San Francisco’s climate is a masterpiece of complexity, defined by its position on a peninsula between the cold Pacific Ocean and the warmer waters of San Francisco Bay. Known officially as a Mediterranean climate with cool summers, the weather here rarely hits extremes but frequently surprises visitors with its rapid fluctuations.
The Daily Rhythm: The Marine Layer Cycle
A typical day in San Francisco follows a predictable metabolic cycle driven by the "marine layer." As the sun heats the inland valleys of California, the rising hot air creates a vacuum that pulls cold, moist sea air through the Golden Gate gap.
- Morning: Much of the city begins the day under a thick gray blanket of advection fog.
- Midday: The sun eventually "burns off" the fog, usually clearing from the eastern Bay side first and moving toward the coast.
- Afternoon: This is typically the sunniest part of the day, though a brisk wind often picks up, coming off the ocean.
- Evening: As temperatures drop, the fog—affectionately named "Karl" by locals—rolls back in, cascading over the Twin Peaks like a slow-motion waterfall.
Seasonal Trends and the "Indian Summer"
San Francisco’s seasons are famously shifted, leading to the local saying that "winter is summer and summer is winter."
- Spring (March – May): A transition period where the winter rains taper off. This is often the windiest time of year, but it offers some of the clearest, most pollen-rich views of the city.
- Summer (June – August): While the rest of the country swelters, San Francisco stays cool. This is the peak of the fog season, colloquially known as "June Gloom" and "Fogust." Temperatures often hover in the low 60s°F (16-17°C), and the damp air can make it feel much colder.
- Autumn (September – October): Locally known as the "Indian Summer," this is the city's true summer. The inland heat weakens the marine layer, allowing the city to bask in consistent sunshine and its highest temperatures of the year, often reaching the 70s°F or 80s°F.
- Winter (November – February): This is the rainy season. While it rarely frosts and almost never snows, the air is damp and crisp. Most of the annual 20 inches of rain falls in short, intense bursts during these months.
The Capital of Microclimates
San Francisco is famous for its microclimates, where weather can change drastically within a few blocks. The city’s 40+ hills act as barriers to the wind and fog.
- The Fog Belt: Western neighborhoods like the Sunset and Richmond are frequently shrouded in mist and are significantly cooler.
- The Sunbelt: Neighborhoods to the east and south of the hills, such as the Mission District, Noe Valley, and Potrero Hill, are often 10°F to 15°F warmer because the hills block the incoming marine layer.
- The Financial District: Tall skyscrapers create "wind canyons," making the downtown area feel significantly gustier than residential districts.
Historical Context and Local Adaptation
Historically, San Francisco’s climate was even foggier; research indicates a roughly 33% decrease in summer fog hours over the last century due to climate change. Despite this, locals have adapted to the "natural air conditioning" provided by the Pacific.
You will rarely see a local without a light jacket or sweater, regardless of how sunny it looks in the morning. The weather is experienced as a tactile force; the air feels "heavy" and salted when the fog is in, and "dry and sharp" when the offshore winds (Diablo winds) blow from the east in the autumn.
Unique Weather Phenomena
- Karl the Fog: The fog is so central to the city's identity that it has its own persona and social media presence. It isn't just a cloud; it's a moving, living element of the skyline.
- Diablo Winds: Occurring mostly in the fall, these hot, dry winds blow from the interior toward the coast, creating high fire danger but also providing the city's rare "balmy" nights.
- Tule Fog: While less common than the coastal advection fog, this thick ground fog sometimes drifts in from the Central Valley during winter mornings, creating a different, more stagnant type of haze.