South San Francisco, California Climate
The warmest month is September, with an average high of 72°F; the coldest is January, with an average low of 45°F. A normal year brings 31.9 in of precipitation.
Temperature through the year
The normal daily high and low and the record for every date at San Francisco International (SFO), since 1945. The line marks today.
- Normal low
- Normal high
- Record high
- Record low
Normals for Sep 27
- Normal high
- 72°F
- Normal low
- 53°F
- Record high
- 96°Fin 1958
- Record low
- 47°Fin 1948
- Chance of a wet day
- 7%
- Daylight
- 11 h 57 min
Month by month
Averages for 1991-2020. Wet days have at least 0.01 inch of precipitation. Freeze nights have a low of 32°F or colder; degree days are on a 65°F base. Each month links to its own page.
| Month | Avg high | Avg low | Record high | Record low | Precipitation | Wet days | Freeze nights | Heating DD | Cooling DD |
|---|---|---|---|---|---|---|---|---|---|
| January | 58°F | 45°F | 74°F (2021) | 26°F (1949) | 6.14 in | 10.8 | 0.3 | 412 | 0 |
| February | 60°F | 46°F | 78°F (2022) | 30°F (1950) | 6.48 in | 11.9 | 0.1 | 335 | 0 |
| March | 61°F | 46°F | 89°F (2026) | 31°F (1966) | 4.45 in | 9.7 | 0.0 | 350 | 2 |
| April | 64°F | 48°F | 92°F (1989) | 36°F (1955) | 2.36 in | 6.3 | 0.0 | 280 | 3 |
| May | 67°F | 49°F | 97°F (1984) | 39°F (1950) | 1.00 in | 4.3 | 0.0 | 221 | 9 |
| June | 69°F | 51°F | 105°F (2000) | 43°F (1954) | 0.25 in | 1.7 | 0.0 | 167 | 11 |
| July | 69°F | 52°F | 99°F (1985) | 44°F (1951) | 0.01 in | 0.3 | 0.0 | 140 | 12 |
| August | 71°F | 53°F | 99°F (2020) | 45°F (1954) | 0.08 in | 1.3 | 0.0 | 108 | 20 |
| September | 72°F | 53°F | 104°F (2017) | 41°F (1950) | 0.11 in | 1.6 | 0.0 | 94 | 29 |
| October | 70°F | 52°F | 99°F (1987) | 37°F (1949) | 1.50 in | 3.5 | 0.0 | 146 | 28 |
| November | 64°F | 50°F | 85°F (1967) | 31°F (1954) | 2.86 in | 6.7 | 0.0 | 248 | 5 |
| December | 59°F | 46°F | 75°F (1958) | 24°F (1972) | 6.64 in | 10.6 | 0.2 | 388 | 0 |
| Year | 65°F | 49°F | 31.88 in | 69 | 1 | 2,891 | 120 |
Precipitation
Rain and melted snow together. The chance of a wet day is the share of days with at least 0.01 inch of precipitation.
31.9 in in a normal year, most in December.
Chance of a wet day
- Chance of a wet day
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Pacifica 4 Sse. A median date has an even chance of a later freeze in spring, or an earlier one in fall.
A freeze (32°F or colder) happens in only 20 percent of years at Pacifica 4 Sse, so there is no typical freeze season.
- Median at 36°F
- last January 25, first January 4
- Median at 32°F
- last January 11, first January 10
Plant hardiness zone
2023 USDA Plant Hardiness Zone Map GIS data (USDA-ARS and Oregon State University’s PRISM Climate Group), at the town’s center on the map’s 800 m grid. Zones are 10°F apart, half zones 5°F.
South San Francisco is in USDA zone 10b 10b: the coldest night of the winter averages 35.4°F, within zone 10b’s 35 to 40°F.
- Zone
- 10b
- Average coldest night
- 35.4°F1991-2020
- Range of the half zone
- 35 to 40°F1.7 to 4.4°C
Growing degree days this year
Observed: NOAA GHCN-Daily highs and lows at San Francisco International (SFO) (USW00023234), through September 22, 2026; GHCN trails the calendar by a day or more. Normal: NOAA’s 1991-2020 daily normals at the same station, NCEI’s own for base 50°F and the corn method, and for bases 41°F and 32°F computed here from NCEI’s daily mean and its standard deviation (Thom’s method).
Since January 1, South San Francisco has gathered 2,897 growing degree days on a 50°F base, 405 more than normal by September 22, 2026: about 29 days ahead of the normal season.
