For postcards and poetry, there’s nothing like the way the fog rolls in and out over the coast of Northern California. There’s the dense white mass stretching thousands of miles out to sea, implacably calm, soundless, colorless, almost featureless. There are the advance wisps that jet through the mountain gaps and curl into Livermore and Santa Rosa. There’s the cooling wall that marches slowly in to plug up the Golden Gate, whistling past tourists, sucking the color from the bridge, leaving just two blinking red lights at the top of the towers. There’s the single narrow column that seems to know the exact route down San Francisco’s Geary Street, plunging the Financial District and the Alcatraz lighthouse into misty darkness while the sun still shines on Potrero Hill.
There is romance and mystery in fog, flowing down the shrouded city streets, shadowing Dashiell Hammett’s quick-talking detectives, churning into the “gray primordial vastness” that bedeviled the sailing ships of Jack London. To borrow a line from noir, it would be a shame if anything happened to the fog.
That’s because fog is not just a moody background; it’s a vital control on Northern California weather. Without it we’d have no redwood trees and no salmon, and we’d have uncomfortable people living in what we could call Nevada-by-the-sea. It would be hot and, with climate change, getting hotter. If you’ve been to Las Vegas in July, you know what that would do to your enjoyment of life, not
to mention your air conditioning bill.
But it’s only in the last few years that scientists have started to get a handle on the conditions that make fog happen here, and how fog might have changed—and be changing—as the climate does. Fog, even as a scientific entity, is elusive and wily and ethereal, defying our attempts at easy prediction.
Fog is a low cloud. Northern California summer fog—also known as a marine layer—is a type known as “marine advection fog,” different from the nine or ten other types that form in different ways and feature different types of clouds. Marine fog forms when wind pushes a warm, moist air mass over a cold surface, in this case, the ocean. As the air cools, it loses its ability to hold as much water, so
droplets condense, form a cloud and hang around in the sky. That’s a relatively simple equation, and works anywhere in the world, including your bathroom when you take a hot shower and pass warm moist air over a cold mirror.
Summertime fog is probably the most vital regulator of the climate in Northern California, and changes in the fog could spell big changes for the species—humans included—that depend on it. But before anyone can offer any kind of confident prediction about the environmental future of Northern California, they need to understand fog, which means understanding the factors that create that warm air and that cold surface, and move all the parts around in the right way.
And there are a lot of moving parts. To name just a few, the formation of summer fog is influenced by the difference in temperatures between the Central Valley and the coast, by large-scale pressure patterns over the entire Pacific Ocean and western United States, by the difference in pressure between, say, Nevada and Seattle, by sea surface temperatures, and by the difference in sea surface temperatures between different parts of the Pacific. Like a Facebook romance, it’s complicated.
Just understanding all that stuff in order to come up with a good description of how and why marine fog forms in Northern California is difficult enough. But now all the factors that conspire to create the fog are changing with global warming. Sea surface temperatures, for example, are heating up, although they also change every year on their own, thanks to regular events like this year’s El Niño or longer-term gyroscopic swings like the Pacific Decadal Oscillation.
Air temperatures are heating up, too, but more so in some places than in others, and it’s not a steady uphill climb everywhere. Coastal maximum summer temperatures, for example, have increased on average over the last hundred years, but decreased during the last thirty, while the inland daily temperature average increased only slightly—meaning that in some places the average temperature rose slightly, while in others it decreased. And since fog is affected by the difference in temperatures between places, but also affects the temperature… yikes.
With so many factors in play, it’s hard to know if the state is in for more fog or for less. One recently published scientific paper argues that we used to have fog more often: Fog was 33 percent more frequent in the first quarter of the 20th century than during the last half, for reasons that appear to
be unrelated to global warming. Another study suggests that warming may actually result in more fog for the coast.
So what will happen to fog? Now that is a good mystery.
Dave Reynolds, the climatologist in charge at the National Weather Service’s Northern California
forecasting office, lives on the boundary of the Mark Twain witticism “Climate is what you expect, weather is what you get.” Reynolds models and monitors the big-picture weather and climate patterns of the entire globe, then tries, along with his colleagues, to predict the Bay Area’s weather.
Our weather, fog included, is determined by the globalclimate—stuff like whether a jet of air blowing off the Himalayas (yes, those Himalayas) makes it all the way to California or gets stopped just short by a high-pressure ridge over the coast. In the summer, the forecasters are looking at worldwide phenomena in the service of one big task: “Forecasting the depth of the marine layer”—the fog—“is the number-one challenge from May 15 to October 15,” Reynolds says. “That drives how warm it’s going to be inland, how cool it’s going to be on the coast, how far the clouds will drive in.”
