From Bay Nature magazineJul-Sep 2013

Fog and redwoods: Demystifying the mist

by on June 30, 2013

Fog coming in, San Mateo County
Fog forms when a warm air mass comes in contact with the cold waters of the Pacific Ocean. Here fog pours in from the ocean on a summer evening to cover the San Mateo coast south of Half Moon Bay. Photo: (c)2001 William K. Matthias

Fog means survival for many Bay Area plants and animals. What will happen to this life-giving airborne moisture in an era of global warming? 

The Grove of Old Trees sits on a hill in Sonoma County in full view of the ocean. “This is where I come to chase the fog,” says Emily Burns. But on this day in March, her prey is elusive. Buttercups gleam in the sunshine beside a fortress of redwoods scraping an azure sky.

Burns has been studying this 28-acre remnant old-growth forest for almost a decade—especially the plants’ relationship with the area’s bounteous, bone-chilling fog. She points out a vigorous sword fern—the subject of her current research—and introduces me to some redwoods over 200 feet tall.

A young biologist, Burns is a pioneer in the field of fog. She has risen before dawn, climbed to the tops of these trees, and measured the effects of fog on their uppermost shoots. In 2009 she published a landmark paper describing how plants in the Grove of Old Trees use fog—part of a growing body of evidence demonstrating fog’s central role in sustaining California’s coastal ecosystems.

Emily Burns in a redwood forest

Emily Burns has been studying the relationship of fog and redwood forests for nearly ten years. Photo: Joan Hamilton

These days, Burns is on a broader quest. As head of conservation planning at the San Francisco–based nonprofit Save the Redwoods League, she’s trying to figure out what the climate-changed future may hold for redwood forests. And fog is a major wild card: Some theories suggest more fog is likely, others less. So Burns and a small group of other biologists are crossing the traditional borders of their field—working with physicists, hydrologists, and meteorologists—to demystify the mists. The job involves everything from measuring fern fronds to teasing out the atomic composition of redwood leaves in a lab. Some of the most important work is centered right here, among the ancient redwoods of the California coast.

Researcher high in a redwood tree

Emily Burns in the canopy of redwood trees in Humboldt County. Photo: Jim Campbell-Spickler

The Bay Area‘s climate is like few others in the world, especially in summer: We can be shivering at 50 degrees along the coast and Bay shore, but then drive a few miles inland and be sweating at over 100. If we don’t like the cold and damp, we can go east—or up. Or stay where we are and wait until noon.

Fog provides us with natural air-conditioning, lower energy bills, and more diverse vegetation and wildlife than we should expect in a place that receives an average of only 20-some inches of rain a year. In summer, coast redwoods can get more than half of their moisture from fog. Scientists once assumed the trees had to soak up fog’s drippings from the soil. But then, in 1998, UC Berkeley biologist Todd Dawson showed that they could also absorb moisture through their leaves. Burns, a student of Dawson’s, built on that knowledge, showing that eight other plants in the forest, including sword ferns, huckleberries, and tanoaks, were capable of “foliar uptake” too.

Fog can be surprisingly wet. Three years ago, physicist Daniel Fernandez of California State University, Monterey Bay, began measuring its water content. His record haul from a one-square-meter fog collector: 39 liters, or nearly 10 gallons, in a single day.

Fog near Tilden Park, Berkeley

Fog pours in over the East Bay hills at Tilden Regional Park in Berkeley at dusk. Photo: Brad Perks

Coastal fog is formed by a complex interaction of land, ocean, and atmosphere. But the basics are as simple as a glass of iced tea. Just as moisture in the air condenses when it comes in contact with the cold glass, fog forms when a warm air mass comes in contact with cold ocean water. On a hot day in the Central Valley, the fog moves inland, pushed by the prevailing winds and pulled by the rising warm air in the valley.

It’s an elegantly timed hydration system. In a daily, or “diurnal,” cycle fog often provides moisture at night and burns off during the day, when plants need sun for photosynthesis. Over the course of a year, fog is (conveniently) most abundant during our nearly rainless summers.

How much fog we have in any given year is determined by factors still poorly understood. The difference between inland temperatures and temperatures along the coast plays a part. But there are other factors, too, including atmospheric shifts of the jet stream and air pressure out at sea, as well as an ocean cycle called the Pacific Decadal Oscillation, in which water temperatures in the eastern and western Pacific seesaw between colder and warmer every 20 or 30 years. Add human-caused climate change, and even bold scientists grow cautious. “I don’t predict the future, only the past,” jokes researcher Jim Johnstone of the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington. “That’s fairly safe.”

