What Is Tule Fog? California's Densest Fog Explained
Tule fog is a thick radiation fog unique to California's Central Valley. Learn how it forms, why it spills into the Bay Area, and how it differs from coastal fog.
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Bay Area weather reporting shaped by fog, terrain, wind, and microclimates.
Explore explainers, local guides, and deeper weather stories in the same cleaner visual language as the homepage.
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Fog Science
How Bay Area fog works, where it forms, and why some neighborhoods get far more of it than others.
Karl the Fog: San Francisco's Fog Name and Why It's So Foggy
Karl the Fog is San Francisco's summer fog. Cold Pacific water makes it, and summer heat inland pulls it through the Golden Gate, so the city is foggiest in summer.
6 min read →
Marine Layer vs. Tule Fog: What's the Difference?
Marine layer and tule fog look similar but form differently. One is a summer coastal phenomenon. The other is a winter inland hazard.
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What Is the Marine Layer? The Bay Area's Coastal Fog Engine Explained
The marine layer is the most important weather feature in the Bay Area. This mass of cool, moist Pacific air brings fog, wind and 30-degree temperature differences.
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Why Bay Area Fog Is Most Intense in Summer (Not Winter)
Summer fog dominates the Bay Area because extreme inland heat creates a powerful pressure gradient that pulls cold ocean air. Winter brings clouds and rain but less intense fog.
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Why Is San Francisco Foggier Than Oakland?
San Francisco's unique geography funnels cold ocean air directly into the city, while Oakland, across the bay, gets marine air that has already thinned and warmed. See why two cities 10 miles apart have such different summers.
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Why Some Bay Area Neighborhoods Are Foggier Than Others
Elevation and ocean exposure determine neighborhood fog in SF. Twin Peaks sits above fog while the Sunset District is perpetually cold and gray.
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When San Francisco's Summer Fog Burns Off: The Thermal Low Effect
San Francisco's summer fog burns off by late morning in most neighborhoods, then returns by 4 p.m. as the Central Valley thermal low peaks. The pattern varies dramatically by neighborhood.
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How Fog Forms Along the Northern California Coast
The fog that blankets the Northern California coast every summer is driven by cold upwelling water, northwest winds, and inland heat. Here is how it all works.
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Why Is It So Foggy Today? How Fog Forms, and Why the Bay Area Gets So Much
Fog is a cloud on the ground, and most foggy days come down to a few setups. They are a clear, calm night, moist air over cold water, recent rain or an inversion. Here is how to tell which one you are in, and when it will lift.
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Why Is the Pacific So Cold in California? Currents, Upwelling and Fog
The cold California Current and summer upwelling keep the Pacific off San Francisco between about 52°F and 58°F all year. That cold water is the Bay Area's fog machine.
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Is Bay Area Fog Declining? Climate Change and Fog Trends
A widely cited study found coastal summer fog became about a third less common over the 20th century. The causes involve ocean temperatures, upwelling and inland heat, and the future trend is still uncertain.
8 min read →
Radiation Fog: How Ground-Level Fog Forms on Clear, Calm Nights
Radiation fog forms in place on clear, calm nights as the ground radiates heat into space and chills the air above it. It is the fog of Bay Area valleys, not the coast, and it burns off predictably by mid-morning.
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What Is Advection Fog? How Marine Air Creates Coastal Fog
Advection fog forms over the cold Pacific Ocean and is carried inland by wind. It is the reason San Francisco summers are gray and cool while inland cities bake. Understanding it explains where and when Bay Area fog is most likely.
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Coastal Fog vs. Inland Fog: Two Different Bay Area Phenomena
The Bay Area runs two fog systems simultaneously. Coastal fog rolls in from the Pacific on summer afternoons. Inland fog forms in valley floors on winter nights. They look similar but behave completely differently.
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What Is Freezing Fog? When Bay Area Fog Turns to Ice
Freezing fog coats surfaces in rime ice when fog and below-freezing temperatures combine. It is rare in the mild Bay Area, but it does occur at high elevations and in the coldest inland valleys, and it creates invisible road hazards.
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Haze vs. Fog: What's the Difference?
Fog is water droplets at ground level, completely harmless to breathe. Haze is smoke, dust, or pollution particles, which can be a serious health hazard. Both reduce visibility and make the sky look gray, but the right response is very different.
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Morning Fog in the Bay Area: Why It Forms and When It Clears
Morning fog in the Bay Area is either the tail of the summer marine layer or overnight radiation fog in the inland valleys. The type determines when it clears, and they behave very differently.
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Steam Fog: Why Fog Rises Off Bay Area Water on Cold Mornings
Steam fog rises off the Bay and inland reservoirs when cold winter air flows over warmer water. The "steaming" effect is caused by rapid evaporation and immediate condensation, the opposite mechanism from the marine layer.
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Upslope Fog: Why Bay Area Hills Are Foggier Than the Flats
The same marine air covers the Oakland hills and downtown Oakland, but the hills logged 76% more fog hours in a year. Upslope fog is why: wind pushes moist air uphill, and the climb cools it into fog.
