San Francisco weather in September
In September, San Francisco has daytime highs averaging 23 °C, nights around 13 °C, with 0.4 wet days, and a sea temperature of 14 °C. Figures are 1991–2020 climate normals.
- 22.7°C Average high
- 13.2°C Average low
- 0.4days Wet days
- 2mm Rainfall
- 14.4°C Sea temperature
- 12h Daylight
Daily temperatures through September
How September develops
Averages for the first, middle and last part of the month, from daily 1991–2020 normals. Chance of rain is the share of years in which that calendar day recorded more than 1 mm.
| Days | Avg high | Avg low | Chance of rain |
|---|---|---|---|
| 1–10 September | 23.0 °C | 13.4 °C | 1 % |
| 11–20 September | 22.1 °C | 13.2 °C | 2 % |
| 21–30 September | 23.0 °C | 13.0 °C | 2 % |
What to expect
| Measure | Typical September | Share of the month |
|---|---|---|
| Days above 25 °C | 7.0 of 30 | 23 % |
| Days above 30 °C | 1.3 of 30 | 4 % |
| Days below 0 °C | 0 of 30 | 0 % |
| Days with more than 1 mm rain | 0.4 of 30 | 1 % |
Conditions in September
| Measure | Average |
|---|---|
| Mean temperature | 17 °C |
| Record high (1991–2020) | 39 °C |
| Record low (1991–2020) | 9.3 °C |
| Relative humidity | 75 % |
| Wind speed | 18 km/h |
| Cloud cover | 33 % |
Where San Francisco ranks in September
| Ranking | Position | Value |
|---|---|---|
| The driest places in North America in September | 2nd of 50 | 1.7 mm |
| Places with the best weather in North America in September | 3rd of 50 | 94/100 |
September compared with the months around it
| Month | Avg high | Rainfall | Cloud |
|---|---|---|---|
| July | 21.4 °C | under 1 mm | 38 % |
| August | 21.9 °C | 1 mm | 39 % |
| September | 22.7 °C | 2 mm | 33 % |
| October | 21.2 °C | 21 mm | 38 % |
| November | 16.9 °C | 46 mm | 51 % |
See all twelve months for San Francisco
- September is the warmest month in San Francisco.
- It is the 3rd driest month of the year.
- It ranks 1st for least cloud cover.
- Days last about 12 hours from sunrise to sunset.
- It rains on 0.4 of 30 days — 1 % of the month.
- Nights are about 9.5 °C cooler than the middle of the day.
- The hottest September day on record since 1991 reached 39 °C, 16 °C above the usual daytime high.
- Beach score: 56 / 100 — see how this is calculated.
QuestionsSeptember in San Francisco, answered
How warm is San Francisco in September?
Daily highs average 23 °C and nights fall to about 13 °C, a spread of 9.5 °C between afternoon and dawn. That makes September the 1st warmest month of the year here.
Does it rain much in San Francisco in September?
September averages 2 mm of rain, spread over about 0 of the 30 days — 1 % of the month. Cloud cover averages 33 % of the sky.
Is the sea warm enough to swim in September?
The sea averages 14 °C off San Francisco in September. Most people find water below 20 °C bracing and above 25 °C comfortable for a long swim; this figure is a monthly surface average, so a sheltered bay on a still afternoon can be warmer.
How long are the days in September?
San Francisco averages 12 hours between sunrise and sunset in September. Day length is computed from latitude and date, so it is exact rather than modelled — but it says nothing about how much of that time is sunny.
Data sources and method
- ERA5 / ERA5-Land reanalysis — Copernicus Climate Change Service (C3S), ECMWF. Copernicus Licence
- Open-Meteo — Open-Meteo.com. CC BY 4.0
- NOAA OISST v2.1 — NOAA Physical Sciences Laboratory. Public domain
- GeoNames — GeoNames.org. CC BY 4.0
- Normal period
- 1991–2020
- Dataset version
- era5-1991-2020-scores1.1
- Last processed
Measured values come from ERA5 reanalysis and are reproduced unchanged. Calculated by us: daylight hours, beach and comfort scores, monthly rankings, threshold day counts, chance of a wet day by date, ten-day averages within the month. Full formulas are on the methodology page.
Contains modified Copernicus Climate Change Service information 2026. Neither the European Commission nor ECMWF is responsible for any use that may be made of the Copernicus information or data it contains.