September is one of the warmer months of the year in Ottawa, with average highs near 20.8°C and lows around 9.5°C (mean 15.2°C). It is about average for rainfall, averaging 85 mm of rainfall and 78% relative humidity across the period.
September is one of the warmer months of the year in Ottawa. Across 45 years of records, the warmest September in Ottawa was 2015 — when the month averaged 18.3°C — while the coldest was 1986 at 12.5°C. Typical September days reach 20.8°C with nights near 9.5°C. The long-term trend is warming, about 0.7°C per decade.
September is about average for rainfall in Ottawa, averaging 85 mm of rainfall. The wettest September on record was 2010 with 162 mm, while the driest was 2023 at 25 mm.
Relative humidity in Ottawa during September averages 78%. The most humid September was 1982 at 83%, and the least humid was 2025 at 69%.
September receives about 3.9 kWh/m²/day of solar energy in Ottawa. The sunniest September was 2016 at 4.5 kWh/m²/day, and the cloudiest was 1990 at 3.1.
Average wind speed in Ottawa during September is 0.3 m/s. The windiest Septembers were 2023, 2024 and 2025, among others at 0.3 m/s, while the calmest were 2023, 2024 and 2025, among others at 0.3 m/s.
Ranked against the other 11 months using 1981–2025 averages.
| Metric | 1980s | 1990s | 2000s | 2010s | 2020s | Change |
|---|---|---|---|---|---|---|
| Avg Temp | 14.1°C | 14.2°C | 15.5°C | 16.1°C | 16.2°C | +2.1°C |
| Avg High | 19.6°C | 19.7°C | 21.3°C | 21.7°C | 22.2°C | +2.6°C |
| Avg Low | 8.6°C | 8.7°C | 9.8°C | 10.3°C | 10.4°C | +1.8°C |
| Avg Rain | 92 mm | 88 mm | 92 mm | 86 mm | 57 mm | -35 mm |
September temperatures in Ottawa have risen by about 2.3°C between 1981–1985 and 2021–2025.
September has ranged from a record low of -2.2°C (1991) to a record high of 33.3°C (2002).
Rainfall in September is about average for rainfall, averaging 85 mm, with a wettest year of 162 mm.
Daytime highs and overnight lows have shifted differently — highs rose about 0.8°C per decade while lows rose 0.6°C per decade, widening the gap between day and night.
One of the best times to visit Ottawa
September is one of the best times to visit Ottawa. Expect days averaging 20.8°C and nights around 9.5°C (mean 15.2°C) — warmer than Ottawa's yearly average — with higher rainfall (85 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 | 23.4°C | 16.8°C | 10.2°C | 27.9°C | 3.0°C | 34 mm | 69% | 0.3 m/s | 4.4 |
| 2024 | 23.5°C | 17.5°C | 11.6°C | 28.4°C | 4.9°C | 56 mm | 77% | 0.3 m/s | 4.3 |
| 2023 | 23.8°C | 17.5°C | 11.2°C | 32.3°C | 5.8°C | 25 mm | 74% | 0.3 m/s | 4.3 |
| 2022 | 20.8°C | 15.1°C | 9.5°C | 28.3°C | 3.0°C | 69 mm | 77% | 0.3 m/s | 3.8 |
| 2021 | 21.8°C | 16.3°C | 10.9°C | 26.4°C | 5.5°C | 105 mm | 76% | 0.3 m/s | 3.8 |
| 2020 | 19.8°C | 14.2°C | 8.7°C | 26.1°C | 1.3°C | 56 mm | 77% | 0.3 m/s | 3.6 |
| 2019 | 21.0°C | 15.1°C | 9.1°C | 26.5°C | 4.1°C | 81 mm | 78% | 0.3 m/s | 3.9 |
| 2018 | 22.2°C | 16.8°C | 11.4°C | 30.2°C | 4.0°C | 56 mm | 78% | 0.3 m/s | 3.8 |
| 2017 | 22.6°C | 17.1°C | 11.7°C | 29.5°C | 5.0°C | 61 mm | 81% | 0.3 m/s | 4.3 |
| 2016 | 23.3°C | 17.2°C | 11.1°C | 32.3°C | 3.4°C | 54 mm | 72% | 0.3 m/s | 4.5 |
| 2015 🔥 | 24.4°C | 18.3°C | 12.1°C | 31.9°C | 5.2°C | 85 mm | 76% | 0.3 m/s | 4.2 |
| 2014 | 20.7°C | 14.8°C | 8.9°C | 27.8°C | -1.1°C | 79 mm | 80% | 0.3 m/s | 3.8 |
| 2013 | 19.8°C | 14.1°C | 8.5°C | 28.1°C | 3.0°C | 94 mm | 78% | 0.3 m/s | 3.9 |
| 2012 | 21.2°C | 15.2°C | 9.1°C | 28.7°C | 2.7°C | 121 mm | 74% | 0.3 m/s | 3.8 |
| 2011 | 22.0°C | 16.6°C | 11.1°C | 26.8°C | 3.1°C | 67 mm | 80% | 0.3 m/s | 4 |
| 2010 🌧️ | 20.2°C | 15.3°C | 10.4°C | 31.8°C | 5.2°C | 162 mm | 82% | 0.3 m/s | 3.3 |
| 2009 | 20.0°C | 14.1°C | 8.3°C | 24.8°C | 2.4°C | 59 mm | 78% | 0.3 m/s | 4.4 |