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Precipitation Fog: Why Bay Area Rain Brings Mist, Not Fog
Rain can make fog by saturating the cool air near the ground, but at Bay Area airports it happens in under 1% of rainy hours. Rain brings mist, and the dense fog comes on the clear, calm night after the storm.
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What Is a Fogbow? The Bay Area's Rainbow in the Fog
A fogbow is a rainbow made in fog instead of rain. Fog droplets are too small to produce vivid colors, so the arc appears white or ghostly pale. The Bay Area's summer marine layer, sitting below sunny coastal hills, sets up good fogbow conditions.
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Fog vs. Mist: What's the Difference?
The only official difference between fog and mist is visibility. Below 1 kilometer is fog, and above is mist. The Bay Area's marine layer almost always qualifies as fog. Here's how all the related terms compare.
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Ocean Fog: How Sea Fog Forms and Moves Onshore
Bay Area fog begins over the cold Pacific Ocean, where warm moist air meets the California Current. It then travels onshore through the Golden Gate. This is ocean fog, and it follows different rules than any other kind.
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Central Valley Fog: Tule Fog and California's Most Dangerous Winter Drives
Tule fog is one of California's most dangerous weather hazards on the road. The Central Valley's flat bowl traps it for days, and it sometimes drifts west through the Delta into the Bay Area's inland valleys.
6 min read →Category
Bay Area Climate
The big-picture forces that shape Bay Area weather: microclimates, wind, storms, and ocean effects.
Why the Bay Area Has So Many Microclimates
A 10-mile drive in the Bay Area can mean a 20-degree temperature change. Here is why the region has so many distinct microclimates.
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Bay Area Coastal vs. Inland: 30-Degree Temperature Differences
The Bay Area coast is typically 20-30 degrees colder than inland valleys on the same summer day. Here is the science behind one of the world's most dramatic short-distance temperature gaps.
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Bay Area Microclimates and Rainfall Distribution
Mount Tamalpais receives 70 inches of rain per year. Livermore, 30 miles away, gets 14 inches. The difference comes down to orographic lift, mountain ranges, and rain shadows. Those same forces create Bay Area microclimates.
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Why the East Bay Gets More Rain Than the Peninsula
Berkeley gets about 26 inches of rain a year and Palo Alto about 15. The bay side of the Peninsula sits in the rain shadow of the Santa Cruz Mountains. The East Bay hills squeeze extra rain out of storms crossing the bay.
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Why Is San Francisco So Cold in Summer?
San Francisco summers average highs of about 66°F to 68°F while the rest of California bakes. The cold Pacific Ocean, persistent marine fog, and coastal pressure dynamics combine to keep the city cool when you least expect it.
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Why the Bay Area Has So Much Wind
The cold Pacific Ocean, the scorching Central Valley, and geographic corridors like the Golden Gate all conspire to make the Bay Area genuinely windier than most regions. Here is the physics behind it.
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North Pacific High and Bay Area Spring Winds
May is when the Bay Area wind machine turns on. The North Pacific High strengthens offshore, inland California heats up, and marine air accelerates through the Golden Gate and local wind corridors.
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The Diablo Winds: The Bay Area's Hot, Dry, and Dangerous Wind Events
Diablo Winds are hot, dry, downslope winds that descend from the Sierra Nevada and Central Valley toward the coast, heating to 90-110°F and dropping humidity below 10%. They are the engine behind the Bay Area's most catastrophic wildfires.
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What Are Atmospheric Rivers? How They Drench the Bay Area
An atmospheric river is a narrow band of concentrated water vapor, a river of moisture in the sky. An average one carries about as much water vapor as the Mississippi River carries at its mouth. Strong ones can drop 5 to 10 inches on the coastal mountains in a day or two.
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How Atmospheric Rivers Impact the Bay Area
Atmospheric rivers are the primary cause of major flooding in the Bay Area, dumping 5-15 inches in 1-3 days. Here is what you need to know about where they hit hardest and how to prepare.
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Pineapple Express Storms: Tropical Moisture Invades California
A Pineapple Express is an atmospheric river originating near Hawaii, carrying warm tropical moisture directly to California at jet-stream speed. These storms are responsible for some of California's most dramatic winter floods.
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Bomb Cyclones: When Rapidly Intensifying Storms Hit the Bay Area
Bomb cyclones are storms that intensify at extraordinary speed, sometimes dropping 24+ millibars of pressure in a single day. When one reaches the Bay Area, the results can include record rainfall, damaging gusts and flooding.
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El Niño 2026-27: What It Could Mean for Bay Area Weather
NOAA expects a very strong El Niño in late 2026 and says a wetter Bay Area winter and spring is more likely. A wet season is still not guaranteed.
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San Francisco Weather by Neighborhood: Sunniest to Foggiest
In summer 2025, the Mission averaged 11.4 hours of sun a day while the Outer Sunset averaged 7.7 and had fog on 92% of mornings. Here is every SF neighborhood, ranked.
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San Francisco Microclimates: Neighborhood Weather Guide
San Francisco microclimates make the Outer Richmond foggy, the Mission sunny, and SoMa breezy. This neighborhood guide explains the city block by block.
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Berkeley Hills: A Weather Divide Between Coast and Valley
The Berkeley Hills split the East Bay into two climate zones. Berkeley's flatlands sit in the marine layer while Orinda and Lafayette on the other side often reach the upper 80s and 90s. The distance between them is less than 8 miles.
