サンフランシスコ 9月の天気

San Francisco-forum threads on September weather are consistently upbeat, calling it one of the better months of the year here, mild days in the 70s once morning fog burns off by late morning most days. Posters describe the fog itself as a near-daily fixture along the waterfront and bridge in the early hours, thinning out by 11am before a clear, mild afternoon takes over. The summer fog pattern that keeps downtown cool in June and July has largely broken by this point, and the heavier weather systems that arrive later in autumn haven't started yet, leaving September genuinely dependable. Regulars still recommend layers over shorts, since spots right on the water - the Wharf, Ocean Beach, the bridge itself - run noticeably cooler than the rest of the city regardless of the season. The record: 21.6C days feeling like 21C, 14C nights, just 2.5mm across 0.5 days, a 16.8C sea, 75 percent humidity and 11.2 hours of sun.

September and October are the actual summer — the fog retreats and it's 22°C and clear. June and July are the grey ones, whatever the latitude suggests. Always carry a layer; crossing from the Mission to Ocean Beach can cost you ten degrees.

San Francisco-forum threads on July fog are consistent: this is the deepest fog season of the year, especially near the water, and the pattern is for it to clear downtown and most neighborhoods by late morning before rolling back in during late afternoon - though how much any given day burns off depends entirely on which part of the city you're in. Posters are firm that this isn't something to plan around avoiding; it's part of what the city is in summer, and it doesn't stop sightseeing. The advice is uniform: long pants and a jacket, dressed in layers. The record explains why the fog wins: 19.1C days, 12.6C nights, essentially no precipitation across the whole month, and a 14.8C sea - San Francisco's summer is dry, cool and grey rather than the postcard version most visitors expect.

Forum posters describe March as one of the most changeable months in San Francisco - it could be warm, cold or rainy, with morning lows typically in the mid-to-upper 40sF and afternoon highs in the low-to-mid 60sF. Posters are consistent that any lingering rain is generally the tail end of the rainy season, with the major winter storms usually done by late March, though posters note it doesn't necessarily turn warm either - even in a typical year, only 2-3 days in the back half of the month might bring genuinely warm weather. Posters consistently emphasize real block-to-block variability, with temperatures feeling 5-10 degrees different between a sunny and shady side of the same street, and a 70F day cooling down to 50F by night. Posters generally recommend serious layers plus a lightweight raincoat regardless of the specific forecast.

San Francisco forum regulars describe February as storm-and-clear, not steady rain: winter here comes in “discrete storms lasting a few days followed by clear weather,” roughly 40% rainy at most, with shorter storms than January. Expect “lows in the 40s at night” and “50s during the day” — layers from polar fleece to a light winter jacket, plus an umbrella and windbreaker for when a storm rolls through. The upside regulars keep repeating: smaller crowds and very low rates. One practical warning for anyone adding Big Sur — Pacific storms this month can close stretches of Highway 1 to rock slides. Our record: 15.3°C days, 6.4°C nights, 5.8 rain days and an 11.2°C sea — cool and mostly dry, with real storm risk stacked into a handful of days rather than spread evenly.

Someone asked what time of day the fog rolls in during late December and got corrected, repeatedly: "fog is more of a summer phenomenon. It is more about clouds and rain in winter." One poster explains the mechanism — "fog is a function of the cold air over the Pacific being drawn to hot inland temps; in winter inland temps aren't hot, and ergo no typical fog" — so if December fog appears at all, it "usually doesn't roll in until around or after dark." What replaces it is storms, and the honest answer to any timing question in December is "it depends entirely on the storms, not time of day." Regulars suggest a Golden Gate webcam over a forecast. The other thing to plan for: "dark is pretty early, around 5pm." Our record: 12 rain days, 126mm, 9.6 daylight hours.