Reynolds needs to be accurate about the depth of the fog, because the rest of his forecast depends on that, and because a lot of people—not just San Franciscans complaining about a 75-degree heat wave—depend on his forecast. Firefighters need to know whether they can do controlled burns in the mountains. Boaters need to know the marine navigation conditions. Air traffic controllers need to know whether they can open all the runways at SFO. Water resource managers need to know what demand will be like and how hot it will be. Energy grid monitors need to know how many air conditioners will be running. Hospitals need to know if the heat or air quality will be a health issue.
Now, for the forecaster’s challenge: In Northern California in the summer, warm, moist air masses are usually blowing across the Pacific from the tropics, borne of, and propelled by, sea surface temperature variations in the western equatorial Pacific. Meanwhile, a general high-pressure area
over the Northern Pacific and a low-pressure area caused by high inland air temperatures causes wind to blow roughly south along the coast of the western United States, churning up the ocean and creating an upwelling of cold, nutrient-rich water called the California Current. On a typical summer
day, when the warm air meets the cold water—poof!—marine advection fog appears over the coast. When there’s such a temperature difference between inland and coast, the ocean cooled air will try to expand into areas where it’s warmer. In the summer, the inland areas get hot, and so the cold, foggy
air marches inland, conquering local heat areas as it goes.
It doesn’t take much of a change in the big picture to affect the fog. Say the high pressure area moves a bit, or the sea surface temperatures change—and remember, these are sea surface temperatures out near, like, Tahiti, and pressure areas covering thousands of square miles off the coast of Seattle—and the fog could go away for a week. You can, perhaps, understand why Reynolds has a tough job.
Reynolds’ workstation sits in the middle of a NORADesque command center, with banks of monitors displaying maps of various resolutions, each grid covered in splotches of color to represent different aspects of the weather. Reynolds sits surrounded by displays of the factors that cause fog: sea surface temperatures, inland temperatures, pressures, pressure gradients. Computers show the current state of all these factors and then forecast how they’ll change on an hourly scale over the next day, week, or month.
While the models are generally pretty accurate at predicting the next 24 hours, both computer weather and climate prediction models are not good at forecasting clouds more than a day or two in advance. There is a reason for this, and it is that long-term clouds, from a mathematical and
computational perspective, suck. Try modeling the physics of a bunch of water droplets condensing in air around an almost infinitely variable local topography with its own microclimates and slight temperature gradations and pressure gradations and elevation changes, all of it depending on subtle
differences in far-flung pressure and sea surface temperatures and…blast, you’ve just burned down your computer.
Even worse, Northern California’s coastal climate is particularly tricky to predict. We live right on the border between different types of climate areas, the wetter northwest and the drier southwest. The farther you get from here, the more confident climate predictions become.
Since computers struggle to predict weather, say, a full month ahead, Reynolds is left trying to predict a warm or cool summer based on a reading of all the other variables. He prefers looking at sea surface temperatures. “The colder the waters are over the ocean, the higher the probability
you’re going to get marine stratus,” Reynolds says. “It’s our view [that] the colder the ocean temperatures, the higher the probability of a cold summer.”
For climate change researchers, who are looking at much longer trends, there’s little hope of deterministically modeling individual cloud behavior. Generally, for a climate scientist to describe something as an actual change, rather than just a decadal shift, a thirty-year observation period
is required. But running a thirty-year simulation of how climate change may affect fog is just too uncertain, time-consuming, and hard on currently available computers.
(Undaunted, scientists at the Lawrence Berkeley Laboratory are working on the requirements for a one-of-a-kind computer that will try to model long-term global cloud behavior. It will be called the “Green Flash” and use special software being built at an atmospheric science research lab in Colorado, with contributions from Lawrence Berkeley engineers. They hope to have it up and running in a few more years.)
In the meantime, land managers often need scientists to take a stab at predicting the future of fog, anyway. For example, the Farallon Islands National Marine Sanctuary has been working on a climate change scenario for the islands, and last year the organization asked Reynolds to provide a
spring-and-summer cloud forecast. He wasn’t comfortable doing it but, when asked to say something, went with the occasionally advanced theory that, if inland areas heat up faster than the coastal areas over the next few decades—and recently, they have—fog should increase. (The pressure gradient caused by higher inland temperatures would increase both the winds that stir up the California Current to create fog, and the breeze that then blows fog in from the ocean.)
“The physics of it says that stratus should increase,” Reynolds says. “All things staying the same.” And then, at the idea of all things staying the same, he pauses for a minute to laugh.