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Livermore Valley: The Hottest Part of the Bay Area
Livermore averages about 89°F in July and reaches the 100s in heat waves while San Francisco sits in the 60s, about 40 miles away. Its inland valley position keeps most ocean cooling out.
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Peninsula Microclimates: Palo Alto vs. Half Moon Bay
Palo Alto averages about 80°F in July while Half Moon Bay, less than 20 miles away over the mountains, sits in the low 60s in fog. The Santa Cruz Mountains divide the Peninsula into two distinct weather worlds.
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Wine Country Weather: Napa and Sonoma Explained
Napa Valley's hot days and cool nights come from the same bay air that keeps San Francisco foggy. July highs climb from about 80°F in Carneros to the low 90s in Calistoga, about 30 miles north.
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Half Moon Bay Weather: Coastal Fog and Pacific Exposure
Half Moon Bay averages about 65°F in summer under persistent Pacific fog. While Palo Alto hits 82°F, Half Moon Bay sits in thick marine layer 15 miles away. Fall is the best season for sunny visits.
6 min read →
Muir Woods Weather: Redwood Canyon Microclimate
Muir Woods runs much cooler than inland Marin and Sonoma towns in summer. The redwood canopy, fog drip and deep canyon create a cool, moist microclimate regardless of what is happening outside.
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Bay Area Summer Heat Waves: When Inland Temps Soar
Bay Area heat waves arrive when atmospheric blocking shuts down the marine layer. Livermore can hit 108°F while San Francisco stays at 72°F 40 miles away. These events are brief, intense, and increasingly frequent.
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Urban Heat Island Effect in San Francisco and the Bay Area
San Francisco's urban heat island is weaker than in most cities, because the marine layer limits daytime heating and slows nighttime cooling everywhere. The effect shows up most clearly on clear, calm winter nights.
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Bay Area Seasons and Its Mediterranean Climate, Explained
The Bay Area has a Mediterranean climate, but its seasons do not follow the calendar. Fog peaks in summer, the warmest weather comes in fall, and rain falls from November to March.
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Napa Valley Weather by Month: When to Visit Wine Country
Napa Valley runs from cool rainy winters to summers in the mid 80s to near 90°F to the golden October harvest window. Each month has distinct character. Here's what to expect and when to visit for your preferred experience.
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Mount Tamalpais Weather: Fog, Rain, and Summit Views
Mount Tamalpais intercepts the marine layer directly. The ocean-facing slopes are some of the Bay Area's foggiest ground, while the summit, at about 2,570 feet, often rises above the fog into sunshine. Always bring a layer.
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Santa Cruz Weather: The Bay Area's Warmest Beach Town
Santa Cruz runs 10-12°F warmer than the Bay Area's Pacific coast in summer, averaging 70-74°F with afternoon fog clearance. It's the closest thing the Bay Area has to a traditional California beach town.
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Bay Area Beach Day Trips: Best Weather Months by Location
Pacific coast beaches stay cold and foggy in summer, while sheltered East Bay beaches can be sunny and near 70°F. Santa Cruz is the most dependable warm beach within a day trip. Here's how to match beach type to weather.
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San Francisco Bay Water Temperature: Today & Monthly
San Francisco Bay water ranges from about 52°F in winter to 69°F in August at Alameda. The ocean outside the Golden Gate stays near 52°F to 58°F all year.
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Temperature Inversions: Why Cold Air Gets Trapped in Bay Area Valleys
A temperature inversion is warm air sitting above cool air, which stops the air from mixing upward. It explains the marine layer's sharp fog ceiling, winter tule fog that lasts for days and the Bay Area's worst air quality episodes.
6 min read →
Fata Morgana: Why the Farallon Islands Seem to Float
The same warm-over-cold layer that makes Karl the Fog also makes the Farallon Islands float, which is why mirages hide in the foggiest months.
9 min read →
How Bay Area Hills and Mountains Create Localized Rain
Bay Area rainfall varies from about 14 to more than 50 inches within 30 miles, almost entirely due to orographic enhancement. The hills and mountains force moist Pacific air upward, wringing out rain on windward slopes that flat areas never receive.
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Bay Area Sea Breezes vs. Land Breezes Explained
The sea breeze is the Bay Area's most important summer weather pattern. Pacific air flows through the Golden Gate, across the bay and into the Central Valley, cooling inland valleys in the late afternoon and evening.
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How High Does Bay Area Fog Go? Which Elevations Stay Above It
The marine layer fog has a ceiling set by the temperature inversion base. Hillside neighborhoods above that line, typically 800-1,500 feet, can sit in sunshine while the flatlands below are completely socked in, sometimes with a 20°F temperature difference.
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Bay Area Heat Domes: When High Pressure Traps Extreme Heat
A heat dome forms when a high-pressure ridge stalls over California, suppressing the marine layer and allowing the ground to heat progressively over multiple days. These events can push San Francisco above 100°F, and a warming climate is making the hottest ones hotter.
