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Let’s talk about the one thing almost every first-time visitor asks about: Will the fog ruin my trip? Short answer: nope. Longer answer: San Francisco fog is part of the magic. It can make the Golden Gate Bridge look like it’s floating, turn the coastline into a moody movie scene, and create those “wow” moments when the sun suddenly breaks through like a spotlight.
Fog also explains why you can be in a t-shirt in one neighborhood and grabbing a hoodie two miles later. That’s not you being unprepared — that’s San Francisco being San Francisco.
Although widely attributed to Mark Twain, there is no concrete evidence that he actually said, “The coldest winter I ever spent was a summer in San Francisco.” This quote has taken on a life of its own, becoming a popular phrase associated with San Francisco’s cool and foggy summers. Despite the misattribution, the sentiment resonates with many who have experienced the city’s unique microclimate.
So, whether Twain said it or not, it remains a playful nod to the city’s renowned fog and its ability to create a chilly ambiance even during the warmer months — especially near the Golden Gate Bridge and the ocean-facing neighborhoods.
San Francisco experiences a unique type of fog called advection fog. As warm, moist air from the Pacific Ocean collides with the cooler California wind current near the coast, the fog rolls in. This low-rolling fog, known as Karl, forms when the warm air passes over colder surfaces like the San Francisco Bay or nearby land.
Unlike radiation fog, which often burns off in the morning, Karl can linger — especially near the coast and around the Golden Gate Bridge, where the marine layer loves to drift and swirl through the opening like it owns the place.
The iconic Golden Gate Bridge has two fog horns that blast for 2.5 hours each day, on average. These fog horns play a crucial role in guiding sailors safely through the wall of fog. Since the bridge’s opening in 1937, these horns have become synonymous with San Francisco’s identity.
Located at the middle of the bridge and the south tower on the San Francisco side, each horn emits a different tone at different times. During the summer (aka fog season), these horns can sound for over five hours a day — and if you’re near the waterfront, it’s one of those “only in San Francisco” soundtracks you’ll never forget.
Fog is essentially water droplets suspended in the air, mixed with dust and other microscopic particles. When humid air cools and reaches its dew point, it becomes saturated with water vapor. As the air continues to cool, it loses its ability to hold the vapor, causing it to condense around tiny particles and form droplets.
Those droplets accumulate to create fog — and when the marine layer gets pushed through the city’s hills and gaps, it can “spill” into neighborhoods in a way that feels almost alive.
According to the National Weather Service, fog is defined as visible moisture that forms below a height of 50 feet, while clouds are located above this height. Fog blankets the ground, giving it a distinct appearance, whereas clouds fill the sky.
So, next time you find yourself walking through the misty streets of San Francisco, you’ll know you’re basically inside a cloud — which sounds dramatic, because it is.
Coastal Redwood trees, the magnificent giants found along the California coast, have a unique relationship with fog. During summer, these trees can derive over half of their moisture from the fog. They absorb fog droplets through their leaves and even soak up the water that drips from the fog-saturated soil.
That misty moisture is one reason coastal forests stay so lush even when summer rain is basically nonexistent — and it’s also why day trips to places like Muir Woods feel so green and otherworldly compared to the city.
“Fog spilled from the heights of San Francisco like the liquid it almost was. On better days it spread across the bay and took over Oakland street by street, a thing you saw coming, a change you watched happening to you, a season on the move. Where it encountered redwoods, the most local of rains fell. Where it found open space, its weightless pale passage seemed both endless and like the end of all things. It was a temporary sadness, the more beautiful for being sad, the more precious for being temporary. It was the slow song in minor that the rock-and-roll sun then chased away.”
Jonathan Franzen, Purity
Many people have a misconception that fog contains a significant amount of water. However, fog is actually made up of droplets with plenty of space between them. You can put your hand through it, and although it may drizzle on your arm, it won’t feel like swimming in water.
Fog appears denser than it actually is — which is part of why it photographs so well. It turns backgrounds into soft gradients, makes lights glow, and gives landmarks (especially bridges and towers) that mysterious, cinematic look.
Fog can be a “views killer” in one spot and a total non-issue five minutes later. That’s why exploring with a flexible route is such a win — you can chase the clear angles, swap viewpoints on the fly, and still get those big San Francisco moments (even when Karl is feeling dramatic).
San Francisco’s fog has gained an online personality with the name “Karl.” @KarltheFog, a social media account, has amassed a massive following, with over 356,000 followers on Twitter and more than 250,000 on Instagram.
Locals affectionately refer to the fog as “Karl,” and the online persona has become a Bay Area celebrity. The name Karl, inspired by Tim Burton’s film “Big Fish,” adds a touch of whimsy to the fog’s enigmatic presence — and honestly, it’s hard not to love a weather system with a fan club.
While fog can appear in any month, the fog season in San Francisco typically extends from April to October. August, aptly known as “Fogust,” is the peak of foggy weather. During summer, the fog creates microclimates, with stark temperature variations between coastal and inland areas.
