San Francisco’s Fog: Holding Strong or Fading Fast in a Changing Climate?
In mid-July 2022, while cities across the U.S. were baking under scorching heat — with temperatures in many places soaring into the 90s and even crossing the 100-degree mark — San Francisco offered a stark contrast. The weather map showed the city peaking at just 65 degrees. It was a classic foggy summer day in the City by the Bay. In fact, that month’s average temperature hovered around 62 degrees, consistent with the Julys that followed.
But with climate change looming large, some are beginning to wonder if tourists will stop scrambling for sweatshirts upon arrival and instead feel perfectly at ease strolling in shorts and T-shirts. The future of San Francisco’s beloved fog has become a hot topic in recent years. Media reports and expert opinions have pointed to a possible thinning of the coastal cloud layer, which could signal a broader warming trend.
Still, most scientists agree: while some changes are evident, a fully fog-free San Francisco remains unlikely — at least for now. Rachel Clemesha, a project scientist at the Scripps Institution of Oceanography at UC San Diego, says the data just doesn’t support the idea of fog vanishing altogether. “There are always years with more or less cloud cover,” she notes. “The past few years are still within that natural range. San Francisco is inherently foggy — losing that would be dramatic.”
Tracking Fog: Elusive Data, Uncertain Trends
When it comes to tracking fog along the California coast, data is inconsistent and often incomplete. In Southern California, there’s some evidence of a decline in the marine layer, especially in highly urbanized regions. But statewide patterns remain unclear.
Peter Weiss, a lecturer and researcher in environmental sciences at UC Santa Cruz, emphasizes the uncertainty. Despite growing concerns about diminishing fog, he says the supporting data is “very spotty.” Part of the challenge lies in fog’s fleeting nature. “It’s an ephemeral phenomenon,” Weiss explains. There’s also no universally accepted method of measurement. Some studies rely on visibility data from airports, others use satellite imagery, and some researchers prefer to gauge water content directly.
Airport data, which stretches back to 1950, shows a roughly 5% decrease in fogginess through 2012. But the reasons behind that decline are still murky. “Nobody’s exactly sure why,” Weiss admits. “It may be linked to sea surface temperatures, which fluctuate naturally. While ocean warming due to climate change is real, it’s also episodic. Some evidence suggests less fog follows periods of warmer sea surfaces, but this remains an active area of research.”
Wider Consequences for California’s People and Nature
Though the scientific picture is still coming into focus, many experts agree that if fog continues to retreat, the consequences could be far-reaching. Nearly 70% of California’s 39 million residents live in coastal counties — areas that also contribute 80% of the state’s GDP. Reduced fog could affect everything from energy usage to public health and agriculture.
California’s redwood forests, for instance, depend heavily on fog, drawing up to 40% of their annual water intake from it. A sustained drop in cloud cover could stress these ancient giants and threaten entire ecosystems. Daniel Fernandez, an environmental science professor at Cal State Monterey Bay, is among those seeking to better understand the implications. His team is applying for funding to study how climate change might alter fog patterns and impact fog-dependent life.
“If fog decreases, we could see significant species die-off in some areas,” Fernandez warns. “It would also change how people live. In fog zones, people don’t rely on air conditioning. That comfort — the cooler air, the reduced energy demand — might disappear.”
The Science Behind the Mist
Fog in California forms when moist, warm air drifts over the cold ocean waters stirred by upwelling currents. It’s most common in summer and is especially dense in Northern California, where colder waters, higher latitudes, and unique coastal geography combine to make fog a near-daily feature.
In San Francisco, that fog can range from a light blanket that cools down the day to a heavy shroud that soaks hair and fogs up glasses — making residents feel like they’re literally living inside a cloud.
UC Davis professor Ian Faloona recently ran regional climate models that show a downward trend in cloud cover. Still, he cautions, the evidence is subtle. “It’s not conclusive,” he says. But he does point out that overall, California is warming fast — nearly 1.8°F per decade. Even San Francisco, though still foggy, has warmed by 1.6°F since 1970.
So while the city may not lose its fog overnight, change is in the air — and it’s worth watching.