In 2010 Johnstone coauthored a paper on this topic published by the National Academy of Sciences. Basing his findings on historic temperature data and measurements of fog ceiling heights from airports along the California coast, he concluded that fog had decreased by 33 percent over the past 100 years. “Coast redwood may be increasingly drought-stressed under a summer climate of reduced fog frequency and greater evaporative demand,” the paper concluded.

log and ferns at Butano State Park

Sword ferns grow over the base of a downed redwood at Butano State Park. Frank S. Balthis

Redwoods in trouble? That got land managers and conservationists’ attention. But it wasn’t the whole story. The previous year, a team led by San Jose State meteorology professor Robert Bornstein published a paper in the Journal of Climate. Basing his findings on records of air temperatures in Los Angeles and San Francisco, Bornstein argued that “summer daytime temperatures on the coast have been cooling since 1970,” which suggests more fog rather than less. The likely reason? Global warming is adding energy to the diurnal cycle. Hotter inland temperatures are drawing in more fog to coastal areas.

Before his paper was published, Johnstone pored over Bornstein’s numbers and found that “the calculations were perfectly good.” Johnstone decided that their differing conclusions stemmed from sampling in different places and looking at different time periods. “It matters a lot when you start looking for a trend,” Johnstone says.

Recent weather data seems to bolster Bornstein’s case. Even Johnstone admits that “the coastal ocean has cooled and fog frequency has been considerably above normal, particularly in the past three summers.” But he points to evidence suggesting that those changes are more likely due to “natural variation,” age-old cycles in and over the Pacific, than to recent warming in inland areas. “Fog does not fit neatly into a greenhouse warming story,” he says.

Scientist at fog monitor

Alicia Torregrosa of USGS and the Pacific Coastal Fog Team collects data from the fog monitor set up at Pepperwood Preserve near Santa Rosa. Photo: David Andersen, Pepperwood Preserve

So how much have fog levels varied in the past? In 1998, UC Berkeley’s Dawson used analysis of isotopes—atoms of a particular element, such as carbon, with differing numbers of neutrons—to show that redwoods were absorbing fog directly through their leaves. His latest work uses isotope analysis to dig deeper, taking advantage of redwoods’ long lives. “Redwoods live 2,000 years or more,” says Dawson. “We can analyze their tree rings to go way, way back in time.”

This is not your father’s tree-ring analysis, however, which simply showed how much a tree had grown each year. “Locked into the tree’s cellulose molecules are carbon and oxygen atoms,” Dawson says. By examining the mix of carbon and oxygen isotopes in the cellulose of a given tree ring, Dawson can distinguish rain from fog and a hot year from a cool one.

“If rain falls, the oxygen isotopes that are incorporated into the tree rings are different from the oxygen isotopes from fog water,” Dawson says. “So I can tell you whether the tree built that ring using water dominated by summer fog or winter rainfall. Taking that information and combining it with the isotope information we get from carbon, I can tell you whether it was a warm year or a cool year—and how trees responded to that combination of water and temperature.”

“We’re trying to look back before the Industrial Revolution, when we started putting CO2 into our atmosphere and changing the temperature,” Dawson says. “Are there natural cycles between foggy and less foggy years? Or have some of those changes in fog been linked to recent, human-induced climate events? The tree rings will tell us.”

Fog and redwoods at sunset

View of redwoods in Sonoma County at the Grove of Old Trees. Photo: Emily Burns, Save the Redwoods

Another way to understand what’s happening to forests and fog is to start measuring here and now. Working with Dawson and other scientists at UC Berkeley and Humboldt State University, Save the Redwoods League began collecting comprehensive data on forest health in 2009. Through its Redwoods and Climate Change Initiative, the league has established study plots in 11 redwood forests along the coast and five giant sequoia forests in the Sierra Nevada. They’re mapping every plant in these one-hectare plots, and noting the size, age, and growth of the largest redwoods and sequoias. They’ll re-measure and re-map each year, for as long as they can keep the effort going.

So far, they’ve found no drought-induced mortality or declines in growth within the 450 mile-long native range of coast redwoods. The league has also confirmed these forests’ vital role in climate stabilization. With their expansive foliage and great girth, these old growth trees are actually growing faster—that is, adding more biomass—than younger ones, and hence storing more carbon. According to Burns, “These trees are pulling a lot of carbon out of the atmosphere. I think the data is going to show that North Coast redwood forests [which contain the largest trees] are holding more carbon than any other forests on the planet.”

