The Elusive Future of San Francisco’s Fog

The ecological, economic and social effects of fog are profound, perhaps no more than in Northern California. Changes would be life-altering. But understanding fog is one of science’s toughest tricks. Quantifying the changes and determining possible causes, including global warming, is climatology’s version of chasing ghosts.

That day on the bridge, computer models predicted the fog would push through the Golden Gate in an hour or two. Mr. Dzierman trusts his gut. He had a hunch it would stay away, defying the familiar summertime cycle.

Just then, a slight breeze, an invisible puff of chill, the kind you might get by opening the freezer door.

“Feel that?” he said, inhaling deeply. “Mother Nature’s air-conditioning. Yeah, that feels good.”

It was a tease.

The day remained gloriously sunny, unusually hot and fog-free, the kind of San Francisco summer’s day that some fear is becoming more frequent. The bridge’s five fog horns went unused, for now.

While coastal fog isn’t unique to the California coast, few places in the world are so deeply associated with the ethereal movements and cooling spritz of fog’s peek-a-boo routine. Fog pours through the Golden Gate and crawls up and down the wrinkled hills of the city and the nearby coast. It cloaks and chills. Millions are affected by it, if only by the invisible cool breezes that presage the fog’s arrival.

Fog is why one neighborhood is notoriously chilly, another is surprisingly sunny, and the airport is where it is. It is why realtors talk about neighborhood fog patterns as much as square footage and schools. It is why fewer than half of Bay Area residents have air-conditioning, and partly why they use less water than most Americans.

Summer fog is why the mighty coastal redwoods grow where they do, surviving California’s dry season thanks to refreshing gulps of cold, wet air. It is why, until recently, few people worried about wildfires along the coast.

In June, July and August, as most of the Northern Hemisphere, and most of California, feels the full force of summer heat, the average daily high in San Francisco is below 70 degrees Fahrenheit, or about 21 Celsius, coolest of any major city in the continental United States.

Fog is a companion, part of the rhythm of summertime, flitting in and out of lives like a family member. But it does more than astonish ill-prepared tourists and dazzle photographers and poets. It nourishes the natural world. It enriches the area’s cultural identity. It might even be an untapped resource in California’s growing anxiety over water.

Which is why a decrease in California’s coastal fog, or the prospect of it disappearing entirely one day, is not a sunny proposition, particularly in and around San Francisco Bay.

The general consensus among the small cadre of scientists who study coastal fog is that it is decreasing, not just in California, but around the world. However, the reasons aren’t clear.

Fog may be the most difficult meteorological phenomena to capture, calculate and predict. Unlike temperature, precipitation, humidity or wind, there is no reliable gauge for it. There is not even a practical definition of it.

Most will say that fog is a cloud that touches the ground, which sounds simple enough. But fog is movement in three dimensions, dipping and rising, forming and disappearing.

Sometimes a thin layer hugs the water below the Golden Gate Bridge, blinding mariners. Sometimes it settles about 200 feet higher, blinding drivers. Sometimes it shrouds the top of the bridge’s towers and the airspace above, blinding pilots. Sometimes it does it all. Which of those things is fog?

It arrives like a whisper and disappears like a magic trick. It is there one moment and gone the next.

Early on a late-spring morning, Peter Weiss, a scientist at the University of California, Santa Cruz, and three of his students, calling themselves “the fog squad,” were erecting fog catchers.

The idea, simpler than the execution, is to harvest water from passing fog. A fog catcher is a stretched piece of mesh that, when it works, becomes saturated by fog moisture, dripping into a gutter, leading to a cistern.

The idea is not new. Fog from the ocean is a dependable feature in several places around the globe, mostly on the west coasts of major continents. Villages in places like Peru and Chile, sometimes with almost no rain throughout the year, have for centuries sustained themselves largely on fog water. Its use is growing in places like Morocco.

The question in a changing climate is whether fog water is a viable resource, if not a solution, for populated places expecting a drier future. Places like California.

