Weather in San Francisco in December

Shoulder season. December is shoulder season in San Francisco — cheaper, quieter, and a genuine gamble. Highs sit at 13.9 °C and the day is short; this is an indoors-with-excursions month, not an outdoors-all-day one.

December ranks 12 of 12 — near the bottom of San Francisco's year. July is the month that works, and the swing between the two is 46 points, which is a different holiday rather than a slightly better one.

This is wet-season rain: 12 wet days and 5 genuinely heavy ones, which in this climate means concentrated downpours rather than all-day grey. The pattern is usually a clear morning and a loud afternoon, so front-load the day and stop fighting the 3pm hour. Layers, and a genuinely waterproof outer — at 13.9 °C the difference between a good day and a miserable one is entirely down to what you are wearing.

What the averages hide

The averages here are honest — no hidden extremes worth planning around. Across the twenty years measured, the hottest December day reached 20.6 °C and the coldest night fell to -0.3 °C, and the wettest single day dropped 95 mm — roughly 69% of the month's rain in one day.

December is moderately variable: 3 mm in the driest year against 318 mm in the wettest, around an average of 138 mm. Roughly two Decembers in three land near that average, and the third does something else entirely.

Wind is light at 8.1 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 82%, which at 13.9 °C means damp rather than muggy — the kind of cold that gets into a coat and stays there.

How December sits in the year

Temperatures hold steady either side: 17.8 °C in November, 13.9 here, 13.9 in January. Nothing about the timing within the month matters much. Rain has already shifted: 5 days more than November had.

Daylight in December

December sits at the dark end of San Francisco's year — 9.6 hours of daylight against 14.8 at midsummer, holding steady all month. Sightseeing days are genuinely shorter, and an itinerary built for summer will not fit.

Sea temperature

The sea sits at about 13.2 °C in December — too cold to swim without a wetsuit. The sea peaks at 15.7 °C in September and bottoms out at 12.2 °C — water lags the air by a month or two, which is why the best swimming often comes after the best weather.

How busy it gets

Off season. The genuine off season: cheapest by a distance, with the caveat that some tours and restaurants close entirely.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

December in San Francisco versus nearby

If San Francisco is one option among several, the nearby comparison is worth making. Napa is close enough to be a coin toss — within a degree on temperature, the same number of rain days. San Jose is close enough to be a coin toss — within a degree on temperature, 1 day less rain.

So: December is a compromise. July is 46 points better and is what San Francisco is actually like at its best — worth moving the trip for if that is possible, and worth adjusting expectations for if it is not.

Every month in San Francisco

See all twelve months in San Francisco, or where else to go in December.