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SunnyDays

ERA5 almanac — 8 years of real data

Montréal in September

Montréal in September: 30% of days see measurable rain, 70°F average high, 22% of days historically clear (n=240, ERA5 2018–2025).

Montréal in September is historically a poor month for sun: only 22% of days clear across 8 years (n=240, avg high 70°F). July is the clearest month for Montréal historically. Historical average, not a forecast.

What you are reading: 8-year historical averages from the ERA5 reanalysis, not a forecast. A month that is historically 90% clear can still be overcast on your specific trip. For this week’s actual forecast, see the live Montréal page. Both can be true at once: “historically strong in this month; this particular week is not.” We label which is which.

Early vs. Late September

Clear-sky percentage for Montréal in September, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period % Clear days Sample (n) Avg high
Early September
1st–15th		 28% n=120 70°F
Late September
16th–end		 17% n=120 70°F
Full month average 22% n=240 70°F

Sunshine & warmth in September

A second measure using actual sunshine hours (ERA5 sunshine_duration), not the daily weather code. A Lowcountry day with 12 hours of sun and a 25-minute afternoon storm is counted below as a warm, sunny day — because it was. How we define this.

Sunshine-based metrics for Montréal in September. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period Warm & sunny (≥72°F, >6 h sun) Sample (n)
Early September
1st–15th		 51% n=120
Late September
16th–end		 25% n=120
Full month average 38% n=240
10.8 h median sunshine per day 74% of days exceed 8 h sunshine 30% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 22%. Sunshine-based “warm & sunny”: 38%. The 16-point gap is from brief afternoon convective storms that stamp the whole day as “rain” in the WMO code but leave the bulk of the sunshine untouched.

Bottom line

Montréal is mostly overcast in September historically: only 22% of days are clear across 8 years (n=240), avg high 70°F. If you want sun in Montréal, July is historically much better. Most visitors this month will see more clouds than sun.

→ See all destinations in July — historically the best month for sun in Montréal.

Every month in Montréal

Compare across all months with data. Numbers are historical clear-sky %.

Historical clear-sky % for Montréal, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 0% n=248 All destinations →
April 2% n=240 All destinations →
May 17% n=248 All destinations →
June 31% n=240 All destinations →
July 35% n=248 All destinations →
August 31% n=248 All destinations →
September ← this page 22% n=240 All destinations →
October 4% n=248 All destinations →
November 0% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Montréal in September?

Sometimes. Montréal sees measurable rain on 30% of days in September historically (n=240 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Montréal good for sun in September?

Not usually. Montréal is historically overcast in September: only 22% of days are clear (avg high 70°F, n=240 days, ERA5 2018–2025). July is historically far better for sun here.

How Montréal compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Montréal this month — same passport tier, similar flight distance, both sides from the same 8-year ERA5 record.

All percentages are historical averages from the ERA5 reanalysis via Open-Meteo, 8 full years (2018–2025). “Clear” means avg high ≥ 72 °F, precipitation < 1 mm, and a non-precipitation weather code. This is history, not a forecast. The live 7-day outlook for any destination is on its own page and may differ from the long-run average. Data from Open-Meteo.