“I would tend to think of it as a small drop in the bucket,” said Dan Fernandez, a professor in the Department of Applied Environmental Science at California State University, Monterey Bay, and a leading researcher in fog-catching. “But we need a lot of small drops in the bucket to deal with what we have coming.”

The idea of catching fog comes from nature. It is how the tallest trees in the world, California’s coastal redwoods, survive.

Decades ago, Todd Dawson stumbled into the uncrowded world of fog research. Traipsing among the redwoods as an undergraduate at the University of California, Santa Cruz, he wondered why they were habitually moist, and the ground around them so soggy, even mossy, even in mid-summer, California’s dry season, when rain is rare.

Fog, obviously. But little had been done to quantify fog’s importance to California’s ecosystems.

“That kind of haunted me,” he said.

Years later, as a professor of integrative biology at the University of California, Berkeley, Dr. Dawson studied redwoods and concluded that 30 to 40 percent of their annual moisture arrives in the form of fog.

The high canopies of redwoods are giant filters for drifting fog. The trees get drenched and nourished; water drips to the forest floor. There it sustains other plants and helps keep stream beds from drying, aiding species from lichens to ferns, newts to salmon.

“Fog is a big water subsidy,” Dr. Dawson said. “And I said, ‘My goodness, we’re ignoring this. We’ve been missing a big part of the hydrologic budget of the redwood forest.’”

In 2010, Dr. Dawson and a graduate student, James Johnstone, published another study with an attention-grabbing conclusion: Using observational data at airports in the coastal redwood region — from central California to its northern border, including the Bay Area — they found that the frequency of fog, measured by fog hours per day, had dropped 33 percent since the middle of the 20th century.

Fog was disappearing.

The ramifications extend far beyond a single species of tree. Does the long-term survival of redwoods and their ecosystems depend on fog? (Probably.) What about fog-cooled agricultural areas, like the Salinas Valley to the south of the Bay Area, or the grape-growing Napa Valley to the north? (Shifts in fog would affect them, surely.)

From a more human standpoint, what would happen to water use, power grids, solar farms and wind turbines? How would the future migrations of human populations be affected? And would San Francisco be, well, San Francisco without fog?

“Less fog is a game changer for a lot of things,” Dr. Dawson said.

For fog catchers, ambitions are realistic, motivations are simple: Why not try to squeeze water from air, especially in dry, dire times like these?

Instead of building fake redwoods, researchers build fog catchers. Results are highly variable — by day, by location, by type of mesh, by the fickle gods of fog.

The most that Dr. Fernandez has collected from a single catcher, made of one square meter of special mesh, was 37 liters of water in a day. Any location that dependably captures an average of more than a liter a day during the summer, he said, might be worth expanding.

“Fog is great,” Dr. Fernandez said. “It’s not rain. But it’s something.”

Dr. Weiss, a friend of Dr. Fernandez’s and a fellow researcher in nearby Santa Cruz, received a grant to study larger collection systems, beginning this summer. On the university’s hillside farm overlooking the Pacific, Dr. Weiss and his students created mockups using wood, mesh and guy wires. Sturdiness is vital; fog is wind, and the bigger the catcher, the more it acts like a sail.

The mesh, a bit like a weed fabric or a furnace filter, is specifically designed to capture fog. But researchers concoct variations. What works in a lab does not always work in nature.

Maybe fog will never be enough to quench the thirst of city dwellers and all their drinking, flushing and washing. But could a farm that gets zero rain in the summer, and uses no outside irrigation, survive on fog? Maybe, depending on the crop and the fickleness of the area’s fog.

“You could translate it from square meters of mesh to food calories, something like that,” Dr. Weiss said.

In 2016 and 2018, a Bay Area distillery called Hangar One produced small batches of Fog Point vodka, made from fog water. Bottles sold for up to $100 each. But Dr. Fernandez and Dr. Weiss want to believe that fog water can be more than a gimmick. If it can nourish redwood trees for thousands of years, maybe it can help humans in coming decades.

Someday, they say, the foggy parts of California might be covered in fog farms, not unlike the solar farms found in sunnier spots. That depends on many factors, not least the future existence of fog.