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Bay Area Wind Chill: Why a Breezy Coast Feels So Cold in Summer
Wind steals body heat fast, and along the Bay Area coast it blows hardest on summer afternoons. That is why July visitors to Ocean Beach are so often underdressed, especially once the sun goes down.
5 min read →
Bay Area Humidity vs. Dew Point: What's the Difference?
Bay Area fog can read 90% relative humidity without feeling muggy, because the dew point is only 50-55°F, modest by global standards. During Diablo wind events, dew point crashes below 30°F, creating genuinely desiccating conditions.
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When Is the Bay Area Warmest? Warmest Month by City
The Bay Area is warmest in September near the coast and in July inland. San Francisco's July averages only 66.3°F, cooler than September's 70.2°F.
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When Is San Francisco Fog Season? The Foggiest Months, Explained
July is San Francisco's foggiest month, not June or August. The North Pacific High reaches peak strength, deepening the marine layer and making afternoon burn-off unreliable. Some July days see no sun at all, while September fog is brief and reliably clears by noon.
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Why Are East Bay Cities Hotter Than the Coast?
On a summer afternoon, San Francisco can sit at 62°F in fog while Oakland is in the low 70s. Walnut Creek reaches the upper 80s, and Livermore passes 90°F. All four cities share the same metropolitan area, but geography determines which ones the marine layer reaches and how much cooling it delivers.
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What's the Rainiest Month in the Bay Area?
In San Francisco, December and January are nearly tied as the rainiest months at about 4.5 inches each. The Pacific storm track takes until December to fully position itself for Bay Area storms. In the Marin watershed, the same months can deliver 15-20 inches.
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Is San Francisco Really 20 Degrees Colder Than Inland?
On an average July afternoon, downtown San Francisco runs about 20 degrees cooler than Walnut Creek. The "20 degrees colder" claim is local shorthand, and on strong marine push days, the gap far exceeds 20 degrees. Understanding when the gradient is largest reveals how the Bay Area's marine layer works.
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San Francisco Weather by Month: A Complete Month-by-Month Guide
San Francisco's warmest months are September and October, not July and August. This month-by-month guide covers temperatures, fog, rain, and what to actually expect.
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San Francisco Weather in June: Fog Season Sets In, Here's What to Know
June is when San Francisco's summer fog sets in. Western neighborhoods stay near 60°F under the marine layer while the Mission District hits 70°F in sunshine three miles away.
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San Francisco Weather in October: Highs Near 70°F, Little Fog
October is San Francisco's real summer. Average highs are near 70°F, fog retreats, and sunshine dominates. But it's also fire season, with Diablo wind events possible.
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San Francisco Weather in March: The Shoulder Season Explained
March is San Francisco's shoulder season between wet winter and dry summer. Expect 62°F highs, 3.1 inches of rain, green hills, wildflowers, and minimal fog.
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San Francisco Weather in April: Spring Sun, Wind and Fog
April is San Francisco's hinge month. The rain fades, afternoons turn windy, foggy mornings return and the ocean hits its coldest.
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San Francisco Weather in May: Dry, Bright and Getting Foggy
May is San Francisco's last clear window before summer fog. The rain is nearly done, afternoons turn windy, May Gray returns to the west side and the ocean hits its coldest.
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San Francisco Weather in July: Fog and Cool Summer Days
July is summer everywhere but San Francisco. The west side sits under fog that lingers all afternoon, the Mission stays sunny and inland towns run nearly 30 degrees warmer.
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San Francisco Weather in August: Fogust and Late-Summer Sun
August is "Fogust" on the west side, where the low cloud deck peaks. Downtown clears more often, the ocean warms and the last week of the month can turn hot.
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San Francisco Weather in September: The City's Warmest Month
September is San Francisco's real summer, with the warmest highs of the year, less fog and the warmest ocean. The city's all-time record of 106°F came in September, and heat and smoke are worth watching.
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San Francisco Weather in November: Temps, Rain and Tips
November is the month San Francisco flips from Indian summer to winter storms. The first soaking storm arrives around November 3 in a typical year.
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San Francisco December Weather: Rain, Temps and Holiday Odds
December is San Francisco's wettest month, but the rain comes in a few big storms. Rain fell on only 27% of Christmas Days from 1991 to 2020.
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San Francisco Weather in January: Cold, Wet, Unpredictable
January is San Francisco's coldest and most unpredictable month. It brought no rain in 2015 and 17.64 inches in three weeks in 2022-23.
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San Francisco Weather in February: Rain and Early Spring
February is San Francisco's all-or-nothing month. It brought no rain in 2020 and 14.89 inches in 1998, with the first signs of spring in between.
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San Francisco Rainfall: How Much and How Often It Rains
San Francisco averages 22.89 inches of rain on about 71 days a year. December is the wettest month, and summer is almost completely dry.
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Bay Area Rainfall Totals by City: Who Gets the Most Rain?
Annual rainfall in the Bay Area varies from about 14 inches in San Jose to more than 50 inches in the Santa Cruz Mountains. The hills and distance from the coast drive most of that range.
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What Is the Delta Breeze? The Cool Air Current That Saves the Bay Area from Summer Heat
The Delta breeze carries cool Pacific air through the Carquinez Strait toward the Delta and Sacramento on summer afternoons and evenings. It often cools hot inland towns by 20 degrees or more.
