The fog that defines San Francisco's summers does not form over the city. It forms over the cold Pacific just off the coast, under a low cloud deck that can stretch far out to sea, and the wind carries it ashore. This ocean fog, also called sea fog or marine fog, is the source of the Bay Area's most famous weather. Knowing how it forms over open water and how it travels to shore explains the daily pattern that residents and visitors live with each summer. It follows different rules from the radiation fog that forms in inland valleys on clear winter nights.
How Ocean Fog Forms
Ocean fog forms when warm, moist air moves over cold water and is chilled from below to its dew point. Fog formed this way, by air moving sideways over a cold surface, is called advection fog. The California coast is one of the best places in the world for it. The California Current carries cool water south along the coast from the North Pacific. In spring and summer, strong north winds also push surface water offshore, and colder water rises from below in a process called upwelling. As a result, the ocean off San Francisco is usually in the low to mid 50s in July, much colder than the air moving over it.
When moist air passes over that cold water, it cools to its dew point and fog forms. The resulting fog layer is often 1,000 to 2,000 feet thick. It can block the sun completely and cut visibility to near zero at the surface. It is not a thin haze but a real cloud that happens to sit at sea level.
From Ocean to Shore: Fog Transport
Once it forms, sea fog travels with the wind. Along the California coast, the main summer wind blows from the northwest, roughly parallel to the shore, but part of the flow pushes the fog toward land. The fog bank holds together as it moves over the water. When it reaches the warm coast, one of two things happens. Sunlight and warm ground can heat it from below until it lifts and evaporates, or it can flow inland wherever a gap in the hills lets it through.
The Golden Gate is the Bay Area's main entry point for fog. This sea-level gap in the Coast Ranges lets the fog flow straight into the bay without climbing over any hills. Once through the Gate, the fog spreads across the bay, pulled inland by the pressure difference between the cool coast and the hot Central Valley. On strong days, that flow carries it to Oakland and up against the East Bay hills.
Why Northern California Sea Fog Is So Thick
Sea fog forms along many cold-water coasts around the world. Examples include the Grand Banks off Newfoundland, the coast of Namibia along the Benguela Current, and the coasts of Peru and Chile along the Humboldt Current. The California coast produces some of the thickest and most persistent sea fog of all. It has a cold current, strong summer upwelling and a hot Central Valley that pulls marine air inland. It also has the Golden Gate, a gap that lets the fog travel far inland instead of stopping at the shoreline.
The marine layer that results can be anywhere from a few hundred to more than 2,000 feet deep. When it is deep, it feels like a complete overcast rather than a thin coastal haze. On a July satellite image, the fog deck shows up as a solid white sheet over the coast and the bay. The Central Valley and the Sierra Nevada sit in sunshine beside it. From above, it looks like what it is, an ocean cloud that has drifted over the land.
How much sea fog forms depends heavily on the ocean temperature. When the water off the coast runs warmer than usual, the contrast with the air shrinks and fog tends to form less readily. Strong El Niño events can warm the coastal ocean, and stretches of cold, upwelled water can strengthen the fog. Researchers are still sorting out how much these ocean swings, and long-term warming, change Bay Area fog from year to year. Either way, the fog that San Francisco lives with each summer is part of a pattern that spans the whole Pacific.
Deep Dive: How Does Sea Salt Shape Ocean Fog?
Every fog droplet starts on a speck of dust, smoke or salt, because water vapor needs a surface to condense on. Over the open Pacific, most of those specks are tiny crystals of sea salt flung into the air by breaking waves. The kind and number of specks decide what the fog looks like and how it behaves.
Why Are Ocean Fog Droplets Larger Than Inland Ones?
The specks that start droplets are called condensation nuclei. Sea salt makes excellent ones, because salt pulls water toward it and starts a droplet even before the air is fully saturated. Over the ocean, these nuclei are fairly large but not very numerous.
Training material from the COMET program at the University Corporation for Atmospheric Research makes the comparison. Droplets in marine clouds are much larger than those over land, and fewer of them form. Over land, pollution and dust supply many more small nuclei. The same amount of water then spreads over far more droplets, so each one stays smaller. A 2002 field study of sea fog in the northwestern Pacific found the same thing. Coarse sea-salt particles, not finer pollution particles, served as the main nuclei.
Droplet size changes how fog feels and behaves. Large droplets collide and merge more easily, so marine fog often produces drizzle, the fine mist that wets windshields along the coast. Air drifting off the cities around the bay can add many small pollution particles to the mix. In 1977, the physicist Sean Twomey showed that more nuclei make clouds with more, smaller droplets that reflect more sunlight. In 1989, Bruce Albrecht showed that such clouds also tend to drizzle less.
How Do Satellites See Fog at Night?
Fog is easy to spot from space in daylight, but at night it can look almost invisible. The fog top is nearly the same temperature as the ground or sea around it, so an ordinary infrared image barely shows it. Forecasters get around this by comparing two infrared wavelengths.
Small water droplets give off less energy at a wavelength of 3.9 micrometers than at about 11 micrometers. At night, a fog deck therefore looks colder in the 3.9 channel than in the 11 channel, while clear ground looks about the same in both. Gary Ellrod of NOAA described a nighttime fog product built on that difference in a 1995 paper in Weather and Forecasting. The GOES weather satellites over the Pacific still use the same principle.
That technique is why forecasters can watch the marine layer spread along the coast overnight. On a nighttime fog image on a summer evening, you can see the sea fog pressing against the coastline. It often pushes through the Golden Gate hours before it reaches your street.