Ask a scientist to explain why the California coast experiences fog in the summer, and the answer will come in paragraphs, not mere words.

“It’s fascinating to study, because sometimes fog feels very local,” said Rachel Clemesha, who studies coastal clouds as a project scientist at Scripps Institute of Oceanography at the University of California, San Diego. “You’re in the fog here, but you see that it’s sunny over there. But it’s also tied to global climate.”

Fog is a niche of science, and the field of study is not a crowded one. One reason is the elusive nature of fog itself. Another is that it affects relatively few geographic areas, each differently.

But the biggest might be that it crosses so many disciplines, from sea to land to air — oceanography, meteorology, geography, biology and chemistry among them. Research is fractured.

Consider that Dr. Dawson is a plant scientist, and his monumental conclusions that fog is on the decline came from studies of redwoods.

For something so seemingly transient and capricious, California’s coastal fog is the result of sprawling atmospheric and oceanic phenomena, a delicate balance of powerful forces. What millions experience on land has its origins far out at sea.

Every summer, a high-pressure system, the North Pacific High, parks itself off the California coast, altering wind patterns and churning up the cold-water California Current that hugs the coast. The rest is all saturation points and dew points: Water vapor rises from the sea, turning into droplets that cling to microscopic airborne particles, like sea-salt aerosols, to form clouds. But these clouds stay low to the ocean, held there by a temperature inversion that creates a “marine layer.”

Sometimes this marine layer is low enough to be considered fog.

“Fog is so complex,” said Alicia Torregrosa, an environmental scientist with the U.S. Geological Survey who has done deep studies on California’s coastal fog. “And the complexity has to do with thermal relationships.”

The start, really, is the North Pacific High. Dr. Weiss calls it “the birth of fog.”

As the world warms, however, this balance could be at risk, with unpredictable consequences.

“What I want to know is, do we expect more or less clouds with climate change,” Dr. Clemesha said. “I still don’t know enough to say confidently. It’s complicated.”

Otto Klemm, a professor of climatology at the University of Münster in Germany, is more certain. He has studied data from airports all over the world.

“Fog has decreased, more or less everywhere,” he said, attributing the link both to climate change and to lower levels of air pollution, as water droplets have fewer particulates to cling to. “Of about 1,000 stations, 600 or 700 show a statistically significant decrease. All over Europe, all over North America, South America — everywhere.”

But back to California, and the fog along the coast.

“We know that the planet is warming, we know that oceans are getting warmer,” said Travis O’Brien, an assistant professor of earth and atmospheric sciences at Indiana University who has studied the California coast extensively. “What we don’t know is what’s happening in the really near coastal environment. In particular, ocean temperatures right along the coasts are a big question mark, and they have long been thought to be really important for coastal fog.”

Research can be contradictory or counterintuitive. A 2017 study using observational records of ships off the California coast suggests that fog is heavier than it used to be. So maybe it’s just not coming onshore the way it once was.

In Southern California, during the periods colloquially called “May Gray” and “June Gloom,” research shows an increase in cloud-base height — low clouds persist, but are now less likely to be in the form of ground-touching fog — because of the urban heat-island effect. Some experts surmise something similar is happening in San Francisco, too.

And maybe fog has made a comeback. Dr. O’Brien updated data from area airport observations and found that the declining fog levels in the second half of the 20th century have appeared to stabilize. Other researchers, using satellite technology, concluded this month “that the number of foggy days fluctuates considerably year-to-year with no discernible positive or negative trend occurring between 2000 and 2020.”

No one is quite sure why that is.

True to form and character, fog varies from day to day, minute to minute, place to place. To say that fog is increasing or decreasing depends on so many variables. Is this valley, this hill, this beach, this city as foggy as it used to be? How can you really tell?

There may be no place with a deeper connection to fog than the California coast, especially the 150 miles or so from Monterey to Point Reyes National Seashore, where the lighthouse, shrouded in fog about 200 days a year, is one of the foggiest places on the continent.

