Climate averages 1991–2020
San Mateo weather
in September.
A warm, very dry month: the warmest and the 3rd driest of the year. Meets the pleasant-weather rule
Long-term climate patterns, not a forecast for a particular trip.
September is a warm month in San Mateo: daytime highs average 24.0°C and nights cool to about 14.2°C. By daytime temperature it is the warmest month of the year.
Expect about 1.8 mm of rain on fewer than one day, which makes September a very dry month (the 3rd driest of the 12 months). Daylight, from sunrise to sunset, lasts about 12 h 28 min on an average September day.
The nearest other city in this atlas is Redwood City (12 km away). In September its highs average 24.6°C, 0.7°C warmer than San Mateo, with 1.8 mm of rain.
Among the 498 cities listed in the United States, San Mateo ranks joint 356th of 498 for the warmest September highs and joint 17th of 498 for the driest September (rainfall, lowest first). The mean September high of those 498 cities is 26.9°C; San Mateo is 2.9°C cooler than that mean.
From September to October, average highs fall by 2.2°C to 21.7°C, and monthly rainfall rises from 1.8 mm to 23 mm.
The annual picture
September in context
Monthly rainfall
Average total · mmAverages from hourly ERA5 reanalysis data. Actual weather varies from year to year. How these numbers are calculated
Every month, side by side
San Mateo climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 13.9°C6.7°C | 6.7°C | 99 | 9.4 | 9.910 h |
| February | 14.8°C7.5°C | 7.5°C | 107 | 9.5 | 10.811 h |
| March | 16.1°C8.5°C | 8.5°C | 82 | 9.1 | 11.912 h |
| April | 17.4°C9.3°C | 9.3°C | 41 | 5.6 | 13.113 h |
| May | 19.3°C10.9°C | 10.9°C | 20 | 3.3 | 14.214 h |
| June | 21.8°C12.4°C | 12.4°C | 3.9 | 0.9 | 14.715 h |
| July | 23.2°C13.7°C | 13.7°C | 0.5 | 0.2 | 14.514 h |
| August | 23.6°C14.2°C | 14.2°C | 0.7 | 0.1 | 13.614 h |
| September, selected | 24.0°C14.2°C | 14.2°C | 1.8 | 0.4 | 12.512 h |
| October | 21.7°C12.9°C | 12.9°C | 23 | 3.0 | 11.311 h |
| November | 17.0°C9.9°C | 9.9°C | 51 | 6.0 | 10.210 h |
| December | 13.7°C7.2°C | 7.2°C | 94 | 9.3 | 9.610 h |
Rainfall is the average of the 30 separate monthly totals. A rain day is a local calendar day with at least 1 mm of precipitation. Daylight is the calculated sunrise-to-sunset length, not measured sunshine.
Compare places
September in nearby cities
- Redwood City in SeptemberUnited States · 12 kmHigh 24.6°C · Low 14.5°C · 1.8 mm rain
- Daly City in SeptemberUnited States · 20 kmHigh 20.1°C · Low 12.6°C · 1.5 mm rain
- Hayward in SeptemberUnited States · 25 kmHigh 26.5°C · Low 14.3°C · 1.7 mm rain
- San Francisco in SeptemberUnited States · 25 kmHigh 22.7°C · Low 13.0°C · 1.5 mm rain
- Union City in SeptemberUnited States · 27 kmHigh 27.4°C · Low 15.1°C · 1.9 mm rain
- Oakland in SeptemberUnited States · 27 kmHigh 26.2°C · Low 13.8°C · 1.4 mm rain
- Mountain View in SeptemberUnited States · 29 kmHigh 26.1°C · Low 15.0°C · 1.9 mm rain
- Fremont in SeptemberUnited States · 30 kmHigh 27.2°C · Low 15.4°C · 1.9 mm rain
Read the numbers correctly
Climate, not a promise.
These patterns help compare months. They cannot tell you the weather on a specific day of your trip.
Sources, method and limitsHow these numbers are made (what the highs, lows and rain days mean, the sources and the model cell used for San Mateo) is explained once on the San Mateo year overview and on the Sources, method and limits page.
Redwood City lies in the same rainfall cell, so its rainfall figures are identical to these; temperatures differ because they come from smaller 0.1° cells.