San Francisco does not follow the weather calendar that most American cities use. Summer feels like the coldest season to many visitors, winter is mild and rainy, and the warmest days of the year arrive in September and October. The city averages a high of about 70°F in September compared with about 66°F in July, a reversal that surprises nearly everyone on a first visit. This guide covers what to expect month by month. It draws on decades of weather station data from downtown San Francisco and the coastal stations that capture the city's fog patterns.
January and February: The Heart of the Rainy Season
January averages about 4.4 inches of rain across roughly 11 rainy days, and February is close behind at 4.4 inches, though both swing widely from year to year. Temperatures are mild for winter, with highs of 58-60°F and lows that rarely drop below 46°F. Snow is essentially nonexistent in the city itself, though you can occasionally see it dusting the peaks of Mount Tamalpais or Mount Diablo across the bay. The rain comes in multi-day stretches driven by atmospheric rivers, the moisture plumes that carry tropical water vapor directly into the Bay Area coast. Between storms, January can deliver strikingly clear, calm days with visibility stretching to the Farallon Islands.
March and April: Transition to Dry Season
March is the last reliably rainy month, averaging about 3.2 inches. By April, rainfall drops to 1.6 inches and the dry season begins. Temperatures start climbing, with March highs averaging 62°F and April reaching 63°F. The hills turn bright green from winter rains, and wildflowers bloom across Lands End and the Marin Headlands. These are some of the best months for hiking in the Bay Area, because the rain has greened everything up but the summer fog has not yet started. April in San Francisco is genuinely pleasant, with more sunshine than June or July.
May and June: The Fog Arrives
May is a month of transition between spring and summer. Early May often feels like an extension of spring, with clear skies and highs around 64°F. By late May, the marine layer, the cool, damp ocean air that brings summer fog, begins moving in. As the Central Valley heats up, its air pressure drops, and cold Pacific air gets pulled through the Golden Gate with increasing force. By June, fog is a near-daily pattern, and it keeps building until it peaks in July.
June highs average only about 66°F, but that number hides a wide range across the city. The Outer Sunset and Richmond District may sit at 55°F under thick fog while the Mission District hits 72°F in sunshine. June is the month when San Francisco's microclimate extremes are most visible.
July and August: Peak Fog Season
July and August are the months that produce the classic "I left my heart in San Francisco but I wish I'd brought a jacket" experience. Average highs are 67 degrees F, but afternoon fog pushes through the Golden Gate almost daily, dropping temperatures in western neighborhoods to the mid-50s. The Fourth of July fireworks at Crissy Field are famously obscured by fog more often than not.
Wind is the other defining feature of a San Francisco summer. Afternoon winds through the Golden Gate regularly hit 20-30 mph, making Ocean Beach genuinely cold even on days when Walnut Creek hits 100°F. The same pressure difference drives both the fog and the wind, because the hotter the Central Valley gets, the harder it pulls marine air into San Francisco. That natural air conditioning keeps the city mild, but it makes layers essential all year.
September and October: The Real Summer
September and October are San Francisco's warmest months, and locals know this is the real summer. The marine layer weakens as the temperature difference between the coast and the Central Valley narrows. Average highs reach 70.2 degrees F in September, and individual days in the low 80s are common. The Indian summer pattern can push temperatures into the upper 80s or even 90s during occasional offshore wind events.
October is nearly as warm, but it brings the first hints of the coming wet season. Highs average close to 70°F, and the month can deliver both the warmest single day of the year and the first significant rain. It is also fire season, when Diablo wind events can push temperatures well above normal while dropping humidity to single digits. October weather in San Francisco is unpredictable in the best way, since a week of 80-degree beach weather can be followed by the season's first rainstorm.
November and December: The Rains Return
November marks the transition back to the wet season. Average rainfall jumps to 2.6 inches after October's 0.9 inch, and temperatures cool to highs of 64 degrees F. The first winter storms typically arrive in November, sometimes bringing the most dramatic rainfall events of the season when atmospheric rivers make early appearances. December averages 4.8 inches of rain and highs of 58 degrees F, making it the wettest month of the year.
Despite the rain, winter in San Francisco is remarkably mild by national standards. The coldest overnight lows average around 46°F, and temperatures below freezing are essentially unheard of in the city. Between storms, December and January can bring brilliantly clear days with cool, crisp air. That winter light makes the bay and the bridges look their best. Winter here is less about cold than about rain, and about the green that rain brings to hills that spent six months turning gold and brown.