Millions live within 50 miles of the coast along that stretch. Daily high temperatures might fall short of 60 degrees near the ocean but be more than 100 degrees inland, all depending on fog’s reach.

In between, many feel the effect of fog but rarely see it — a breath of cool afternoon air, maybe, or a bracing chilled wind that upends outdoor plans in the evening.

But in some places, the fog puts on a show.

It clings close to the ground, like a stalking cat. Sometimes it pauses, sometimes it pounces. It slips through topographic gaps. It peeks over the top of hillsides and slinks into valleys. Sometimes it comes in wisps. Sometimes in waves.

Sometimes the fog reaches one valley, one town, one neighborhood — but not the next. Sometimes it smothers everything in its path. “There’s rarely a July Fourth fireworks celebration that anybody actually sees,” said Phil Ginsburg, general manager of San Francisco Recreation and Park Department.

Some locals love the fog. Some hate it. All deal with it. Fog is such a part of the landscape that it even has a name: Karl, personified through a wry, anonymous Twitter account.

“This is my town,” it responded to a follower recently. “You’re just living in it.”

Alexander Clark has been selling real estate in San Francisco for 20 years.

“Literally every single property I sell, the topic of fog comes up,” he said.

San Francisco is not a big city, roughly seven miles by seven miles, containing about 870,000 residents. Surrounded on three sides by water, it is the fingernail on a thumb-shaped peninsula.

The city is pinched by hills, from Mount Davidson and Twin Peaks near the geographic center, to aristocratic Nob Hill and Telegraph Hill in the northeast corner. Beyond providing San Francisco its steep streets with clattering cable cars, the hills create both a divide and a maze for the wind and fog coming from the ocean.

It creates microclimates, maybe nanoclimates. Mr. Clark was at a friend’s house on 48th Avenue, near the beach, on a summer day. “We wore down coats in the front of his house,” he said. “And you go in the backyard and kids are doing Slip ‘N Slide.”

A decade ago, Mr. Clark drew up a simple fog and wind city map for his real estate blog to give potential buyers a basic sense of neighborhood patterns. “People know the Sunset, Richmond, you’re going to have fog,” he said. “But where it really starts to play is in some of these areas where people aren’t quite sure. They’re like, ‘Hey, so what’s the fog situation? Where’s the fog line? I want to buy in the sun.’”

To try to understand fog’s effect on property values, he and a colleague crunched numbers. San Francisco’s foggy neighborhoods tend to be slightly less expensive than the median prices, though there are exceptions, like the tony Sea Cliff neighborhood.

But because there are so many factors that make or break a neighborhood — transportation, restaurants, backyards or the lack of them — proving that people pay less to live in foggier parts of town is difficult. So it comes down to feelings.

“Some people come out to an open house and there’ll be fog, or the fog will just be rolling in, and they’ll say, ‘Yeah, we’re out,’” Mr. Clark said. “I’ll say, ‘That’s fine. You’re not the right buyer.’”

Few people have spent more time living and working in the fog than Toby Kanzawa, a fourth-generation San Franciscan. Mr. Kanzawa works as a gardener at Golden Gate Park, for many years as a supervisor in Section 6 on the far west end, near the ocean.

He firmly believes there is less fog now.

“It’s something we always talk about,” he said. “We’ve noticed it out here.”

Golden Gate Park stretches 3 miles from west to east, from the Pacific Ocean to the central part of San Francisco, near the famous Haight-Ashbury of hippie lore. There can be people bundled in down jackets and beanies at the west end, sunbathing on the east.

Signs of fog’s influence are sprinkled around the park, even when the fog is nowhere to be seen. The Kwanzan cherry tree in front of the Japanese Tea Garden is twisted and tilted from the wind and fog that whips from the west. The nearby azaleas, rhododendrons and magnolias sometimes emerge from foggy summer and bloom again in San Francisco’s sunny fall.

“Some plants are fooled into thinking it’s a second spring,” said Steven Pitsenbarger, the gardener at the Japanese Tea Garden.

Elsewhere, the fog effect is seen in the natural grasslands, which turn brown in the rainless summers — except for telltale green rings around shrubs and trees. It is not just redwoods; plants of all sizes serve as fog catchers.

