September is one of the warmer months of the year in Riverside, with average highs near 32.4°C and lows around 17°C (mean 24.7°C). It is one of the drier months, averaging 8 mm of rainfall and 43% relative humidity across the period.
September is one of the warmer months of the year in Riverside. Across 45 years of records, the warmest September in Riverside was 2020 — when the month averaged 27.8°C — while the coldest was 1986 at 20°C. Typical September days reach 32.4°C with nights near 17°C. The long-term trend is warming, about 0.4°C per decade.
September is one of the drier months in Riverside, averaging 8 mm of rainfall. The wettest September on record was 1983 with 47 mm, while the driest were 1993, 2018 and 2020, among others at 0 mm.
Relative humidity in Riverside during September averages 43%. The most humid September was 2023 at 62%, and the least humid was 2020 at 28%.
September receives about 5.8 kWh/m²/day of solar energy in Riverside. The sunniest September was 2014 at 6.3 kWh/m²/day, and the cloudiest was 2023 at 5.1.
Average wind speed in Riverside during September is 1.7 m/s. The windiest Septembers were 1985 and 1986 at 1.9 m/s, while the calmest were 2011 and 2020 at 1.4 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 23.8°C | 24.5°C | 24.9°C | 25.1°C | 25.5°C | +1.7°C |
| Avg High | 31.4°C | 32.2°C | 32.9°C | 32.8°C | 32.8°C | +1.4°C |
| Avg Low | 16.1°C | 16.7°C | 16.9°C | 17.4°C | 18.1°C | +2°C |
| Avg Rain | 17 mm | 7 mm | 4 mm | 6 mm | 6 mm | -11 mm |
September temperatures in Riverside have risen by about 0.7°C between 1981–1985 and 2021–2025.
September has ranged from a record low of 6.4°C (1982) to a record high of 45.3°C (2020).
Rainfall in September is one of the drier months, averaging 8 mm, with a wettest year of 47 mm.
One of the best times to visit Riverside
September is one of the best times to visit Riverside. Expect days averaging 32.4°C and nights around 17°C (mean 24.7°C) — warmer than Riverside'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 | 31.6°C | 24.8°C | 18.0°C | 37.6°C | 13.7°C | 12 mm | 54% | 1.8 m/s | 5.7 |
| 2024 | 33.4°C | 25.8°C | 18.3°C | 43.9°C | 11.6°C | 2 mm | 42% | 1.6 m/s | 5.8 |
| 2023 | 28.9°C | 22.4°C | 15.8°C | 36.7°C | 12.4°C | 6 mm | 62% | 1.7 m/s | 5.1 |
| 2022 | 33.7°C | 26.6°C | 19.6°C | 42.1°C | 11.2°C | 16 mm | 48% | 1.7 m/s | 5.7 |
| 2021 | 33.1°C | 25.3°C | 17.5°C | 40.3°C | 11.2°C | 2 mm | 41% | 1.6 m/s | 5.9 |
| 2020 🔥 | 35.9°C | 27.8°C | 19.6°C | 45.3°C | 15.6°C | 0 mm | 28% | 1.4 m/s | 5.9 |
| 2019 | 32.2°C | 24.5°C | 16.9°C | 39.2°C | 7.8°C | 3 mm | 45% | 1.7 m/s | 6 |
| 2018 | 33.1°C | 25.1°C | 17.0°C | 39.2°C | 14.2°C | 0 mm | 38% | 1.6 m/s | 6.1 |
| 2017 | 31.2°C | 23.9°C | 16.6°C | 42.1°C | 9.8°C | 4 mm | 50% | 1.8 m/s | 5.7 |
| 2016 | 31.4°C | 23.6°C | 15.9°C | 38.4°C | 10.0°C | 3 mm | 42% | 1.8 m/s | 5.9 |
| 2015 | 32.7°C | 25.6°C | 18.5°C | 39.2°C | 11.8°C | 29 mm | 49% | 1.6 m/s | 6 |
| 2014 | 33.4°C | 26.2°C | 18.9°C | 40.3°C | 12.5°C | 4 mm | 44% | 1.7 m/s | 6.3 |
| 2013 | 32.8°C | 24.9°C | 17.0°C | 38.3°C | 9.0°C | 4 mm | 39% | 1.6 m/s | 6.1 |
| 2012 | 35.0°C | 27.2°C | 19.4°C | 38.4°C | 14.6°C | 3 mm | 33% | 1.5 m/s | 6.1 |
| 2011 | 33.0°C | 25.2°C | 17.4°C | 39.7°C | 11.7°C | 5 mm | 38% | 1.4 m/s | 5.5 |
| 2010 | 33.5°C | 25.0°C | 16.4°C | 41.6°C | 9.9°C | 2 mm | 36% | 1.6 m/s | 5.9 |
| 2009 | 34.6°C | 26.3°C | 18.1°C | 39.9°C | 9.2°C | 1 mm | 36% | 1.6 m/s | 6.2 |
