San Francisco weather and climate guide

7-day forecast for San Francisco

Tonight in San Francisco, CA brings slight chance drizzle. Across the 8-day outlook, highs average 74°F and lows average 57°F, ranging from 55°F to 78°F. No day in the period shows a meaningful chance of precipitation.

Current conditions

Observed at SAN FRANCISCO DOWNTOWN (SFOC1) at 7:43 PM PDT.

Why summer is cold

Three things combine. First, the California Current carries cold water south along the coast. Second, summer north-northwest winds drive coastal upwelling, pulling even colder water up from depth — ocean surface temperatures off the city run in the low to mid 50s in July, colder than in winter. Third, the Pacific high pressure system creates a temperature inversion that traps a shallow layer of cool marine air near the surface.

When hot air over California's Central Valley rises, it creates a thermal low that pulls marine air inland through the only sea-level gap in the coastal mountains — the Golden Gate. That air is cool and saturated, so as it moves over the cold water and then over the city, it condenses into the advection fog that defines San Francisco summers.

The paradox worth understanding is that the hotter it gets inland, the foggier and colder the city gets. A 105 °F day in Sacramento strengthens the pressure gradient, accelerates the marine push, and delivers 58 °F with wind and fog to the Outer Sunset. The two are directly causally linked.

The result is that September and October, not July and August, are the city's warmest and clearest months. By then the inland heat has faded, the marine layer weakens, and the fog retreats. Visitors who plan a summer trip and pack for California summer are consistently caught out.

Microclimates are real and extreme

San Francisco is 49 square miles of hills sitting between cold ocean and warmer bay, and the fog does not distribute evenly. Hills block it. The result is temperature differences within the city that are larger than the differences between many entire cities.

On a typical summer afternoon, the Outer Sunset and Richmond districts sit in fog at 58 °F with 20 mph wind. The Mission District, sheltered by Twin Peaks, sits in sun at 75 °F. That is a 17-degree difference across about four miles. Bernal Heights and Potrero Hill are similarly favored; the western half of the city is not.

Cross the bay and it gets more extreme. Oakland runs warmer than San Francisco, and Walnut Creek — 25 miles east, behind the coastal hills — can be 100 °F on the same afternoon the Sunset is at 58. Any Bay Area forecast that gives one number is nearly useless, which is why local forecasts always break out coast, bay and inland valleys separately.

The practical advice locals give is real: dress in layers, always carry a jacket, and check the temperature for your specific neighborhood rather than for the city.

Winter is the wet season

Roughly 90% of the annual 23 inches falls between November and April, delivered by Pacific frontal systems and, in the biggest events, by atmospheric rivers. These narrow corridors of concentrated water vapor can carry more water than the Mississippi River and are responsible for both the region's worst flooding and a large share of the Sierra snowpack that supplies its water.

The winter of 2022–23 illustrated the pattern well, with nine atmospheric rivers in three weeks producing widespread flooding, downed trees and power outages after several years of severe drought. Californian precipitation is characteristically feast or famine; the state's year-to-year variability is the highest in the country.

Winter temperatures are extremely mild — January averages 57 °F by day and 46 at night. Frost is rare in the city and snow effectively does not happen. The hazards of winter here are wind, saturated-soil tree failures, urban flooding and landslides on steep slopes, not cold.

Wildfire smoke changed the fall

San Francisco itself has minimal direct wildfire risk — it is dense, largely non-flammable construction with little wildland interface. Smoke is a different matter. Fires in the Sierra, the North Bay and the coastal ranges send smoke into the Bay Area regularly from August through October, and the sinking-air inversion that produces the fog also traps smoke near the surface.

September 9, 2020 is the reference date: smoke from multiple large fires, lofted high and spread over a marine layer, blocked enough sunlight that streetlights stayed on all day and the sky was orange at noon. Air quality index readings across the region reached hazardous levels during the 2018 Camp Fire, when smoke from 150 miles north settled over the Bay for two weeks.

This has become a normal part of the annual cycle rather than an anomaly, and checking AirNow alongside the forecast in late summer is now routine practice for residents.

Practical notes on fog, wind and the bridge

Fog here comes in two forms. Summer advection fog forms over cold water and is pushed inland by wind — it can persist all day and is often thickest in the afternoon on the west side. Winter tule fog is different: it forms in the Central Valley on cold, calm, clear nights through radiational cooling, and it occasionally spills into the Bay. It is denser and more dangerous for driving.

Wind is a constant. The Golden Gate acts as a venturi, and afternoon winds of 20 to 30 mph through the gap and across the western neighborhoods are ordinary in summer. Combined with 58 °F air, that produces a wind chill effect that makes the city feel far colder than the thermometer reads — the main reason visitors buy so many souvenir sweatshirts.

One geographic note that catches people: the Golden Gate Bridge is frequently in fog when downtown is clear, because the bridge sits directly in the marine air channel. If you want a clear photograph, morning before the marine push, or September and October generally, gives far better odds than a July afternoon.

Weather hazards to know about

• Atmospheric river flooding

  November through April

  The main winter hazard. Heavy sustained rain causes urban flooding, landslides on steep terrain and tree failures in saturated soil.

• Wildfire smoke

  August through October

  Smoke from regional fires settles under the marine inversion and can reach unhealthy or hazardous air quality for days at a time.

• High wind

  Year-round, strongest in summer afternoons and winter storms

  The Golden Gate channels wind through the city. Combined with cool air, it produces a substantial wind chill effect in summer.

• Coastal hazards and sneaker waves

  Year-round, worst in winter

  Ocean Beach has strong rip currents and sneaker waves that kill people every year. The water is cold enough for rapid incapacitation.

• Dense fog

  Summer advection fog; winter tule fog

  Visibility hazards for driving, particularly winter tule fog spilling in from the Central Valley.

What to wear and pack, by season

Spring (March to May)

Layers plus a light waterproof shell. Variable, with the fog pattern beginning to establish by May.

Summer (June to August)

This is the counterintuitive one. Pack a real jacket, long pants and layers. Highs near 68 °F with wind and fog, not beach weather.

Fall (September to October)

The warmest and clearest season. Light clothing works, though evenings still cool quickly.

Winter (November to February)

A waterproof jacket and layers. Mild but wet; you will not need a heavy coat or boots for cold.