If you have lived in the Bay Area for decades, you may feel that summers are less foggy than they used to be. Research gives that impression some support. The best-known study found that summer fog along the Northern California coast became about a third less common over the 20th century. The causes are still debated, though, and the link to climate change is more complicated than the idea that a warmer world simply means less fog. Three things interact to shape the fog, which are the temperature of the ocean, the heat inland and the winds that carry ocean air ashore. Understanding how they work together helps explain what has changed and what might come next.
What Research Shows About Fog Decline
The most widely cited study was published in 2010 by researchers at UC Berkeley. They used records of low cloud from coastal airports, which go back to 1951, and longer temperature records that track how foggy summers were. Together, those records suggested that summer fog in the coast redwood region had become roughly 33 percent less frequent since the early 1900s.
The researchers tied the change to the temperature contrast between the cold ocean surface and the warmer land. When the coastal ocean is unusually cold compared with the land, fog forms easily. When that gap narrows, fog becomes less common. Over the century, the gap had narrowed, and the fog had thinned along with it.
Other research has complicated the picture. Some studies have found that summer afternoons along parts of the California coast have actually cooled since the 1970s, even as inland areas warmed. One explanation is that stronger inland heat now pulls in more ocean air. Satellite maps of fog and low cloud, which cover only the past few decades, show large swings from year to year and no simple trend. Most scientists agree that fog has varied a great deal, but the size and direction of the recent trend are still being studied.
Why Is Fog Declining? The Competing Explanations
Bay Area fog depends on cold ocean water. Each summer, steady northwest winds along the coast push surface water offshore, and colder water rises from below to replace it. That process, called upwelling, keeps the ocean near San Francisco in the 50s. The cold water chills the air above it and builds the marine layer, the shallow blanket of cool, damp air that turns into fog.
Anything that warms the coastal ocean can weaken that process. The North Pacific goes through long warm and cool phases, lasting a decade or more, that raise or lower coastal water temperatures. Global warming also adds heat to the ocean over time. If the water near the coast cannot get as cold as it once did, the marine layer forms less easily. The California Current and the strength of upwelling both play a part.
The winds that drive upwelling can change as well. They come from the North Pacific High, a large area of high pressure over the ocean. When the high shifts or weakens, coastal upwelling slows, the water warms and the fog thins. A stronger high can do the reverse and help the fog. Scientists are still working out how much each of these factors explains the fog changes seen so far.
What Fog Decline Means for the Bay Area Ecosystem
Coast redwoods are the plants most closely tied to the fog. They evolved in a climate where summer fog shades the forest and drips water onto the ground during the dry months. That fog drip can supply a meaningful share of a redwood forest's summer water. Less fog means more sun, warmer air and drier soil in the dry season, which adds stress to trees already dealing with drought.
Fog also keeps coastal areas cool, and without it temperatures in coastal scrub, grassland and forest rise on summer days. That can shift when plants flower, change where fog-loving species can survive and dry out vegetation that might otherwise stay damp through the summer. San Francisco's famously steady summer climate, which lets most homes go without air conditioning, depends on that fog as well.
The coastal grasslands and headlands of Point Reyes National Seashore and the rest of the Marin coast are shaped by fog too. Many of their plants rely on summer fog for moisture. A lasting drop in fog would change these places in ways that are hard to predict and hard to undo.
Will Bay Area Fog Continue to Decline?
The future is genuinely uncertain, and different research groups have reached different conclusions. Some expect fog to keep thinning as the ocean warms. Others point out that a hotter interior could strengthen the pull of ocean air and the winds that drive upwelling, which would help keep the fog going.
The uncertainty comes from forces that push in opposite directions. A warmer ocean works against fog, while stronger inland heat can work for it. Climate models also disagree about how the North Pacific High will change. The overall effect on future Bay Area fog is not yet known.
What is clear is that a warmer world does not automatically mean a fogless Bay Area. The relationship runs through the ocean and the atmosphere in ways that make confident forecasts difficult. The scientists studying the question are open about that uncertainty.
Deep Dive: How Do Scientists Measure a Century of Fog?
No one has measured California fog continuously for a century, so scientists piece the record together from several kinds of evidence. Airport weather logs reach back to the mid-1900s. Temperature records and the chemistry of redwood tree rings stretch further. Satellites add a detailed map of recent decades.
What Can Airport Records Show?
Airports have logged the height of the cloud base every hour for decades, because pilots need it. James Johnstone and Todd Dawson of UC Berkeley used those logs from Arcata and Monterey, which reach back to 1951. They published the study in 2010 in the Proceedings of the National Academy of Sciences. They counted hours with a cloud or fog ceiling below about 400 meters, or roughly 1,300 feet.
To reach back before 1951, they used the link between fog and the summer temperature contrast along the coast. Foggy summers keep the coast cool compared with inland areas. Records from 114 temperature stations along the Pacific coast showed that the contrast had shrunk over the century. Press accounts of the study reported a roughly 33 percent drop in fog frequency since the early 1900s.
What Do Tree Rings and Satellites Add?
Redwoods record their water supply in the wood they grow each year. Fog water and winter rain carry slightly different mixes of oxygen isotopes, atoms of the same element with different weights. In a 2013 study in the Journal of Geophysical Research, Johnstone, John Roden and Dawson found that the isotopes in redwood rings respond to summer fog and drought. The rings could eventually extend the fog record back centuries.
Satellites fill in the map. Alicia Torregrosa and colleagues at the U.S. Geological Survey classified about 26,000 hourly GOES satellite images from the summers of 1999 to 2009. Their maps showed fog and low cloud averaging from under 2 to about 18 hours a day depending on location. Places such as Montara, on the San Mateo coast, averaged more than 14 hours a summer day.
Each method has limits. Airports sample only a few points, temperature contrasts are an indirect measure and the satellite record is short. Researchers trust the long-term picture most where several methods agree, which is why the size of the recent trend is still debated.
What You Actually Notice Day to Day
From year to year, the changes you notice mostly come from natural swings in the Pacific. El Niño years often bring warmer coastal water and a weaker marine layer. La Niña years with cooler water can be foggier. Marine heat waves, such as the warm "blob" off the West Coast from 2014 to 2016, can also thin the fog for a season or two. Swings like these are often larger than any long-term trend.
As a result, it is hard to tell from personal experience whether a given summer was clear because of long-term change or a short-term ocean pattern. Both kinds of change matter. Long records suggest that coastal fog has varied a great deal over the past century. The long-term question is still open, even when one summer feels clearly different.
If you want to see the Bay Area's summer fog at its most typical, visit in June, July or August, when it is most persistent. September and October, when the fog usually pulls back, offer the best chance of warm, clear days on the coast. How those seasonal patterns change in the coming decades will say a lot about the future of the region's unusual climate.
