Travel & Day Trips

Why is San Francisco So Foggy? The Science

It’s no secret that San Francisco is known for its iconic fog, often playfully named “Karl” by locals. The city’s trademark mist gives the bustling metropolis an air of mystery and allure, even as it envelopes its famous landmarks like the Golden Gate Bridge.

But why is San Francisco so foggy? The answer lies in a combination of unique geographical and atmospheric factors.

The Role of the Pacific Ocean

The Pacific Ocean plays a significant role in San Francisco’s propensity for fog. Ocean water, particularly the Pacific, is known for its cold currents.

The California Current, an eastern boundary upwelling system, brings cold water from the northern Pacific along the Californian coast. As warm, dry inland air meets this cold, moist ocean air, the result is the formation of advection fog.

San Francisco’s Unique Geography

San Francisco’s geographical layout further amplifies this effect. The city is positioned on a narrow peninsula, with the cold Pacific Ocean on one side and the warmer San Francisco Bay on the other. This unique placement between two large bodies of water greatly enhances the conditions for fog formation.

The city’s famous hills also play a role. As the cool, moist ocean air is pushed inland, it is forced upwards by the city’s hilly terrain, further cooling it down and triggering condensation. The result is the frequently seen fog that rolls over the hills and engulfs the city.

The Influence of the Marine Layer

Another critical factor in San Francisco’s fog phenomenon is the marine layer, a relatively stable layer of air that forms over the Pacific Ocean. This layer varies in depth and is typically cooler than the air above it due to contact with the cold ocean water. When the marine layer moves inland, it can condense into fog as it interacts with the warmer, dryer air above it.

In the summer months, the inland areas of California heat up faster than the coastal areas. The resulting pressure difference pulls the marine layer (and therefore the fog) inland, typically in the late afternoon and evening. This is why fog is most common in San Francisco during the summer.

The Role of the Central Valley

Lastly, the Central Valley of California also contributes to the foggy conditions in San Francisco. As this vast, flat region heats up during the day, it creates a low-pressure area that draws the marine layer and its fog inland through the Golden Gate—essentially the only sea-level gap in the coastal mountains.


So, why is San Francisco so foggy? It’s a result of a perfect storm of geographical and atmospheric conditions: the cold Pacific Ocean currents, the unique peninsula on which the city sits, the city’s hilly terrain, the marine layer, and the thermal influences of the Central Valley.

This iconic fog often results in cool temperatures and obscured views, but it also gives the city a certain charm that locals and visitors alike have come to appreciate and even love. Next time you see “Karl” rolling in over the hills, you’ll know exactly why he’s come to visit.

Thomas Smith

Thomas Smith is a food and travel photographer and writer based in the San Francisco Bay Area. His photographic work routinely appears in publications including Food and Wine, Conde Nast Traveler, and the New York Times and his writing appears in IEEE Spectrum, SFGate, the Bold Italic and more. Smith holds a degree in Cognitive Science (Neuroscience) and Anthropology from the Johns Hopkins University.

Leave a Reply

Back to top button