The initiative will eventually help shape the league’s response to global warming. “These are the best old-growth forests we have,” Burns says. “We want to understand if there are climate change impacts. And if we see an area that’s a refuge from climate change, we are going to ramp up efforts to protect it.” The search for refugia does not mean turning away from efforts to preserve drier, more southerly forests, such as those along the Big Sur coast, however. “We don’t know the magnitude or even the direction of the climate changes we’re facing,” Burns says. “So we’re not planning on abandoning any forest land. That would be a big mistake.”

Sunlight through redwoods

Shafts of sunlight slice through the fog and limbs of the redwood forest at Purisima Creek Redwood Open Space Preserve on the Peninsula. Photo: Karl Gohl

Beyond the redwoods, fog is a powerful influence on other landscapes along the California coast. When U.S. Geological Survey landscape ecologist Alicia Torregrosa heard about a possible decline in fog, she first thought about salmon. Flows in the salmon creeks she was studying just north of San Francisco are sometimes very low in the summer. With less fog, those creeks might heat up more quickly, creating, she says, “a double whammy” for fish.

Colleagues suggested other concerns. What about maritime chaparral, with its diverse array of fog-dependent plants found nowhere else on the planet? What about fog-fed dune plants and the amphibians adapted to life along the coast? What about the wine industry? What about human health and energy consumption? There were countless reasons to try to understand what was happening to fog.

At first Torregrosa tried to figure it out on her own. “Using existing satellite data I thought I could get nice views over time,” she says. Soon, however, she was climbing “a mountain of atmospheric science research” designed to predict the weather and avoid plane crashes. “There was a lot of coarse data, but nothing biologically relevant,” she says.

So Torregrosa decided to build a fog brain trust. In April 2012, she hosted 36 atmospheric and natural scientists at the inaugural meeting of the Pacific Coastal Fog Team. The event gave scientists studying ocean currents and atmospheric pressure a chance to talk with scientists studying salamanders and manzanitas. There were revelations on both sides. “The wealth of information we found among atmospheric scientists was mind-boggling,” Torregrosa says. “And a lot of them didn’t realize there was such an ecological need for their information.”

Since that first meeting, the Fog Team has been knitting together observations from the middle of the Pacific Ocean to the tops of the tallest redwoods. Before the end of the year, they hope to produce data for resource managers facing decisions about what lands to buy, what lands to restore, and how to minimize damage in an era of climate change. “A better understanding of fog patterns will give them a better understanding of where fog-reliant plants can live,” she says.

Dawson of UC Berkeley is among the team’s biologists. “If we really want to see how the coastal climate is changing and how coastal fog is changing, we need to be making lots of measurements in lots of places,” he says. “No single person can do that. We need a consortium of people with like interests.”

Fortunately, the mighty redwoods don’t appear to be fazed by climate change yet. Looking up at a tree as tall as a 20-story building, Save the Redwoods League’s Burns points to its silvery tips. “Those are the healthy redwood shoots pushing upwards,” she says. “That’s only possible if there’s enough water.”

Such observations are reassuring. But we’re still at the beginning of the story about climate change and fog. We know fog so far has buffered the California coast from the big heat waves and droughts suffered by inland areas. But as to its future in a climate-changed world, “fog is so dynamic and ephemeral,” Burns says. “We don’t yet have the data to get a handle on the whole phenomenon.”

Joan Hamilton is a Berkeley-based freelance writer who focuses on nature and conservation issues. She also produces downloadable audio guides to the outdoors ( 

“Dispatches from the Home Front” is a series of articles highlighting groundbreaking work being done by Bay Area institutions, agencies, and nonprofit groups to comprehend, mitigate, and adapt to the impact of climate change on Bay Area ecosystems. The series is a partnership with the Bay Area Ecosystem Climate Change Consortium ( More at

Funding for “Dispatches From the Home Front” has been provided by the State Coastal Conservancy, The Nature Conservancy, and Pacific Gas & Electric Company.

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M. D. Vaden on July 3rd, 2013 at 8:14 am

One thought I’d offer … probably would help to study other inland Coast Redwoods too, for a broader understanding of how the trees function in relation to fog, or in relation to less fog. As well as other temperature extremes.

For example, there are appreciable established planted speciments growing in Medford, Oregon, and Kerby / Cave Junction, and Portland, where rainfall is as low as 19 inches per year and temps in summer upward of 90 to 100 degrees F.

It would be interesting to find out the variables regarding fog allowing the Sequoia species to get super tall, versus, say, the role of fog associated with seed production continuing to successful germination.

Without dispute, Coast Redwoods live and grow rather quickly inland. It’s just that I’ve never seen one germinate yet in a landscape or city park. (outside Nor Cal or SW OR)

Mary Ellen Hannibal on July 16th, 2013 at 9:03 am

This is a really well done piece on an important topic!