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Bay Area Thunderstorms: How Rare Are They and When Do They Happen?
Bay Area thunderstorms are genuinely rare, averaging 2-5 days per year. The cold Pacific and persistent temperature inversion suppress the instability thunderstorms need.
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Seasonal Guides
Month-by-month breakdowns of what to expect and when to visit for the best conditions.
Why Is September Hotter Than July in the Bay Area?
September, not July, is the warmest month in San Francisco and along the coast. Summer fog keeps the coast cool until it loosens its grip in late August. Here is why.
5 min read →
Best Time to Visit San Francisco for Good Weather and Least Fog
September is San Francisco's warmest month at 70.2°F, and October is close behind. Summer is the foggiest season, so fall is the best time to visit for sun.
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Best Months for Hiking in the Bay Area
Bay Area hiking has two ideal windows, spring wildflower season in April and May and fall clearing season in September and October. Summer is complicated, and winter is muddy. Here is how to plan.
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Bay Area Rainy Season: When It Starts, Peaks and Ends
The Bay Area rainy season runs November through March. In San Francisco, the first rain typically falls around October 14 and the first soaking storm around November 3.
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Snow in the Bay Area: Mount Diablo and Where to See It
Snow in the Bay Area is rare but real. Mount Hamilton and Mount Diablo see snow every few years, when cold storms drop the snow level below about 3,000 feet. Here is where to go, when to expect it, and what the records show.
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Bay Area Indian Summer: Why Fall Brings Heat, Not Cool
September and October are the warmest months of the year in San Francisco and along the coast, and inland valleys stay nearly as hot as midsummer. When the North Pacific High weakens, the marine layer retreats and Indian summer arrives with the year's best weather.
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Bay Area Winter Storms: When They Hit, Storm Types and Rain by Location
The Bay Area's entire annual rainfall arrives in five winter months via Pacific storms and atmospheric rivers. El Niño and La Niña can tilt the odds toward a wet or dry year, but they do not decide it.
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Does It Snow in San Francisco? What Actually Happens in Winter
San Francisco does not snow in any practical sense. The oceanic influence keeps winter lows around 46°F, far too warm for snow. The last notable snowfall was 1976.
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Wine Country Harvest Season Weather: Napa and Sonoma Crush Guide
Wine country harvest season runs from late August through mid-November, with warm, nearly rain-free Septembers. October is the best month to visit for weather and harvest activity.
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Tide Pooling in the Bay Area: Best Low Tides for 2026-27
The best Bay Area tide pool days need a minus tide, daylight and calm surf. NOAA predictions show when all three line up in 2026-27.
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Bioluminescence in Tomales Bay: When and How to See the Glow
Tomales Bay glows when three things line up, which are the plankton, a dark night and calm water. Here is how to pick the night.
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Monarchs in Pacific Grove and Santa Cruz: When to See Them
Monarchs fly only above about 55°F. Monterey airport records show winter days usually cross that line around 10 AM, and some never do.
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Why Blue By-the-Wind Sailors Wash Up on Bay Area Beaches
By-the-wind sailors live far offshore and can't swim. When spring winds turn onshore, they cover Bay Area beaches in blue.
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Northern Lights in the Bay Area: When and Where to See Them
Two things have to line up for the Bay Area to see the aurora, a severe solar storm and a night without fog.
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Why Is Dungeness Crab Season Delayed in California?
The calendar says November 15, but the ocean sets the opener, through toxic algae in warm water and humpback whales near shore.
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Historical Events
Record-breaking storms, heat waves, floods, and snow events that define Bay Area weather history.
Great Flood of 1862: California's Worst Storm and ARkStorm 2.0
In the winter of 1861-62, weeks of atmospheric rivers turned the Central Valley into a lake and gave San Francisco its wettest season on record. The flood is the model for ARkStorm, and climate change has already doubled the odds of a storm like it.
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El Niño in the Bay Area and California: 1997-98 vs. 2026-27
A very strong El Niño is forming for 2026-27. The 1997-98 El Niño doubled San Francisco's rainfall, but the stronger 2015-16 event left the city near normal.
10 min read →
The 2020 Bay Area Heat Dome: August Heat, Lightning Fires and Orange Skies
In August 2020, a heat dome brought record inland heat and rolling blackouts, then dry lightning sparked the SCU, LNU and CZU fires. A Labor Day heat wave and the orange sky of September 9 followed.
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Hottest Temperature Ever in San Francisco and the Bay Area
San Francisco's all-time high is 106°F, set in 2017, and the Bay Area record is 116°F in Livermore in 2022. Downtown San Francisco hits 90°F only about 3 days a year.
5 min read →
Rare Snow Events in the Bay Area
Snow is a generational event at Bay Area sea level, San Francisco last recorded measurable accumulation in 1976. Higher ridgelines above 1,500 feet see occasional dustings, and Mount Diablo at 3,849 feet is the region's most reliable snow location.
6 min read →
Coldest Temperature Ever in San Francisco and the Bay Area
San Francisco's all-time low is 27°F, and downtown has dropped to freezing on only 8 days since 1950. Inland valleys like Livermore have reached 18°F.
