, the Bay Area's inland valleys get a different kind of morning fog. Napa Valley, the Livermore Valley and the Petaluma lowlands often wake to dense ground fog that formed overnight as the land cooled. This fog can be thicker and more striking than the marine layer. It sits low on the valley floor while the hillsides above stay in clear air.
Radiation fog forms when the ground loses heat quickly on clear, calm nights. The air touching the ground cools to its dew point, and its water vapor condenses into fog. The fog keeps building through the night and is usually densest at dawn. On a still winter morning, visibility on the Napa Valley floor can drop well under a quarter mile while ridges 500 feet above sit in sunshine.
Winter inland fog usually clears faster than summer coastal fog, because nothing keeps feeding it once the sun comes up. As the surface warms, radiation fog lifts and evaporates. On most winter days it is gone from the inland valleys by mid to late morning, leaving sunny, mild afternoons. That is part of what makes winter in Napa and Livermore so pleasant, with foggy mornings giving way to clear afternoons in the upper 50s to mid 60s.
How to Tell Which Type of Morning Fog You Have
Season and location are the first clues to which fog you have. Summer morning fog in San Francisco is almost always the marine layer, and winter morning fog in an inland valley is almost always radiation fog. The two kinds also look different from a hillside. Radiation fog hugs the ground in a shallow, flat layer with clear air just above it. Marine layer fog reaches up to a ceiling that may be hundreds or thousands of feet high. Standing in it feels like being inside a cloud rather than under one.
Wind offers another useful clue. Radiation fog forms in still air, while marine layer fog arrives on the wind and usually comes with a noticeable breeze. If it is foggy and breezy, you are almost certainly in the marine layer. If it is foggy and completely still, radiation fog is the more likely cause.
Burn-Off Times by Bay Area Location
Burn-off times vary a great deal by place and season. In San Francisco's Outer Sunset and Outer Richmond, summer fog often lingers until noon or later, and on strong marine push days it may last all day. The Mission District and the Castro sit partly sheltered from the ocean flow, so their fog often clears by 10 or 11 AM.
On days when the marine layer is shallow, fog can fill the lower East Bay while the ridges above sit in clear air. Berkeley and Oakland often clear earlier than western San Francisco, because the fog has thinned and warmed by the time it crosses the bay. In Napa Valley and Livermore, winter radiation fog typically clears between 9 AM and noon.
The most reliable way to judge whether your morning fog will clear is to check the marine layer forecast. When the National Weather Service expects strong onshore flow, plan on a slow or incomplete burn-off. When it expects the onshore flow to weaken, the fog will likely clear by early afternoon. In winter, inland radiation fog almost always clears by noon, whatever is happening on the coast. Dense tule fog in the Central Valley is the exception and can sometimes last all day.
