Hotspots indicate zones in the San Francisco Bay Area we project could be dry in the future due to changes in climate. Base data acquired from a U.S. Geological Survey study of fog belts along the California coast.

Hotspots indicate zones in the San Francisco Bay Area we project could be dry in the future due to changes in climate. Base data acquired from a U.S. Geological Survey study of fog belts along the California coast.




In San Francisco, a typical day between June and September starts with a heavy blanket of marine stratocumulus clouds streaming into the bay. By late morning, the fog dissipates for a few hours of clear skies. Then in late afternoon, it rolls in again, shrouding the city until nightfall. 

The iconic fog can be attributed to a dramatic temperature differential between the Pacific Ocean and the Central Valley. Upwelling along the coast creates the clouds. Then, as temperatures rise inland, the clouds get sucked through the geologic bottleneck of the Golden Gate.

This pattern is beginning to shift. According to UC Berkeley professor Todd Dawson, the amount of fog in the Bay Area has declined by 33% over the past century, a loss of approximately three hours of fog a day. 

Dawson and his team attribute the reduction in summer fog to climate change. Warmer temperatures along the coast are heating up the water surface of the ocean, weakening the upwelling effect, and in turn, decreasing the amount of fog produced. To test this theory, they collected data from 114 temperature stations along the California coast and inland and compared them to historical records. In doing this, they found that the coast-inland temperature contrast has decreased significantly, with some of the sites going from a temperature differential of 17 degrees Fahrenheit historically, to just 11 degrees Fahrenheit today. 

This dramatic shift in the coast-inland temperature differential and resulting decrease in fog could have a profound impact on Bay Area ecologies. Since the Bay Area region only receives an average of about 20 inches of rain a year, many plants rely on the absorption of airborne moisture for survival. These fog-dependent species include a number of coastal forest plants such as redwoods, sword ferns, huckleberries, and tan oaks that can be found in the city. Beyond plants, scientists are also concerned about faunal systems. In the wider region, a reduction in fog might mean that coastal creeks heat up more quickly, creating a concern for spawning salmon. 

San Francisco fog fieldwork, observing coastal ecologies.

San Francisco fog fieldwork, observing coastal ecologies.


While this climatic shift in San Francisco has been slow and incremental, long-time residents are beginning to notice the new weather pattern. Grace Gornall, a Bay Area native who moved to Los Angeles for five years before moving back, compares the new San Francisco summer climate to that of Hollywood. Instead of being cool and crisp, she describes it as being warm and muggy. She states “The temperature, in general, is lighter; you can wear a t-shirt without a jacket. Personally I love it, but I know that that’s not a good thing.” 

A drier, sunnier San Francisco might have broader effects beyond shifting regional ecologies. The fog is often seen as an unexpected nuisance by summer visitors; miserably cold tourists on double decker buses going across the Golden Gate Bridge is a common sight. A reduction in summer fog could translate to increased tourism for the region. 

Sunnier summers might even lead to an increase in Bay Area transplants, a region already suffering from a housing shortage. 

While many scientists believe that coastal fog will continue to decrease in the Bay Area, a few suggest that more fog is likely. For example, a team of researchers at San Jose State, led by Robert Bornstein, looked at coastal air temperatures between Los Angeles and San Francisco, finding that in these locations, summer daytime temperatures have been cooling since 1970, while inland temperatures have been increasing, leading to a wider temperature differential.  

Also, even if Bay Area summers shift towards drier, sunnier weather due to climate change, this does not mean the rest of the year will follow suit. In fact, recent studies are finding that due to climate change, more extreme winter storms are ahead for California. Scientists in MIT’s Joint Program on the Science and Policy of Global Change believe that the state will, on average, experience three more extreme precipitation events per year.