Half Moon Bay may be a chilly 50 degrees, while just 30 miles east, people are wearing shorts. So, if you’re planning a visit, be prepared for the mystique of June Gloom, No Sky July, and Fogust — and dress like you’re packing for multiple cities, not one.
San Francisco’s summer fog is a result of the city’s climate anomaly. While much of the Northern Hemisphere experiences scorching summers, San Francisco’s average July high temperature is a cool 67 degrees Fahrenheit, with evening temperatures dipping into the 50s.
The cold water off the coast (caused by upwelling) combines with warmer ocean breezes, leading to the formation of fog. As warm air rises inland, the fog gets drawn through the Golden Gate and envelops coastal cities, including San Francisco and Point Reyes.
The foggiest time in San Francisco is during the warmer summer months, particularly June through August. Locals even coined the term “June Gloom” to describe the damp, cool, fog-laden days that prevail in early summer. June holds the title for the foggiest overall month in San Francisco.
If your dream is crystal-clear bridge photos every single day, winter often has fewer foggy moments — but you’ll miss out on the full “Karl experience,” which is honestly part of the city’s personality.
“Fog is like your ex. It’s always lurking around when you least expect it.” – Unknown
“Fog: Nature’s own ethereal blanket, keeping San Francisco cozy and mysterious.” – Anonymous
“Fog is like a magician’s trick—it appears, disappears, and leaves you in awe.” – Unknown
San Francisco may be famous for its fog, but it’s not alone. Here are five other foggy spots in North America:
Fog consists of tiny water droplets suspended in the air. On average, fog contains up to 0.5 milliliters of water per cubic meter. To put it into perspective, if you were to condense the fog in an Olympic-sized swimming pool, you would be left with around 1.25 liters of water.
So yes, fog feels like it could fill a bathtub — but scientifically, it’s more “floating mist” than “giant airborne puddle.”
San Francisco’s fog and unique geography played a role in baffling early European explorers. The fog often obscured the opening to San Francisco Bay, causing navigators like Juan Rodríguez Cabrillo and Francis Drake to bypass it. It wasn’t until Juan de Ayala’s expedition in 1775 that Europeans successfully passed through the fog and anchored near Angel Island.
The misty veil of fog may have kept San Francisco Bay hidden, adding an air of mystery to the region’s exploration — and setting the tone for a city that still loves a little drama from the natural world.
As you traverse the streets of San Francisco, let the fog envelop you in its mystique and appreciate its role in shaping the city’s unique character. Embrace the allure of Karl and his whimsical dance, knowing that you’re experiencing a meteorological wonder unlike anywhere else.
Want the full fog experience (or you’re trying to find a spot where it’s clearing)? These locations are famous for fog drama, shifting visibility, and those classic San Francisco moods.
San Francisco Jeep Tours is a top tour activity in San Francisco onboard private tours. The customized city tour routes and entertaining tour guides cover lots of information on the “not to miss” attractions — and the guides can adapt the route based on real-time visibility when Karl is doing his thing.
Book a San Francisco Jeep City Tour, Sunset and City Lights San Francisco Jeep Tour, or the Full Day Combo City and Muir Woods tour to travel to many of the bucket-list and off-the-beaten-track locations in and around San Francisco.
Check out the San Francisco Jeep Tours website for more information on exploring the City by the Bay.
Fog is one of the most talked-about (and misunderstood) parts of San Francisco weather. Here’s how it works, what to expect on tours, and how to plan around it.
It’s a perfect storm of ocean, hills, and heat.
Cold Pacific Ocean currents, coastal geography, and hotter inland temperatures combine to create a strong marine layer. That cool, moist air is funneled through the Golden Gate and spreads across the city, especially in summer, making places near the Golden Gate Bridge and the coast foggier than nearby inland neighborhoods.
It behaves differently depending on where you are.
Fog doesn’t blanket the city evenly. Neighborhoods near the ocean and the Golden Gate often see fog while areas like Mission District or downtown can be sunny. Locals even track seasonal patterns and joke about fog as if it has a personality — especially during summer “Fogust.”
Not necessarily.
Fog can obscure some classic postcard views, but it also creates dramatic, moody photos and a uniquely San Francisco atmosphere. On Private San Francisco City Sightseeing Tours, your guide actively chooses viewpoints based on real-time visibility and may adjust the route to find clearer angles or alternate overlooks.
Timing matters.
Morning departures often have clearer skies, making a 2.5- or 3-hour Private San Francisco City Sightseeing Tour a strong choice if views are your top priority. On clear evenings, the San Francisco Sunset or City Lights Jeep Tour delivers incredible bridge and skyline vistas.
Layers are non-negotiable.
Dress in layers with a warm base, a mid-layer, and a windproof outer shell. A hat or beanie helps too. Weather can change quickly as you move from downtown to the Golden Gate, Twin Peaks, or coastal viewpoints.
Local tip: If you want the simple “what to pack” breakdown, use this guide: Dressing for San Francisco’s Summer (Foggy and Chilly).
Bonus tip: Fog often shifts fast. One viewpoint may be socked in while another just minutes away is perfectly clear — one of the biggest advantages of a flexible, private Jeep tour.
