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SunnyDays

ERA5 almanac — 8 years of real data

Denver in September

Denver in September: 15% of days see measurable rain, 84°F average high, 74% of days historically clear (n=240, ERA5 2018–2025).

Historically one of the better months to visit Denver for sun: 74% of days are clear on average (n=240, avg high 84°F). Every percentage here is a historical average from 8 years of ERA5 data — 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 Denver 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 Denver 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		 70% n=120 84°F
Late September
16th–end		 78% n=120 84°F
Full month average 74% n=240 84°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 Denver 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		 93% n=120
Late September
16th–end		 91% n=120
Full month average 92% n=240
11.8 h median sunshine per day 94% of days exceed 8 h sunshine 15% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 74%. Sunshine-based “warm & sunny”: 92%. The 18-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

Denver clears solidly in September: 74% of days historically (n=240), avg high 84°F. Most trips here in September will find more sun than cloud.

Every month in Denver

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

Historical clear-sky % for Denver, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 2% n=248 All destinations →
April 22% n=240 All destinations →
May 33% n=248 All destinations →
June 56% n=240 All destinations →
July 52% n=248 All destinations →
August 59% n=248 All destinations →
September ← this page 74% n=240 All destinations →
October 38% n=248 All destinations →
November 6% n=240 All destinations →
December 1% n=248 All destinations →

Frequently asked

Does it rain in Denver in September?

Rarely. Only 15% of days in Denver see measurable rain in September, based on 8 years of ERA5 historical data (n=240 days, 2018–2025). This is a historical average, not a forecast.

Is Denver good for sun in September?

Yes. Denver is historically strong for sun in September: 74% of days are clear on average (avg high 84°F, n=240 days, ERA5 2018–2025).

How Denver compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Denver 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.