| 2008 | 21.5°C | 15.3°C | 9.2°C | 31.1°C | 1.4°C | 83 mm | 80% | 0.3 m/s | 3.8 |
| 2007 | 21.9°C | 15.8°C | 9.7°C | 30.5°C | 2.7°C | 50 mm | 75% | 0.3 m/s | 4.4 |
| 2006 | 19.2°C | 13.9°C | 8.7°C | 24.9°C | 2.2°C | 137 mm | 82% | 0.3 m/s | 3.3 |
| 2005 | 22.6°C | 16.7°C | 10.8°C | 29.8°C | 3.5°C | 128 mm | 77% | 0.3 m/s | 4.1 |
| 2004 | 21.7°C | 16.0°C | 10.4°C | 27.3°C | 4.3°C | 118 mm | 80% | 0.3 m/s | 4.2 |
| 2003 | 22.4°C | 16.7°C | 11.0°C | 28.5°C | 5.8°C | 92 mm | 76% | 0.3 m/s | 4 |
| 2002 | 23.8°C | 17.6°C | 11.4°C | 33.3°C | 4.4°C | 71 mm | 74% | 0.3 m/s | 4.3 |
| 2001 | 21.4°C | 15.8°C | 10.3°C | 30.9°C | 4.1°C | 92 mm | 75% | 0.3 m/s | 4.1 |
| 2000 | 18.7°C | 13.3°C | 7.9°C | 27.5°C | -2.1°C | 94 mm | 80% | 0.3 m/s | 3.6 |
| 1999 | 22.4°C | 16.9°C | 11.3°C | 32.3°C | 4.1°C | 133 mm | 77% | 0.3 m/s | 3.5 |
| 1998 | 21.3°C | 15.5°C | 9.7°C | 27.7°C | 3.4°C | 73 mm | 76% | 0.3 m/s | 3.9 |
| 1997 | 19.6°C | 14.2°C | 8.8°C | 26.3°C | 0.5°C | 78 mm | 77% | 0.3 m/s | 3.4 |
| 1996 | 21.0°C | 15.9°C | 10.9°C | 28.4°C | 3.8°C | 113 mm | 80% | 0.3 m/s | 3.7 |
| 1995 | 18.8°C | 12.6°C | 6.3°C | 26.7°C | 1.2°C | 64 mm | 73% | 0.3 m/s | 4 |
| 1994 | 19.8°C | 14.3°C | 8.8°C | 26.6°C | 4.0°C | 55 mm | 80% | 0.3 m/s | 3.8 |
| 1993 | 18.2°C | 12.8°C | 7.4°C | 26.6°C | -0.4°C | 106 mm | 78% | 0.3 m/s | 3.4 |
| 1992 | 19.1°C | 13.6°C | 8.2°C | 25.6°C | 0.0°C | 96 mm | 81% | 0.3 m/s | 3.4 |
| 1991 | 18.4°C | 13.0°C | 7.5°C | 27.2°C | -2.2°C | 67 mm | 74% | 0.3 m/s | 3.8 |
| 1990 | 18.5°C | 13.0°C | 7.6°C | 26.0°C | 1.2°C | 93 mm | 80% | 0.3 m/s | 3.1 |
| 1989 | 20.6°C | 14.8°C | 9.0°C | 27.0°C | -1.1°C | 84 mm | 77% | 0.3 m/s | 3.7 |
| 1988 | 19.5°C | 13.7°C | 7.9°C | 25.1°C | 0.6°C | 76 mm | 77% | 0.3 m/s | 3.9 |
| 1987 | 19.6°C | 14.5°C | 9.4°C | 26.8°C | 1.1°C | 110 mm | 80% | 0.3 m/s | 3.8 |
| 1986 ❄️ | 17.8°C | 12.5°C | 7.3°C | 23.3°C | 1.5°C | 146 mm | 81% | 0.3 m/s | 3.3 |
| 1985 | 20.9°C | 15.1°C | 9.2°C | 28.5°C | 4.2°C | 65 mm | 77% | 0.3 m/s | 4.1 |
| 1984 | 18.7°C | 13.0°C | 7.4°C | 24.8°C | 0.7°C | 29 mm | 78% | 0.3 m/s | 4.4 |
| 1983 | 22.0°C | 16.1°C | 10.2°C | 30.7°C | 3.9°C | 67 mm | 75% | 0.3 m/s | — |
| 1982 | 19.5°C | 14.0°C | 8.5°C | 26.3°C | 3.0°C | 108 mm | 83% | 0.3 m/s | — |
| 1981 | 18.0°C | 13.4°C | 8.7°C | 24.9°C | -1.6°C | 146 mm | 82% | 0.3 m/s | — |
Actual recorded September data for each year, 1981–2025 — sourced from NASA POWER.
The average high in September is around 20.8°C with lows near 9.5°C (mean 15.2°C), based on 45 years of NASA POWER data.
September averages 85 mm of rainfall in Ottawa, about average for rainfall. The wettest on record was 162 mm in 2010.
The hottest September reached 33.3°C in 2002. The coldest dipped to -2.2°C in 1991.
Comparing the earliest years on record (1981–1985) with the most recent (2021–2025), average September temperatures have risen by about 2.3°C. Daytime highs moved from 19.8°C to 22.7°C and overnight lows from 8.8°C to 10.7°C over the same span.
September is one of the warmer months of the year in Ottawa and about average for rainfall, so plan for around 20.8°C daytime highs and 9.5°C at night.
Not quite — September is the 9th coolest month in Ottawa, averaging 15.2°C.
For September in Ottawa, pack light layers you can adjust through the day, with typical highs of 20.8°C and lows of 9.5°C. Bring an umbrella or light rain jacket too.
Relative humidity in September averages 78% — quite humid.
Average wind speed in September is 0.3 m/s. The windiest September on record averaged 0.3 m/s (1981).
The wettest September on record was 2010 with 162 mm. On average, September sees 85 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 Ottawa 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.
