Weather in Ottawa in September

A good month. September is a solid month to be in Ottawa. Highs average 21.9 °C, lows 10.4 °C, and rain falls on 12 days — nothing in the month is working against you.

September is the highest-scoring month of the twelve. The nearest rival is August, and the month at the other end of the year is December.

Rain on 12 days, 3 of them heavy. Enough to need a plan B, not enough to need a plan around it. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

The averages here are honest — no hidden extremes worth planning around. Across the twenty years measured, the hottest September day reached 33.6 °C and the coldest night fell to -0.8 °C, and the wettest single day dropped 98 mm — roughly 113% of the month's rain in one day.

September is statistically steady but not small: the year-to-year variation is modest in relative terms, yet the wettest September on record still delivered 162 mm against 50 mm in the driest. In a month this wet, even a tame spread is a lot of rain in absolute terms — the 87 mm average is a reasonable planning figure, not a promise.

Wind is light at 1.1 km/h, so the temperature on the page is close to the temperature on your skin, and humidity runs at 78%, high enough to notice on the warm days without dominating them.

How September sits in the year

The month is on the way down: 3.5 °C cooler than August, with October dropping another 8.4 °C. The first week and the last are not the same trip.

Daylight in September

12.6 hours of daylight, losing 1.6 hours across the month. That is between Ottawa's extremes of 15.6 and 8.8 hours, so days are long enough not to constrain the plan.

An inland climate

Ottawa has no sea to moderate it, and it shows: 42.4 °C separates the hottest afternoon of the year from the coldest night. That is a genuinely continental range, and it means summer and winter here are barely the same place.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Ottawa versus nearby

If Ottawa is one option among several, the nearby comparison is worth making. Kingston is close enough to be a coin toss — within a degree on temperature, 1 day more rain. Montréal is close enough to be a coin toss — within a degree on temperature, 1 day less rain.

So: September is the answer for Ottawa. If it is booked out or priced out, August is the next best thing at 4 points behind, and December is the month to keep off the shortlist.

Every month in Ottawa

See all twelve months in Ottawa, or where else to go in September.