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Most Rain in a Single Day in the Bay Area
The Bay Area's wettest single days come from strong atmospheric rivers. San Francisco has topped 5 inches twice, and the mountains can collect 10 inches or more, enough to overwhelm creeks and storm drains built for gentler storms.
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Air Quality
How Bay Area weather drives air quality, wildfire smoke events, and what AQI readings mean for your health.
Wildfire Smoke and Bay Area Air Quality
Wildfire smoke can reach the Bay Area from fires hundreds of miles away, and an inversion can trap it near the ground within hours. The 2018 Camp Fire brought nearly two weeks of unhealthy air across the region.
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Spare the Air Days: How Weather Drives Bay Area Air Quality
A winter Spare the Air alert makes burning wood illegal across the Bay Area. Calm, cold, clear weather traps smoke near the ground, which is when the alerts happen.
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AQI Explained: What Bay Area Air Quality Numbers Mean
The Air Quality Index translates complex pollution measurements into a single number with health guidance. In the Bay Area, PM2.5 from wildfire smoke and wood burning drives most elevated readings. Here's how to read AQI and what to do at each level.
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Bay Area Ozone: Why Summer Smog Peaks in the Inland Valleys
Ground-level ozone is one of the Bay Area's most stubborn air quality problems. It forms on hot summer afternoons in inland valleys where sunlight converts vehicle emissions to smog. Livermore consistently records the region's highest ozone levels.
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Safety & Preparedness
Flood zones, fog driving, and storm preparedness for Bay Area residents and visitors.
Flooding Risk Maps: Bay Area Flood Zones
Check how high San Francisco Bay is running, find official live flood maps and gauges, and see which communities face creek, tidal, stormwater and debris flow flooding.
10 min read →
King Tides 2026-27 in the Bay Area: Dates, Times and Flood Spots
The 2026-27 Bay Area king tides peak on December 24 at 7.23 feet at the San Francisco gauge. Alviso, Foster City and the Embarcadero flood first.
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Kelvin Waves in the Bay Area: How El Niño Raises Our Tides
El Niño is sending slow bulges of warm water, called Kelvin waves, to California. NOAA data show them raising San Francisco Bay about a foot in October 2026.
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Sneaker Waves: Why They Kill on Northern California Beaches
A sneaker wave can follow 10 to 20 minutes of small surf and run more than 150 feet up the beach. Off San Francisco, the swells behind them peak in winter.
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Can a Tsunami Hit San Francisco? What History and Maps Show
Tsunamis have reached San Francisco, but NOAA records show the Golden Gate softens the blow while the open coast and harbors take the damage.
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Rip Currents at Ocean Beach: Why San Francisco Says Stay Out
At Ocean Beach, the calmest-looking water is the most dangerous. The rip looks like a smooth gap, and warm days still bring big surf.
10 min read →
Red Tide in San Francisco Bay: Algal Blooms and Fish Kills
The 2022 bloom was the largest in San Francisco Bay's recorded history. Here is what happened, why, and what to do next time.
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Is Earthquake Weather Real? What Bay Area Quake Days Show
Loma Prieta came on an 83°F day, and the 1906 quake on a normal 62°F one. The weather on quake day is whatever you remember.
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Driving in Bay Area Fog: Visibility and Safety Tips
Bay Area marine layer fog can drop highway visibility to 200 feet. Here are the safety rules that matter most, from headlight use to following distance to the foggiest routes.
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What Is a Dense Fog Advisory? Bay Area Fog Warnings Explained
A Dense Fog Advisory means widespread visibility of a quarter mile or less, usually for several hours. Bay Area advisories most often cover inland valleys in winter or Bay-area coastal corridors when the marine layer is especially thick.
6 min read →
Bay Area Fire Season: When Wind Events Collide With Drought
Bay Area fire season peaks in September-November when drought-dry vegetation meets Diablo wind events. The Tubbs, Kincade, and 1991 Oakland Hills fires all struck during fall offshore wind conditions.
8 min read →
What Does a Fire Weather Warning Mean? Terms and Thresholds Explained
A fire weather warning means the National Weather Service expects weather that could make wildfires spread fast. In the Bay Area, forecasters weigh wind, humidity and fuel dryness together before issuing one.
7 min read →
Red Flag Warning vs. Fire Weather Watch: What Each Alert Level Means
Red Flag Warnings and Fire Weather Watches are the two NWS fire weather alerts. One means dangerous conditions are imminent, the other means they are possible. Both demand action.
7 min read →
Bay Area Tornado Watch vs. Warning: Can Tornadoes Actually Hit the Bay?
Bay Area tornadoes are real but rare and mostly weak, about one or fewer in a typical year. Tornado warnings do get issued during strong winter storms. Here is what they mean.
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Flood Advisory vs. Flood Warning in the Bay Area: How to Know When to Act
A flood advisory means minor flooding is occurring or expected. A flood warning means significant flooding is imminent. Know the difference and where Bay Area floods hit hardest.
7 min read →
Coastal Flood Advisory vs. Warning vs. Watch: What Each Level Means
A coastal flood advisory is the lowest of three NWS alert levels for ocean water threatening to push onto shore. Learn the difference between advisory, watch, and warning.