Much of Golden Gate Park is manicured grass, requiring hundreds of millions of gallons of irrigated water annually. While the city moves to recycled water and tries to improve the efficiency of its water use, a future with less fog and cloud cover could have huge implications. Fog season overlaps with the heat of summer, the time of year with the highest temperatures and highest evaporation rates.

Just as the gardeners see the silent signs of fog, some see signs of it dissipating. Mr. Kanzawa sees it in the once-empty, fog-soaked portions of the park now crowded with picnickers and sunbathers. On the crowded sand of Ocean Beach and Baker Beach. In real-estate prices. In the westside roads that were dangerous, if not impassable, on summer nights in his memory.

“It would get super thick,” Mr. Kanzawa said. “And now that’s a rarity. I haven’t seen it like that in a long time.”

Mr. Clark, the real-estate agent, is not so sure about the decline. He just knows that fog is a hot topic with everyone who is moving, either to San Francisco or within it.

“I mean, I ask everybody,” Mr. Clark said, ‘What’s your opinion of fog?’”

The United States Coast Guard station at the foot of the Golden Gate Bridge takes part in more than 300 search-and-rescue missions in a typical year, many of them in the fog.

Petty Officer Second Class Omar Mercado has an unusual, sometimes blinded view of those missions, driving a 47-foot motor lifeboat.

“I hate the fog,” Petty Officer Mercado said. “At first I thought it was really interesting and pretty. But after going out in it five or 10 times, no, not anymore.”

The channel of the Golden Gate is narrow, precariously so for the large container ships, some more than a quarter-mile long, that move among the major ports in the Bay. The tide floods in or rushes out like a spigot turned on full. Waves just beyond the bridge can rise 10 feet or more. The bridge towers, rock outcroppings and the undulated coast create whirlpool-like eddies. The wind almost always whips through at 10 or 15 knots, often harder.

Then, you add fog.

Petty Officer Mercado and his three crewmates recalled one particularly frightening summer night. Visibility barely extended over the bow. The sailors, on high alert, could hear the fog horns coming from above, presumably from the Golden Gate Bridge.

Suddenly, a moving gray wall appeared through the moving gray of fog: a container ship. Disaster was averted by mere feet.

“As a driver, you can get disoriented really fast,” Petty Officer Mercado said. “If you make it here at this station, you can make it at any other.”

The area is littered with shipwrecks and sea tales, not all deep in the past. In 1971, in the middle of a foggy night, a pair of oil tankers, each owned by Standard Oil, collided under the bridge.

The disaster led to the Ports and Waterways Safety Act of 1972 and the Vessel Traffic Service, a sort of air traffic control for congested or tricky ports in the United States. In a dim, windowless room at a Coast Guard station on Yerba Buena Island in the center of San Francisco Bay, the traffic controllers sit in front of large screens at all hours.

Ship traffic that used to be tracked here with large paper maps and magnets on a wall is now done with digital screens and color-coded dots. Blue is a passenger ship or ferry. Yellow is a ship carrying petroleum or other chemicals.

There are more than 30 controllers on staff, most of them civilians with deep local knowledge. Becoming certified takes about a year, and Tom Boone trains them.

Is there anything good about fog?

“No,” Mr. Boone said, without hesitation.

Despite all the effort, accidents happen and fog is the reason. In 2007, in fog, a container ship struck the Bay Bridge connecting San Francisco and Oakland. In 2013 a tanker did the same thing.

Navigation is so tricky that the biggest ships are met miles offshore by bar pilots, local experts who climb aboard and temporarily take control. They must find their way through everything else on the water — smaller ships, sailboats, kayakers, even swimmers. And fog.

Mr. Boone, a Navy veteran who specialized in seamanship and navigation, has sailed around the world and seen plenty of fog-prone ports, from Houston to Hong Kong. “The fog here just seems thicker,” he said.

Back near the Golden Gate, at the Coast Guard station, sailors monitor fog by eye and experience. Stepping out the door, rushing out for another rescue mission, the north tower of the bridge feels close enough to touch.