Figuring out what will happen to the fog in the next century is not merely speculative research for the benefit of postcard photographers. Fog—both the clouds and their cooling effect—enables life of all kinds in California. It makes it a pleasant place to live, serving as natural air conditioning, which is important in an age of energy dependence and global warming. Fog sustains redwood trees, and everything that lives in them, because redwoods are somewhat inefficient plants that lose more water than other plants on sunny days; redwoods need cloud cover so as not to become desiccated.
While redwoods exemplify fog dependence—coast redwoods live only in the narrow coastal fog belt of Western North America—a great many other species need the fog. Salamanders, frogs and other amphibians, for example, are particularly sensitive to both temperature and humidity. Some salamanders suffer heat stress and die at temperatures in the mid-70s. The fog provides vital temperature control, allowing coastal amphibian species to survive (although some species hack it in the hotter Central Valley by hiding out during the day). Other plant species, like huckleberries, prefer shade and cool temperatures to avoid losing too much water.
The Central Coast’s endangered Coho salmon also desperately need the redwoods and the fog. The fish can’t reproduce in streams warmer than about 60 degrees Fahrenheit, a boundary to which many streams come perilously close in the warm summer spawning months. Without the cooling effects of fog and redwood shade, the salmon would face one more problem on a list that’s already near unmanageable: reduced spawning habitat, lowered summertime water flow, reduced food availability, and decreased genetic variability. A recent estimate put the total number of Central Coast Coho at 500 fish, and scientists warn that the subspecies is near extinction. “Redwoods and Coho are inextricable with each other,” says Charlotte Ambrose, a salmon recovery coordinator with the National Marine Fisheries Service. “If we can manage our redwood region for a diversity of ages in the redwood stand, we are likely to be managing for Coho salmon.”
As one way to help, Norman Miller, a climate scientist at Lawrence Berkeley National Lab, has proposed an investigation with the fisheries service to use a climate model to simulate where future stream temperatures might stay low enough to harbor spawning salmon. The project would look at fine-scale water temperature as a function of the presence of fog, while also taking into account tree shading and dew. “The question becomes, how do we quantify regions that don’t get above a temperature like that?” Miller says. “Because stream temperature is a key to salmon recovery.”
The end goal would be to identify good spawning streams in advance, and preemptively work to protect them. “We really don’t have good models in place right now to help us identify where those areas are where we can protect them now, or work with whoever the landowner is now to preserve those areas,” Ambrose says.
There’s also a human factor to fog. We rely on its cooling effects for agriculture—the wine grape-growing conditions of Napa, Sonoma, and Mendocino counties, for example, are shaped by the fog to such an extent that even minor changes could be career-ending for some vintners. Summertime
clouds are also a major transportation issue for boats and airplanes, and have been throughout history. In the era before the Bay Bridge, when you still had to go from Oakland to San Francisco by boat, fog was a menace to navigation, a dark, ominous, terrifying shipwreck-waiting-to-happen. The
San Francisco Bay was once the most shipwreck-prone place on Earth. Fog, in other words, is a weather condition that matters. Just ask the pilot of the Cosco Busan.
Several recent research papers take some steps toward a better understanding of Northern California fog. One is the dissertation of UC Berkeley geography graduate student James Johnstone, now a postdoc at the University of Washington. In a 250-page paper that is considerably more thrilling
than its title, “Climate Variability in Northern California and Its Global Connections,” as well as in a paper published this February in the Proceedings of the National Academy of Sciences, Johnstone set out to define the global patterns that control fog in the summer and rain in the winter in an attempt to see what might happen to redwood trees.
One of the curious things about Northern California fog is that the observational record for it is extremely poor. The National Weather Service started tracking fog measurements at the San Francisco airport in the mid-1990s, but that’s not yet the kind of time scale a climate researcher needs. Miller and others use summertime relative humidity data measured by several offshore buoys along the coast—if the humidity is 100 percent, they assume there’s fog.
Johnstone used regional airport data. Airports take hourly measurements of the height of the cloud ceiling, which is a nice way of identifying foggy days. Johnstone used observations from the Arcata and Monterey airports, which had records going back to 1951. The observational record shows a weak, insignificant decrease in fog between 1951 and 2008, with considerable year-to-year shifts. (In the peak year, 1951, there was more than twice as much fog as in 1997, the minimum year for fog.) But Johnstone wanted to extend his records to cover the entire century, and so he went looking for a way to identify fog by proxy.
He matched up the observed fog records with the big-picture stuff over the same time period, to see how well temperature, pressure, and sea surface temperature worked as predictors of fog. They all did, to some degree. Movement in the North Pacific Pressure High and changes in global sea surface temperatures both appeared related to the presence of fog in Northern California. But temperature variability—the difference between inland and coastal daily maximum temperatures across the western United States—stood out. “It correlates amazingly well with fog,” Johnstone says. “This just blew my mind that they correlate so strongly.”
