September is one of the warmer months of the year in San Mateo, with average highs near 19.3°C and lows around 13.6°C (mean 16.5°C). It is one of the drier months, averaging 8 mm of rainfall and 83% relative humidity across the period.
September is one of the warmer months of the year in San Mateo. Across 45 years of records, the warmest September in San Mateo was 2017 — when the month averaged 18.9°C — while the coldest was 2012 at 14.4°C. Typical September days reach 19.3°C with nights near 13.6°C. The long-term trend is warming, about 0.2°C per decade.
September is one of the drier months in San Mateo, averaging 8 mm of rainfall. The wettest September on record was 1989 with 37 mm, while the driest was 2020 at 0 mm.
Relative humidity in San Mateo during September averages 83%. The most humid September was 2012 at 88%, and the least humid was 1984 at 73%.
September receives about 5 kWh/m²/day of solar energy in San Mateo. The sunniest September was 1997 at 5.9 kWh/m²/day, and the cloudiest was 1991 at 4.1.
Average wind speed in San Mateo during September is 3 m/s. The windiest September was 2019 at 3.7 m/s, while the calmest was 1991 at 2.4 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 16.3°C | 16.3°C | 16.1°C | 16.8°C | 17.1°C | +0.8°C |
| Avg High | 19.3°C | 19.2°C | 19.1°C | 19.6°C | 19.6°C | +0.3°C |
| Avg Low | 13.2°C | 13.5°C | 13.2°C | 14.1°C | 14.5°C | +1.3°C |
| Avg Rain | 15 mm | 4 mm | 7 mm | 5 mm | 6 mm | -9 mm |
September temperatures in San Mateo have stayed relatively stable across the record.
September has ranged from a record low of 10.3°C (1985) to a record high of 34.4°C (2017).
Rainfall in September is one of the drier months, averaging 8 mm, with a wettest year of 37 mm.
One of the best times to visit San Mateo
September is one of the best times to visit San Mateo. Expect days averaging 19.3°C and nights around 13.6°C (mean 16.5°C) — warmer than San Mateo's yearly average — with low rainfall (8 mm), so conditions are generally comfortable for sightseeing and outdoor plans.
| Year | Avg High | Avg Temp | Avg Low | Hottest Day | Coldest Day | Rainfall | Humidity | Wind | Solar |
|---|---|---|---|---|---|---|---|---|---|
| 2025 | 20.1°C | 17.9°C | 15.6°C | 28.8°C | 14.1°C | 11 mm | 86% | 2.9 m/s | 4.7 |
| 2024 | 19.3°C | 16.6°C | 13.8°C | 27.3°C | 12.6°C | 2 mm | 85% | 2.9 m/s | 5.1 |
| 2023 | 18.7°C | 16.6°C | 14.4°C | 20.6°C | 13.2°C | 3 mm | 86% | 3.5 m/s | 5 |
| 2022 | 20.1°C | 17.5°C | 14.8°C | 26.1°C | 13.2°C | 20 mm | 84% | 3.3 m/s | 5.2 |
| 2021 | 18.9°C | 16.3°C | 13.7°C | 25.0°C | 12.5°C | 3 mm | 83% | 3.2 m/s | 5 |
| 2020 | 20.4°C | 17.5°C | 14.6°C | 27.9°C | 12.8°C | 0 mm | 84% | 2.8 m/s | 4.9 |
| 2019 | 20.8°C | 17.9°C | 15.0°C | 30.4°C | 11.8°C | 6 mm | 82% | 3.7 m/s | 5.7 |
| 2018 | 18.4°C | 15.7°C | 13.1°C | 23.8°C | 12.1°C | 1 mm | 85% | 3.4 m/s | 5.2 |
| 2017 🔥 | 22.0°C | 18.9°C | 15.8°C | 34.4°C | 13.2°C | 4 mm | 78% | 3.1 m/s | 5 |
| 2016 | 19.3°C | 16.4°C | 13.6°C | 30.1°C | 12.1°C | 1 mm | 82% | 3.4 m/s | 5.3 |
| 2015 | 20.9°C | 17.9°C | 15.0°C | 29.3°C | 13.4°C | 4 mm | 78% | 3 m/s | 5.2 |
| 2014 | 19.9°C | 17.7°C | 15.5°C | 24.9°C | 13.7°C | 11 mm | 86% | 2.8 m/s | 4.9 |
| 2013 | 19.5°C | 16.9°C | 14.4°C | 26.2°C | 11.5°C | 12 mm | 84% | 3.4 m/s | 5.4 |
| 2012 ❄️ | 16.7°C | 14.4°C | 12.0°C | 24.4°C | 11.1°C | 3 mm | 88% | 2.8 m/s | 4.9 |
| 2011 | 18.7°C | 16.0°C | 13.2°C | 28.3°C | 11.9°C | 3 mm | 86% | 2.5 m/s | 4.9 |
| 2010 | 19.4°C | 16.2°C | 13.0°C | 28.8°C | 11.7°C | 2 mm | 83% | 2.9 m/s | 5.3 |
| 2009 | 19.2°C | 16.4°C | 13.6°C | 24.8°C | 10.9°C | 9 mm | 86% | 3 m/s | 5.2 |
| 2008 | 19.2°C | 16.2°C | 13.2°C | 25.4°C | 11.6°C | 2 mm | 81% | 2.6 m/s | 5.1 |
| 2007 | 19.0°C | 16.4°C | 13.8°C | 25.5°C | 12.0°C | 11 mm | 81% | 3.2 m/s | 5.2 |
