Fog is a quintessential part of the Bay Area, a seemingly simple suspension of water in air that has shaped our culture and ecology since time immemorial. For us, it is the ghostly embodiment of cold weather — and maybe our only chance at a white Christmas. At its core, it only needs two ingredients – cool temperatures and humid air — and it forms when moisture condenses to form a cloud close to the surface of the Earth.
But beyond that, its formation is still not fully understood, shaped by a variety of forces around the globe. Which might be why it is not only the stuff of song and poetry, but also of serious scientific research.
“I think one of the reasons why fog is so cool to look at and study – beyond its absolute stunning beauty – is that it’s just so multi-dimensional,” says Alicia Torregrosa, a physical scientist who led years of research on how fog develops in the Bay Area. “Our coastline is perfectly situated to be a barrier or a gate to this floating water.”
As winds blow in over the Pacific, they churn up the coastal ocean, bringing up frigid waters from the depths towards the surface. Paired with currents coming down from Alaska, this keeps the Bay Area waters in the crisp 50s throughout the year.
Those same winds that cause the icy upwelling also bring in moist air, and as they hit the chilly ocean, the water in the air cools and condenses into fog.
While this misty mix mostly stays close to shore in the winter, the summer weather helps draw it deeper into the Peninsula and sometimes even across the water to the East Bay. Paired with seasonal sea breeze, this creates the windy, foggy summers that are a hallmark of Peninsula weather.
Beyond its fascinating science, the fog has entwined itself with human life in the Bay Area for centuries.
While “there was undoubtedly a connection with fog and all the other elements of our environment,” no cultural expression related to fog survives for the first peoples of the Bay Area, says Gregg Castro, culture director for the Association of Ramaytush Ohlone – a result of Spanish and American cultural suppression, he says. Still, some practices from other tribes survived, like the fog song of the Rumsen Ohlone people of Monterey Bay, which Castro sings not only in the presence of literal fog but “to clear the fog in our hearts and our minds.”
Over time, historical records begin to emerge — and so does evidence that fog has always been divisive. One 1866 writer, in an unsigned report held by the National Parks Service, penned that the Peninsula “can scarcely be called inviting. Her continual rains in winter, and cold winds and fogs in summer, must be very trying to average nerves and lungs.”
Others had a more optimistic outlook on the fog, arguing that the climate made those who could withstand it stronger. One 1898 edition of the now-defunct Bay Area newspaper “The Argonaut” reported on a group of military volunteers who gathered from around the West Coast prior to the Spanish-American War, venturing that the California lands to the south did not “produce as hardy a lot of men as are those who breathe in the winds and fogs of this neighborhood.”
A few short years later, however, fog was a key factor in one of the worst disasters in Bay Area history: the sinking of the steam ship City of Rio De Janeiro. On the early morning of February 22, 1901, a thick fog covered the waters near the mouth of the Bay, but Captain William Ward, otherwise noted to be a careful officer who didn’t take chances with the weather, opted to try and bring his ship in. (According to contemporary news reports, he and his pilot were urged on by an impatient dignitary wishing to catch a train.)
He struck jagged rocks off Fort Point, and as the tide began to ebb, it pulled out the ship, and water rushed into the hull. In the ensuing chaos, the majority of the passengers and crew – many of them Chinese and Japanese immigrants – perished in the frigid water. That day, 128 of the 210 aboard the ship died in what still is the deadliest maritime disaster in the history of the Bay Area. For years, bodies washed up near Fort Point, including that of Captain Ward, identified by a watch chain around his rib cage; others were trapped in the wreck for more than a century.
The grisly tragedy led to the building of Mile Rocks lighthouse in 1906, equipped with what was then known as an “air trumpet” meant to signal the dangerous rocks to coming ships. The era of the foghorn had come to the Bay .
Even with modern safety tech, the fog has brought its fair share of distress. On November 7, 2007, morning fog blanketed the Bay when the Cosco Busan left the Port of Oakland and the pilot – impaired by prescription drugs – had a communication breakdown with the captain of the ship, running into the Bay Bridge. The bridge left an eight-foot-deep gash in the hull, piercing the fuel tanks and spilling more than 50,000 gallons of fuel into the water. Though there were no human fatalities or injuries, the oil spread to 69 miles of shore, closed fisheries and killed more than 6800 birds.
But while fog has been tied to tragedy, it has also built entire ecosystems, and fostered one of the most massive creatures on the planet. Like most plants, the redwood takes in water from its roots, but millions of years of evolution have taught it to drink deep out of thin air. Using specialized leaves, the redwood pulls moisture from the air and collects it into its system, and may even pull in water through its bark. Meanwhile, the needles gather fog into dew, which drips onto the roots. Between these adaptations, the redwood gets up to 40% of the water it needs just from fog. While they can grow without fog, trees in foggy climes tend to fare better, growing into behemoths that live for centuries and grow hundreds of feet tall.
The fog doesn’t just support the tree but the scores of species that teem in and around it. On just the redwoods, amphibians, birds, insects and even other trees, can roost — with some creatures living their whole lives in the trees. The fog – and the shade of thriving trees – helps keep creek waters cool, allowing Coho salmon to swim upstream and spawn. The moisture also helps lower the fire risk of the coastal forests, and for humans and other animals alike, acts as a natural air conditioner.
“That has huge impacts on what can grow in the fog belt – things that can’t grow in Martinez or Sonoma because they can’t deal with the heat,” says Lew Stringer, associate director of natural resources at the Presidio Trust. “Plants and animals have evolved to be in that little niche that’s a ribbon that runs along the coast.”
But the fog has another – perhaps more famous – plant that it helps to cultivate: wine grapes. In the North Bay, another break in the mountains invites fog and winds into the Petaluma Gap, where settlers have grown wine since the 1800s. Together, the fog and wind keep the temperature cool and create grapes that ripen later and grow thicker skins, producing distinctive pinot noirs and chardonnays.
“The gaps that we’ve got in our coastline are portals, are gates where the fog can come in,” says Torregrosa. “It’s this amazing temperature engine, and it can make for some really amazing Pinot Noir Vineyards.”
The cooling, gloomy weather and its benefits have inspired a new generation of fog fans, chief among them the fog itself. Conceived by an anonymous user on Twitter (now X) in 2010 and Instagram a year later, Karl the Fog takes on the personality of the Bay’s iconic weather pattern. Over hundreds of posts, the account takes a comedic, long-form response to the centuries-old history of fog-hate that has garnered an avid fan base of more than 300,000 followers each on Instagram and X. The posts lampoon the love of sunny weather and praise the waves of white that wash over the Bay Area, with one 2017 Instagram post — a picture of cloudless skies over Dolores Park — paired with a call for help: “We’re facing the worst weather possible. Keep us in your prayers.”
Despite the tongue-in-cheek delivery, Karl embodies the way the ethereal phenomenon takes life in our imaginations, a seemingly sentient recurring character in the Bay Area’s story. On occasion, his posts hint at the mystery of the fog and its enduring power to build ecosystems, take down ships and inspire us for centuries on end: “All that is sunny does not glitter, not all those in the fog are lost.”