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Travel Tips
Practical advice on what to pack and how to dress for Bay Area weather.
What to Pack for San Francisco (By Month)
San Francisco's daily temperature swings and year-round fog make packing tricky. The answer is always layers, but the specific combination changes by month. Here is what to bring.
5 min read →
Dressing in Layers: Why San Francisco Weather Demands It
San Francisco's daily swing from foggy 55°F mornings to mid-60s afternoons to cool evenings means one outfit never works. Layers are not optional here. They are the only system that works.
5 min read →
Golden Gate Bridge Fog: Why It's Always Foggy and When to See It Clear
Summer fog pours through the Golden Gate in the afternoon. For clear bridge views, go before 10 AM or visit in October, and check the fog forecast first.
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Napa & Sonoma Day Trip: Best Weather Months to Visit Wine Country
October brings harvest activity, warm afternoons and golden vine leaves to wine country. Spring is the green alternative, and summer is hot by day but cool by night. Here is how to pick the right month for a day trip.
6 min read →Category
Locations
Deep-dives into specific Bay Area and Northern California destinations and their unique weather patterns.
Monterey Weather: Coastal Microclimate at the Bay's Southern Edge
Monterey's protected bay position makes it warmer and clearer than the Bay Area coast to its north. Morning fog burns off by noon most days, fall is the warmest season, and Carmel Valley averages July highs about 11°F warmer than the coast.
6 min read →
Carmel-by-the-Sea Weather by Month and Best Time to Visit
Carmel-by-the-Sea is a fog town in summer and warmest in fall. Twelve miles inland, Carmel Valley runs about 11 degrees warmer in July.
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Big Sur Weather: Best Time to Visit, Fog, Rain and Highway 1
Big Sur is three climates and a road, with a foggy coast, sunny ridges and warm valleys. Its winters are wet enough to close Highway 1.
8 min read →
Sausalito Weather vs. San Francisco: A Mile Apart, Degrees Different
Sausalito is a mile from San Francisco across the Gate, but the Marin Headlands provide partial protection from the marine layer, making mornings often clearer and slightly warmer. The trade-off is serious afternoon wind on the waterfront.
5 min read →
Stinson Beach Weather: When the Bay Area's Best Beach Is Worth the Drive
Stinson Beach can be 15°F warmer than Ocean Beach on the same day, depending on how deep the marine layer is and how hard it pushes ashore. Fall is the most reliable season for warm beach conditions.
5 min read →
Point Reyes Weather: Fog, Wind, and Wild Coast Conditions
Point Reyes is the Bay Area's most exposed coastline. The Park Service calls it the windiest place on the Pacific Coast and the second foggiest in North America. The Inverness Ridge creates a dramatic weather divide within the park.
6 min read →
Walnut Creek Weather: Diablo Valley Heat and Sunshine
Walnut Creek sits in the Diablo Valley, sheltered from most of the marine layer by the hills to its west. July highs average in the upper 80s, heat waves push past 100°F and fall is often the most pleasant season.
5 min read →
Concord Weather: Hot Inland Valley With Afternoon Marine Relief
Concord sits at the north end of the Diablo Valley, near enough to the Carquinez Strait to catch an evening breeze on most summer days. July highs average around 90°F, and heat waves can top 105°F.
5 min read →
San Jose Weather: The Bay Area's Warmest Major City
San Jose is the Bay Area's warmest major city: July highs in the low 80s, mostly sunny skies and winters that rarely bring frost. The marine layer travels 40 miles from the coast before arriving, losing most of its cooling effect.
6 min read →
Oakland Weather: Why It's Sunnier Than San Francisco
Oakland sits across the bay from the Golden Gate, so summer fog has usually thinned by the time it arrives. July afternoons run in the low 70s, about 6°F warmer than downtown San Francisco.
6 min read →
Pacifica Weather: Summer Fog, Cold Surf and Clear Fall Days
Pacifica's coastal valleys open straight to the ocean, so summer fog flows right into town and July afternoons usually stay in the low 60s. The beach is dramatic but cold, and September and October bring the clearest weather.
5 min read →
Mount Diablo Weather: Summit Temperatures, Fog Views and Fire Season
Mount Diablo's summit, at 3,849 feet, often rises above the valley fog. It runs close to 20°F cooler than Walnut Creek on summer afternoons and gets snow every few winters. The clearest views come after winter cold fronts.
6 min read →
Altamont Pass Weather: Why the Bay Area's Windiest Gap Blows
The Altamont Pass is a low gap between the Bay Area and the Central Valley. Summer heat in the valley pulls cool air east through it, which is why the hills have held a wind farm since the early 1980s.
6 min read →
Santa Rosa Weather: Climate, Fire Season and the Coastal Divide
Santa Rosa sits between the foggy coast and the warm interior valleys, with July highs in the low 80s and ocean air that cools most summer evenings. Fall brings the most pleasant weather and the highest fire risk.
6 min read →
Calistoga Weather: Napa Valley's Hottest Town
Calistoga sits at the narrow northern end of Napa Valley, where the cooling breeze from San Pablo Bay barely reaches. Summer afternoons run about 10°F warmer than Napa city, but clear nights cool off quickly.