Temperature records go back much further than the cloud ceiling observations, to the beginning of the 20th century. If the inland-coast temperature differential is reliable enough an indicator of fog, which Johnstone thinks it is, it allows an estimation of what fog has done over the entire century.
This time, there was a stronger trend: Johnstone’s analysis showed that fog in the early 1900s was about 33 percent more frequent than in recent decades since 1951. While 1951 was extraordinarily foggy, by extending the study backward, Johnstone inferred that years like that were much more typical between 1901 and 1925. He credits the long-term reduction in fog to coastal circulation and temperature changes, including the roughly thirty-year cycling of the Pacific Decadal Oscillation.
Johnstone cautions that making any predictions now would be premature. His study shows such considerable year-to-year variability in fog—even decade-to-decade variability—that he concludes crediting anything to global warming is probably still unsupportable. “The future is anybody’s guess,” he says.
One of the few well-known predictions about fog comes from Bereket Lebassi, a graduate student at Santa Clara University, along with a number of local researchers including Norman Miller and San Jose State professor Robert Bornstein, in a paper published last year in the Journal of Climate.
Although Johnstone’s work shows an overall decrease in the difference between inland and coastal temperatures, due in part to uncertain trends in the inland summer temperature maximums since 1901, those inland temperatures have rebounded since about 1970, warming relatively faster than the coast, which has actually seen its daily temperature maximums decrease, according to the Lebassi paper. The authors argue that this is fog cause-and-effect: The increased temperature differential causes more fog, which in turn further cools the coast, which further increases the temperature differential.
“As global warming is warming the interior, that would mean—in theory, and our observations back this up—there would be increased onshore sea breeze activity, bringing in cooling air, and bringing in fog, which would block the sunshine, and therefore the temperatures along the coast would be cooling,” Bornstein says. “And that’s what we found.”
This is a fairly remarkable-sounding idea: The effect of global warming could be to cool coastal California. Bornstein, like Reynolds, says that in theory, in the future the cooling trend could continue, since current global warming predictions call for temperatures to increase more quickly inland than on the coast. If coastal maximum temperatures continue to cool, as they did during the study period, that would be good news for hospitals worried about human heat stress and vintners worried about the extinction of wine grapes in the Napa Valley. Bornstein has started to look at energy consumption, and how more fog in the future might help coastal California keep its energy use down.
Still, the data wasn’t complete enough to nail down any conclusions. (And, Johnstone points out, the increase in fog over the last thirty years amounts to a small uptick in a century-long decline.) “I’m not willing to say anything,” says Berkeley Lab’s Miller. “The real questions behind that are, what changes will we see in the mechanisms that are conducive to fog formation, the controls? You can ask the same question about hurricanes—what are the big controls?”
These controls are moving targets—sea surface temperature gradients, inland temperature gradients, pressure gradients, changes in the upwelling. Sea surface temperatures, for example, are warming everywhere, but asymmetrically. Even the tropical jet stream appears to be changing, shifting slightly to the north, which could alter weather over the entire mid-latitudes. None of it is changing uniformly with respect to fog, meaning that in some sense, researchers have to pick their poison. Will stronger winds lead to a stronger upwelling and therefore colder water—and so more fog? Or will globally warming ocean temperatures cancel that effect out and weaken the upwelling, leading to less fog?
Park Williams, a postdoc in UC Santa Barbara’s geography department who worked on his own fog study for his dissertation, is the first to admit that, after years of studying fog data, the future is still unclear. It’s more important than ever, though, he says, particularly when you look at predictions for temperature changes in California and realize that those are completely dependent on fog. (In the Intergovernmental Panel on Climate Change report, for example, coastal California is a tiny, cooling dot in a sea of red warming areas.)
“Cloud cover along the coast reasonably dictates summer temperatures,” Williams says. “Until we have a more accurate picture of how summer cloud cover may respond, we have no idea what our temperature outlook is for a global warming picture.”
But for now, the fate of fog defies easy—or complete —explanation. It’s the kind of thing that keeps scientists going: a real, complex puzzle that could take years or decades to fully untangle, with tremendous implications for human health, navigation, agriculture, native species, and energy use.
In The Sea Wolf, Jack London compared fog to “the gray shadow of infinite mystery, brooding over the whirling speck of earth.” London wrote that line in 1904, a few decades before the first researcher proposed—and was ridiculed for suggesting—that greenhouse gases might change the climate. We know more now about almost everything, including about fog and what causes it. But we do not understand everything, and so fog remains a gray shadow of infinite mystery.