| 2008 | 34.2°C | 26.1°C | 18.1°C | 39.0°C | 12.2°C | 1 mm | 40% | 1.5 m/s | 5.9 |
| 2007 | 30.5°C | 23.0°C | 15.6°C | 41.7°C | 8.5°C | 10 mm | 46% | 1.8 m/s | 6.1 |
| 2006 | 32.6°C | 24.6°C | 16.6°C | 40.0°C | 8.5°C | 4 mm | 34% | 1.6 m/s | 6.1 |
| 2005 | 30.4°C | 22.6°C | 14.7°C | 36.3°C | 10.0°C | 8 mm | 43% | 1.7 m/s | 6.1 |
| 2004 | 32.6°C | 24.6°C | 16.6°C | 39.9°C | 10.2°C | 2 mm | 41% | 1.7 m/s | 6.2 |
| 2003 | 34.7°C | 26.4°C | 18.2°C | 39.8°C | 12.8°C | 1 mm | 39% | 1.5 m/s | 5.8 |
| 2002 | 33.8°C | 25.8°C | 17.7°C | 42.0°C | 10.9°C | 2 mm | 36% | 1.7 m/s | 5.6 |
| 2001 | 33.5°C | 25.4°C | 17.3°C | 39.0°C | 12.7°C | 2 mm | 41% | 1.6 m/s | 5.9 |
| 2000 | 32.3°C | 24.1°C | 15.9°C | 40.8°C | 11.3°C | 8 mm | 43% | 1.7 m/s | 5.7 |
| 1999 | 31.7°C | 23.8°C | 15.9°C | 37.4°C | 11.9°C | 3 mm | 45% | 1.6 m/s | 5.7 |
| 1998 | 29.3°C | 22.6°C | 15.9°C | 38.1°C | 10.3°C | 13 mm | 57% | 1.7 m/s | 5.4 |
| 1997 | 33.4°C | 25.9°C | 18.4°C | 38.0°C | 12.9°C | 36 mm | 46% | 1.6 m/s | 5.8 |
| 1996 | 31.2°C | 23.3°C | 15.4°C | 36.2°C | 11.2°C | 0 mm | 48% | 1.7 m/s | 5.8 |
| 1995 | 33.5°C | 25.6°C | 17.8°C | 38.5°C | 9.5°C | 2 mm | 40% | 1.7 m/s | 6 |
| 1994 | 31.8°C | 24.0°C | 16.3°C | 37.7°C | 10.6°C | 0 mm | 42% | 1.6 m/s | 5.7 |
| 1993 | 32.5°C | 24.1°C | 15.8°C | 39.5°C | 8.0°C | 0 mm | 38% | 1.6 m/s | 5.6 |
| 1992 | 33.7°C | 25.6°C | 17.6°C | 37.7°C | 11.1°C | 0 mm | 36% | 1.5 m/s | 6 |
| 1991 | 32.4°C | 25.0°C | 17.6°C | 37.4°C | 10.5°C | 7 mm | 46% | 1.5 m/s | 5.5 |
| 1990 | 32.6°C | 24.6°C | 16.6°C | 40.7°C | 11.9°C | 4 mm | 41% | 1.7 m/s | 6 |
| 1989 | 32.3°C | 24.1°C | 16.0°C | 38.7°C | 9.5°C | 13 mm | 37% | 1.8 m/s | 5.9 |
| 1988 | 32.5°C | 24.2°C | 15.8°C | 41.2°C | 9.8°C | 2 mm | 40% | 1.7 m/s | 5.3 |
| 1987 | 32.4°C | 24.4°C | 16.5°C | 39.4°C | 11.7°C | 5 mm | 42% | 1.6 m/s | 5.5 |
| 1986 ❄️ | 27.6°C | 20.0°C | 12.4°C | 38.9°C | 7.6°C | 30 mm | 49% | 1.9 m/s | 5.6 |
| 1985 | 28.3°C | 21.0°C | 13.6°C | 34.9°C | 9.8°C | 12 mm | 50% | 1.9 m/s | 5.5 |
| 1984 | 34.5°C | 27.1°C | 19.6°C | 41.2°C | 12.8°C | 11 mm | 43% | 1.7 m/s | 5.3 |
| 1983 🌧️ | 32.2°C | 25.1°C | 18.0°C | 39.0°C | 11.0°C | 47 mm | 48% | 1.6 m/s | — |
| 1982 | 30.2°C | 23.1°C | 15.9°C | 40.3°C | 6.4°C | 27 mm | 46% | 1.8 m/s | — |
| 1981 | 32.9°C | 25.0°C | 17.0°C | 37.5°C | 11.5°C | 7 mm | 35% | 1.6 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 32.4°C with lows near 17°C (mean 24.7°C), based on 45 years of NASA POWER data.
September averages 8 mm of rainfall in Riverside, one of the drier months. The wettest on record was 47 mm in 1983.
The hottest September reached 45.3°C in 2020. The coldest dipped to 6.4°C in 1982.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 0.7°C. Daytime highs moved from 31.6°C to 32.1°C and overnight lows from 16.8°C to 17.8°C over the same span.
September is one of the warmer months of the year in Riverside and one of the drier months, so plan for around 32.4°C daytime highs and 17°C at night.
Not quite — September is the 10th coolest month in Riverside, averaging 24.7°C.
For September in Riverside, pack light, breathable clothing and sun protection, with typical highs of 32.4°C and lows of 17°C.
Relative humidity in September averages 43% — relatively dry.
Average wind speed in September is 1.7 m/s. The windiest September on record averaged 1.9 m/s (1985).
The wettest September on record was 1983 with 47 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 Riverside 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.
