Climate averages 1991–2020
Ottawa weather
in September.
A mild, wet month: the 4th warmest and the 9th driest of the year.
Long-term climate patterns, not a forecast for a particular trip.
September is a mild month in Ottawa: daytime highs average 20.4°C and nights cool to about 12.0°C. By daytime temperature it is the 4th warmest of the 12 months.
Expect about 95 mm of rain on roughly 10 days, which makes September a wet month (the 9th driest of the 12 months). Daylight, from sunrise to sunset, lasts about 12 h 35 min on an average September day.
The nearest other city in this atlas is Gatineau (7 km away). In September its highs average 20.3°C, about as warm as Ottawa, with 95 mm of rain.
Among the 86 cities listed in Canada, Ottawa ranks joint 31st of 86 for the warmest September highs and joint 60th of 86 for the driest September (rainfall, lowest first). The mean September high of those 86 cities is 19.8°C; Ottawa is 0.6°C warmer than that mean.
From September to October, average highs fall by 7.6°C to 12.8°C, and monthly rainfall rises from 95 mm to 99 mm.
The annual picture
September in context
Monthly precipitation
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
Ottawa climate table
| Month | Average highHigh / low | Average low | Precipitation mm/month | Wet days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | -5.0°C-14.2°C | -14.2°C | 73 | 10.5 | 9.29 h |
| February | -3.3°C-12.9°C | -12.9°C | 58 | 8.8 | 10.410 h |
| March | 2.5°C-7.0°C | -7.0°C | 73 | 9.9 | 11.912 h |
| April | 10.7°C0.6°C | 0.6°C | 90 | 11.1 | 13.513 h |
| May | 18.6°C7.9°C | 7.9°C | 88 | 11.2 | 14.915 h |
| June | 23.4°C13.5°C | 13.5°C | 99 | 11.6 | 15.616 h |
| July | 25.8°C16.5°C | 16.5°C | 98 | 12.4 | 15.315 h |
| August | 24.6°C16.0°C | 16.0°C | 90 | 11.1 | 14.114 h |
| September, selected | 20.4°C12.0°C | 12.0°C | 95 | 10.3 | 12.613 h |
| October | 12.8°C5.6°C | 5.6°C | 99 | 12.2 | 11.011 h |
| November | 5.5°C-0.8°C | -0.8°C | 83 | 11.4 | 9.610 h |
| December | -1.1°C-7.9°C | -7.9°C | 79 | 11.5 | 8.89 h |
Precipitation (rain or snow, as water) is the average of the 30 separate monthly totals. A wet 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
- Gatineau in SeptemberCanada · 7 kmHigh 20.3°C · Low 12.2°C · 95 mm rain
- Gloucester, Canada in SeptemberCanada · 8 kmHigh 20.3°C · Low 11.8°C · 90 mm rain
- Nepean in SeptemberCanada · 9 kmHigh 20.4°C · Low 11.8°C · 90 mm rain
- Orléans, Canada in SeptemberCanada · 16 kmHigh 20.0°C · Low 11.6°C · 96 mm rain
- Kanata in SeptemberCanada · 21 kmHigh 20.4°C · Low 11.8°C · 87 mm rain
- Pierrefonds-Roxboro in SeptemberCanada · 145 kmHigh 20.4°C · Low 12.6°C · 92 mm rain
- Kingston, Canada in SeptemberCanada · 145 kmHigh 20.2°C · Low 14.8°C · 95 mm rain
- Saint-Louis-de-Terrebonne in SeptemberCanada · 152 kmHigh 20.0°C · Low 11.9°C · 94 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 Ottawa) is explained once on the Ottawa year overview and on the Sources, method and limits page.
Gatineau lies in the same rainfall cell, so its rainfall figures are identical to these; temperatures differ because they come from smaller 0.1° cells.