- 2026, observed
- Normal
| Base | Since Jan 1 | Normal to date | Departure | Normal year |
|---|---|---|---|---|
| Base 50°F | 2,897 | 2,492 | +405 (+16%) | 3,286 |
| Corn, 86/50 | 2,978 | 2,662 | +316 (+12%) | 3,552 |
| Base 41°F | 5,265 | 4,831 | +434 (+9%) | 6,500 |
| Base 32°F | 7,650 | 7,216 | +434 (+6%) | 9,793 |
Base 50°F: The common base for warm-season crops, turf and most insect models; USA-NPN’s pest forecasts use it. Corn, 86/50: Modified growing degree days for corn: highs above 86°F count as 86 and lows below 50°F as 50 (Barger 1969; Purdue Extension). Base 41°F: 5°C, the base used for cool-season crops such as alfalfa and canola. Base 32°F: 0°C, the base USA-NPN uses for its accumulated temperature maps and for its hemlock woolly adelgid forecast. A day adds its mean temperature, (high + low) / 2, above the base; the corn method first holds the high and low between 50°F and 86°F. Corn typically needs 100 to 120 of those units from planting to emergence (Purdue Extension).
Humidity and comfort
NASA POWER reanalysis, 1991-2020, for the grid cell around South San Francisco.
Days with a mean dew point of 65°F or more are rare here in every month.
| Month | Dew point | Humidity | Dry | Comfortable | Humid | Muggy | Oppressive | Miserable |
|---|---|---|---|---|---|---|---|---|
| January | 46°F | 84% | 98% | 2% | 0% | 0% | 0% | 0% |
| February | 47°F | 83% | 98% | 2% | 0% | 0% | 0% | 0% |
| March | 48°F | 84% | 99% | 1% | 0% | 0% | 0% | 0% |
| April | 48°F | 84% | 99% | 1% | 0% | 0% | 0% | 0% |
| May | 50°F | 85% | 95% | 5% | 0% | 0% | 0% | 0% |
| June | 52°F | 85% | 91% | 9% | 0% | 0% | 0% | 0% |
| July | 54°F | 86% | 69% | 31% | 0% | 0% | 0% | 0% |
| August | 55°F | 86% | 43% | 56% | 1% | 0% | 0% | 0% |
| September | 55°F | 83% | 52% | 45% | 3% | 0% | 0% | 0% |
| October | 52°F | 80% | 75% | 25% | 1% | 0% | 0% | 0% |
| November | 49°F | 80% | 91% | 9% | 0% | 0% | 0% | 0% |
| December | 46°F | 81% | 95% | 5% | 0% | 0% | 0% | 0% |
Comfort is judged by the day's mean dew point: dry below 55°F, comfortable to 60°F, humid to 65°F, muggy to 70°F, oppressive to 75°F, miserable above.
Clouds
Average share of the sky covered, from satellite (CERES and GEWEX SRB) by way of NASA POWER, 1991-2020.
The clearest month is October, with the sky 47 percent covered on average; the cloudiest is February, at 61 percent.
Wind
The mean wind speed over the whole day, from the MERRA-2 reanalysis by way of NASA POWER. Gusts run well above it.
The windiest month is April, averaging 11.2 mph at 10 meters (33 feet) above the ground.
Daylight and sunshine
The longest day is Jun 20, with 14 h 47 min of daylight; the shortest is Dec 18, with 9 h 33 min.
- Day length
Solar energy
Average daily sunshine reaching the ground on a flat surface, in kilowatt-hours per square meter, clouds included.
How San Mateo County is running
NOAA NCEI Climate at a Glance, from the nClimGrid analysis of every station, updated early each month. A rank of 1 is the highest or lowest of all years since 1895.
August 2026 in San Mateo County: 65.0°F (+0.5 °F against 1991-2020), the 17th warmest in 132 years on record.
August 2026 in San Mateo County: 0.03 in (-0.02 in against 1991-2020), the 40th wettest in 132 years on record.
January to August 2026 in San Mateo County: 59.2°F (+2.1 °F against 1991-2020), the 3rd warmest in 132 years on record.
January to August 2026 in San Mateo County: 15.30 in (-3.38 in against 1991-2020), the 56th driest in 132 years on record.
- Warmest August
- 68.0°F in 2020
- Coolest August
- 58.5°F in 1911
- Wettest August
- 0.41 in in 1976
- Driest August
- 0.00 in in 1895
Every year since 1895
Month pages
The weather in South San Francisco in each month, day by day.
What this means
A normal is the average of the thirty years 1991 to 2020, the period the World Meteorological Organization and NOAA use for comparison. It is an average, not a forecast: most days depart from it, and some by a long way.
A record is the highest or lowest value on the date in every year the station has reported, back to 1945. It rests on one station, so a neighbor a few miles away can hold different ones.
Cloud, humidity, wind and sunshine come from a reanalysis: a weather model run over the past with every observation fed in, averaged over a grid cell about 50 km across. It is consistent everywhere on Earth; it smooths local detail such as a lake shore or a valley.
More for this place
Nearby
Sources
Normals: NOAA NCEI U.S. Climate Normals, 1991-2020, station USC00046599 (Pacifica 4 Sse), public domain. Records: NOAA GHCN-Daily, station USW00023234, every year on file, quality-flagged values removed. Cloud, humidity, wind and sunshine: NASA POWER (MERRA-2, CERES and GEWEX SRB), 1991-2020. The data was obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division: POWER Daily API v2, accessed 2026-09-24. Sun and moon: computed with the NOAA solar equations and Meeus. Rankings: NOAA NCEI Climate at a Glance, public domain.