News Roundup – Burl Heists, The Future of Fog and More | Redwood Planet Media on March 8th, 2014 at 3:19 pm

[…] Coastal redwood forests rely heavily on fog for moisture, but conflicting predictive models paint an uncertain picture of how climate change will effect the presence of fog. Bay Nature covers the work of researchers trying to determine what the future holds for our coastal redwoods. […]

The Biggest Trees On Earth on April 26th, 2014 at 11:53 am

[…] into the Redwood forest decreases the water loss from transpiration, and keeps the soil moist. Researchers suggest the fog is more important than the areas annual […]

Catching Fog | champagnewhisky on May 2nd, 2014 at 4:35 am

[…] of moisture from fog, has been around for millions of years, at least when it comes to plants. Some ferns, redwoods and berry plants absorb up to half their water intake from the moisture in fog – and that’s not from moisture that accumulates on the ground. They absorb the moisture […]

Catching Fog | Gaia Gazette on May 2nd, 2014 at 11:51 am

[…] of moisture from fog, has been around for millions of years, at least when it comes to plants. Some ferns, redwoods and berry plants absorb up to half their water intake from the moisture in fog – and that’s not from moisture that accumulates on the ground. They absorb the moisture […]

Kaylin Northam on December 3rd, 2015 at 3:36 pm

I would be very interested in learning how fog harvesting can be implemented on a mass scale, mostly for use in agriculture during this record-breaking drought here in California. I’m sure fog harvesting can be used on a small scale, maybe for personal farms, but what about our commercial growers? Are there any ideas floating around that take Fog Nets and innovate them for farming? If anyone has any information, please feel free to email me! All help input is appreciated!

Ken Spacek on January 13th, 2016 at 11:15 am

The amount of biomass in the redwood forest regulates the amount of fog water that it receives. The redwood forest has less than 10 percent of it’s natural biomass this condition has resulted in a 90 percent loss storm and fog water retention and recharge of the aquifer. The amount of water stored in the aquifer has been decreasing since the loss of old growth during the 50’s and 60’s, this is a slow process and is not recognized by researchers as it is difficult to compare old growth to young growth. The old growth redwood forest made and maintained the climate they lived in. The biomass stayed constant, shade, fire and bug tolerant species created a extremely stable system. How the hydrological cycle of this forest has changed has not been studied in regards to it’s historic value and needs to be so.

Margaret on August 3rd, 2016 at 8:52 am

I know this article is old. I am interested in assisting your research needs if I can. I am in the redwoods often … Do you have volunteers collecting data?

Joan Hamilton on August 3rd, 2016 at 9:37 am

There may be an ongoing fog/fern study led by Emily Burns at Save the Redwoods League. You might check with her.

Gerald Landry on September 29th, 2016 at 12:13 am

I watched a video on YouTube recently where the host researching the loss of coastal kelp beds suspected that it caused a decrease in the formation of mists and fog. He talked about the sea urchin population going feral in the 60’s which was decimating the kelp. There is one project where volunteers are smashing urchins with hammers and have restored several acres of vibrant kelp forest.
The promotion of eating urchin roe or Uni as it is called in Sushi Restaurants has helped develop California’s largest seafood export. This in turn protects the kelp beds. People also have to be educated that a healthy Sea Otter population who love eating urchins is beneficial.

Gerald Landry on September 29th, 2016 at 12:24 am

Ref: Sea fog (also known as haar or fret) is heavily influenced by the presence of sea spray and microscopic airborne salt crystals. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves. Except in areas of storminess, the most common areas of breaking waves are located near coastlines, hence the greatest densities of airborne salt particles are there.
Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon. Another recently discovered source of condensation nuclei for coastal fog is kelp seaweed. Researchers have found that under stress (intense sunlight, strong evaporation, etc.), kelp releases particles of iodine which in turn become nuclei for condensation of water vapor, causing fog that diffuses direct sunlight.[27]

Fog – Wikipedia, the free encyclopedia

Fog is a visible mass consisting of cloud water droplets or ice crystals suspended in the air at or … Fog commonly produces precipitation in the form of drizzle or very light snow. … The advection of fog along the California coastline is propelled onto land by …. kelp releases particles of iodine which in turn become nuclei for …

Divers hammer thousands of urchins to save Palos Verdes Peninsula ……/divers-hammer-thousands-of-urchins-to-save-p...
Aug 13, 2014 – “We’re killing the sea urchins that have overrun this kelp habitat so much … Even though the urchin’s gonads are eaten as a popular food item … here” learning about the project in hopes they can do the same on their coast.

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