Petty Officer Mercado appreciates the beauty, but sees mostly danger. He considers it foggy when he cannot see Alcatraz or the San Francisco skyline. It is bad fog — mission-altering, risk-adding, response-lengthening fog — when he cannot see the bridge.

Rescues run the gamut: swimmers in distress, sailboats in rough seas, motorboats disabled.

There are stories of the sailboat that collided with a container ship unseen in the fog. Of the motorboat that cut across the tow lines of a Coast Guard lifeboat pulling a disabled ship to safety.

Less fog, Petty Officer Mercado said, “would make our job easier.”

The late San Francisco newspaper columnist Herb Caen had a lifelong love affair with the city’s fog.

“The miraculous thing about the annual summer miracle is that the miracle continues,” he wrote in 1993. “The countryside sizzles, the Midwest fries when it isn’t drowning, the Right Coast is in its ‘it ain’t the heat, it’s the humidity’ miseries, but in San Francisco we are cool, if not, at times, downright cold.”

Caen considered foghorns the music of the gods.

“The fog is on schedule, morning and night, billowing through the Golden Gate despite the indignant protests of the horns under the great bridge,” he added.

Far below the bridge’s deck, on a perch accessible by a tiny elevator, operating engineer Randy Rosenkild examined one of the five foghorns. They are shaped like trumpets. Two larger ones, 48 inches long, face in opposite directions near the base of the south tower. Three slightly smaller ones are tucked below the roadway near midspan.

The larger ones have a specific signal: two seconds on, 18 seconds off, over and over. The others burp something quite different: one second on, two seconds off, one second on, 36 seconds off, repeat.

The idea is that mariners blinded by fog can guide themselves between the distinct sounds.

Their bellows can be heard more than two miles out to sea, and sometimes in nearby places like Sausalito and Berkeley. There is a chorus of other warnings, too, in the soup of fog. Large ships squawk foghorns of their own. Buoys in the water bleat.

Residents sometimes phone the bridge and ask why the foghorns are on at all. It’s not foggy at their house, they say.

Mr. Rosenkild’s job is to make sure the huge horns do their important work. Fall — reliably sunny, rarely foggy — is his favorite time to clamber to their hidden homes under the bridge. Summer is his least.

“If you’ve never walked into thick fog, it will literally take your breath away with the chill factor,” Mr. Rosenkild said. “It’s so cold.”

There is little precision or pageantry to the use of the bridge’s foghorns. When the electrician on duty notes that it is too foggy to see across the mile-wide channel of the Golden Gate, the foghorns are turned on with a click of a computer mouse.

Inside a room on the south end of the bridge, Del West, an electrical superintendent, decided it was time. He warned workers all over the bridge by walkie-talkie, warned them again, then once more.

The bellow of a foghorn can be deafening, or even more dangerous, to people nearby. “It can interrupt your heartbeat,” Mr. Rosenkild said. It didn’t sound like a joke.

Mr. West clicked an icon on the computer screen that read “fog horns.” A moment later, bass tones bellowed from the belly of the bridge.

He logged the time into a book with a green canvas cover that read, “Records.” Electricians leave notes from their shifts there, including the use of fog horns.

Fog researchers might want to see those records, but Mr. West said that most of the log books have been lost to time. No one thought of the importance of tracking fog through history.

Out on the bridge, about 40 full-time painters continued their endless work. Under the thump-thump-thump of passing cars overhead, Mr. Dzierman and his crew sandblasted, primed and painted. Last year they spread 2,590 gallons of International Orange paint around the bridge.

They do their work where the structure needs it most. And more often than not, it is the west side, because of the effect of the fog from the ocean — the wind, the salt, the moisture.

“The west side gets pounded,” Mr. Dzierman said. “Pounded.”

But he’d rather have fog than no fog.

“I’d miss it,” Mr. Dzierman said. “It’s part of the bridge, right? It’s part of San Francisco. It’s part of the Golden Gate.”

The fog bank lurked on the horizon, a ghostly tease.