6 min read →
Napa Valley Weather Guide: Best Times to Visit and What to Expect
Napa Valley spans hot Calistoga (about 90°F July highs) to warm downtown Napa (about 84°F), with dry summers, wet winters, and September-October harvest as the best visit window.
6 min read →
Sonoma Weather Guide: How Wine Country's Climate Differs from Napa
Sonoma County stretches from the foggy coast in the low 60s to inland valleys near 90°F, one of the widest climate ranges in California wine country. Here is how it differs from Napa.
6 min read →
Napa Weather in March: Late Rains, Wildflowers, and Shoulder Season Perks
March is Napa Valley's shoulder season, with afternoons in the mid 60s and mustard blooming in the vineyards. It also brings the last big rains of winter and lower hotel rates than at harvest.
5 min read →
Napa Weather in October: Peak Harvest Season and What It Means for Visitors
October is often Napa Valley's best month, with highs in the upper 70s to low 80s and harvest in full swing. The vines turn fall colors, and there is almost no rain. Fire season is the one weather risk to watch.
5 min read →
Fremont Weather Guide: Summer, Winter, Mission Peak and Nearby Cities
Fremont sits between the southern bay shore and the hills below Mission Peak, with summer highs around 80°F and about 15 inches of rain a year. It is warmer than SF, a little cooler than Livermore and drier than Oakland.
5 min read →
Berkeley Weather Guide: Hills, Fog, Fire Risk, and Microclimates
Berkeley has one of the Bay Area's sharpest climate gradients. The flatlands near the bay are foggy, while the hills just a few miles east are often sunny and get 26-29 inches of rain.
6 min read →Category
Events & Venues
Weather at Bay Area events, venues, and activities, from Giants games to surf season and camping.
Does Weather Affect Flight Delays at SFO and OAK?
SFO is among the most weather-delayed major airports in the country, not from severe storms but from summer morning fog. Low clouds cut its arrival rate from about 45 planes an hour to about 36, causing ground delay programs that cascade across the national network.
6 min read →
Best Weather for SF Giants Games at Oracle Park
Oracle Park sits on the bay waterfront, exposed to the afternoon sea breeze. July games can be cold, around 60°F with a stiff wind, while September games are often warm and sunny. Knowing which games tend to have which weather helps you dress right.
5 min read →
Mavericks Big Wave Season: When Do Half Moon Bay's Waves Peak?
Mavericks at Half Moon Bay is dormant in summer and one of the most powerful waves on Earth in winter. The reef requires northwest swell from North Pacific winter storms, November through March. December through February produce the biggest days, with wave faces regularly reaching 40-60 feet.
6 min read →
Best Surfing Weather in the Bay Area by Season
Fall is the best surfing season in the Bay Area, with early North Pacific swells, easing wind and thinner crowds. Winter brings the biggest waves but also the most onshore wind. Summer waves are small but accessible for beginners if you surf in the morning before the sea breeze builds.
6 min read →
Bay Area Camping Weather: Best Months to Camp by Location
Bay Area camping weather varies dramatically by location and season. Coastal Marin camps are foggy and cool in summer but spectacular in fall. Mount Diablo and inland parks are too hot in summer. Spring and fall are the right seasons. Wherever you go, expect cool nights and bring a warm sleeping bag.
6 min read →
Outside Lands Music Festival Weather: What to Expect in August
Outside Lands takes place in Golden Gate Park in early August, in the middle of San Francisco fog season. Expect temperatures from the mid 50s to the mid 60s, a steady west wind and cold evenings.
5 min read →
Thanksgiving Weather in San Francisco and the Bay Area
Thanksgiving in San Francisco is mild by national standards, with highs around 60°F and almost no chance of freezing. Rain falls on about one Thanksgiving Day in four as the wet season begins. Clear years produce beautiful low-angle November light over the bay.
5 min read →
Fourth of July Weather in San Francisco: Will Fog Ruin the Fireworks?
July 4 falls near the peak of San Francisco fog season, so the fireworks often burst inside a low ceiling of fog. They look bright up close but are hard to see from a distance. Inland East Bay towns are usually clearer. Pack a jacket either way.
5 min read →
Best Weather for Alcatraz Island Tours in San Francisco
Alcatraz sits 1.25 miles offshore in the bay, fully exposed to wind and marine air. Summer visits are typically 58-65°F with 15-25 mph wind. Pack a jacket regardless of what the mainland weather looked like. May and September-October offer the most comfortable conditions.
5 min read →
Bay Area Outdoor Wedding Planning: Best Weather Months by Location
September and October are the gold standard for outdoor Bay Area weddings, Indian summer, warm afternoons, golden light and little rain. May is a strong alternative. Summer at a coastal SF venue carries a real fog-and-cold risk that inland wine country venues avoid entirely.
6 min read →
Levi's Stadium Weather: 49ers Game-Day Guide for 2026
Levi's Stadium, the Bay Area's NFL stadium, is far warmer than San Francisco. September games average highs near 81°F, while January night games fall toward 40°F.
5 min read →
Whale Watching in the Bay Area: Best Weather and Season Guide
The Bay Area has two whale watching seasons, gray whales from December to May and humpbacks from spring through November. September and October combine peak activity with the best weather.
5 min read →