Daly City, immediately south of San Francisco, is known for its relentlessly overcast skies, often called the “Daly City gray.” This persistent meteorological phenomenon results from a convergence of oceanic, atmospheric, and geographical conditions. The heavy, low-lying cloud cover is a predictable feature of the regional climate. This explanation details the three principal components that create and funnel this dense moisture, ensuring Daly City remains one of the foggiest places on the California coast.
The Ingredients: Cold Ocean Water and the Marine Layer
The foundation of Daly City’s fog begins hundreds of miles offshore in the Pacific Ocean with the cold California Current. This current transports cool water from the Gulf of Alaska southward along the West Coast of North America. The water temperature is significantly cooler than the air mass above it, which is essential for fog formation.
As warmer, moist air from the Pacific moves horizontally over this colder water, the air temperature drops rapidly to its dew point. This process is known as advection fog, where warm air is chilled by contact with a cold surface. The resulting cool, moist air mass becomes a thick, low-lying cloud deck called the marine layer.
The water vapor condenses into microscopic droplets, forming the dense fog that hugs the ocean surface. This marine layer typically extends only a few thousand feet high, making it susceptible to being pushed inland by prevailing westerly winds.
The Atmospheric Trap: Temperature Inversion
The marine layer fog is prevented from rising and dissipating by a temperature inversion. Normally, air temperature decreases with altitude, allowing warm air to rise and moisture to disperse. During an inversion, however, a layer of warmer air sits aloft, directly above the cooler marine layer air near the surface.
This elevated warm air acts like a stable lid, effectively trapping the cool, dense fog below it. The inversion layer is often strengthened by the Pacific High, a high-pressure system over the ocean that causes air to sink and warm through compression. The colder ocean water below enhances this temperature contrast, creating a strong ceiling for the marine layer.
Since the cool, moist air cannot rise through the warm inversion layer, it remains compressed close to the ground. This atmospheric lid forces the moisture to stay concentrated, resulting in the dense, low fog that defines the local weather.
Daly City’s Geographical Funnel
While the cold water and the inversion create the fog, Daly City’s specific landscape dictates its intensity. The city is situated on the San Francisco Peninsula, where the coastal mountain ranges create a geographical funnel effect. The marine layer, trapped beneath the temperature inversion, is forced to flow through the lowest available passages in the coastal topography.
To the north, the fog is channeled through the Golden Gate, but a significant portion is also pushed directly inland at a lower elevation. Daly City is positioned to receive the full force of this flow, particularly through the “San Bruno Gap.” This gap is a relatively low-lying area between the San Bruno Mountains to the north and the higher coastal hills, like Sweeney Ridge, to the south and west.
The prevailing westerly winds drive the trapped marine layer through this valley with great force, concentrating the moisture and intensifying the fog directly over Daly City and South San Francisco. This funneling action explains why Daly City often remains blanketed in dense fog, even when nearby inland areas may see clear skies above the inversion layer. The channeling of the marine layer through this geographical gateway ensures the city’s consistently gray climate.