| 2006 | 18.3°C | 15.3°C | 12.4°C | 25.6°C | 10.8°C | 2 mm | 84% | 3 m/s | 4.7 |
| 2005 | 18.0°C | 15.1°C | 12.3°C | 26.4°C | 11.7°C | 5 mm | 85% | 3.1 m/s | 4.8 |
| 2004 | 20.0°C | 16.8°C | 13.6°C | 29.8°C | 11.6°C | 3 mm | 79% | 3.2 m/s | 5.7 |
| 2003 | 20.3°C | 17.2°C | 14.0°C | 28.5°C | 12.2°C | 1 mm | 80% | 3 m/s | 5.1 |
| 2002 | 18.8°C | 15.9°C | 13.1°C | 27.4°C | 11.5°C | 1 mm | 83% | 2.7 m/s | 5.3 |
| 2001 | 17.8°C | 15.2°C | 12.6°C | 30.0°C | 11.5°C | 6 mm | 87% | 2.9 m/s | 4.7 |
| 2000 | 20.2°C | 16.9°C | 13.6°C | 29.9°C | 12.0°C | 35 mm | 82% | 2.8 m/s | 5.2 |
| 1999 | 18.3°C | 15.6°C | 12.8°C | 27.1°C | 10.7°C | 5 mm | 83% | 2.5 m/s | 4.6 |
| 1998 | 18.5°C | 16.0°C | 13.6°C | 23.1°C | 12.4°C | 15 mm | 85% | 3.1 m/s | 4.9 |
| 1997 | 21.5°C | 18.5°C | 15.5°C | 30.5°C | 14.5°C | 3 mm | 81% | 3.3 m/s | 5.9 |
| 1996 | 18.3°C | 15.4°C | 12.5°C | 22.1°C | 11.0°C | 4 mm | 84% | 3 m/s | 4.9 |
| 1995 | 18.6°C | 15.9°C | 13.2°C | 23.5°C | 12.2°C | 2 mm | 87% | 3.2 m/s | 5.1 |
| 1994 | 19.1°C | 16.3°C | 13.5°C | 26.9°C | 12.5°C | 4 mm | 86% | 2.9 m/s | 4.8 |
| 1993 | 18.9°C | 15.9°C | 12.9°C | 24.7°C | 10.6°C | 1 mm | 83% | 3.1 m/s | 4.9 |
| 1992 | 19.4°C | 16.4°C | 13.4°C | 27.1°C | 11.7°C | 1 mm | 83% | 3 m/s | 5.1 |
| 1991 | 18.4°C | 15.7°C | 12.9°C | 24.3°C | 11.7°C | 3 mm | 87% | 2.4 m/s | 4.1 |
| 1990 | 20.5°C | 17.4°C | 14.3°C | 27.0°C | 13.3°C | 6 mm | 84% | 3.3 m/s | 5.4 |
| 1989 🌧️ | 18.4°C | 15.7°C | 13.0°C | 22.8°C | 12.2°C | 37 mm | 85% | 2.7 m/s | 4.7 |
| 1988 | 18.8°C | 15.3°C | 11.9°C | 24.1°C | 11.1°C | 1 mm | 80% | 3 m/s | 4.7 |
| 1987 | 19.3°C | 16.3°C | 13.3°C | 23.4°C | 11.9°C | 1 mm | 82% | 3.3 m/s | 4.5 |
| 1986 | 17.9°C | 15.4°C | 12.8°C | 22.8°C | 11.6°C | 27 mm | 83% | 3.3 m/s | 4.9 |
| 1985 | 18.7°C | 15.9°C | 13.0°C | 24.2°C | 10.3°C | 14 mm | 81% | 2.8 m/s | 4.9 |
| 1984 | 21.4°C | 17.8°C | 14.1°C | 26.1°C | 11.4°C | 3 mm | 73% | 3.2 m/s | 5.6 |
| 1983 | 21.2°C | 18.2°C | 15.1°C | 29.5°C | 13.1°C | 22 mm | 80% | 3 m/s | — |
| 1982 | 18.9°C | 15.9°C | 12.9°C | 21.9°C | 11.5°C | 25 mm | 81% | 3.6 m/s | — |
| 1981 | 19.0°C | 16.0°C | 13.0°C | 23.1°C | 11.5°C | 8 mm | 83% | 3.3 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 19.3°C with lows near 13.6°C (mean 16.5°C), based on 45 years of NASA POWER data.
September averages 8 mm of rainfall in San Mateo, one of the drier months. The wettest on record was 37 mm in 1989.
The hottest September reached 34.4°C in 2017. The coldest dipped to 10.3°C in 1985.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 0.2°C — but days and nights have moved in opposite directions. Overnight lows rose from 13.6°C to 14.5°C while daytime highs eased from 19.8°C to 19.4°C, narrowing the gap between day and night.
September is one of the warmer months of the year in San Mateo and one of the drier months, so plan for around 19.3°C daytime highs and 13.6°C at night.
Not quite — September is the 12th coolest month in San Mateo, averaging 16.5°C.
For September in San Mateo, pack light layers you can adjust through the day, with typical highs of 19.3°C and lows of 13.6°C.
Relative humidity in September averages 83% — quite humid.
Average wind speed in September is 3 m/s. The windiest September on record averaged 3.7 m/s (2019).
The wettest September on record was 1989 with 37 mm. On average, September sees 8 mm.
All figures are derived from NASA POWER (Prediction Of Worldwide Energy Resources), based on MERRA-2 satellite reanalysis, covering 1981–2025.
Explore how the weather in San Mateo behaves each September, based on 45 years of NASA POWER climate data from 1981 to 2025. This page breaks down average temperatures, rainfall, humidity, wind and solar radiation for September specifically, along with year-by-year records and how the month has changed over the decades.
