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SunnyDays

ERA5 almanac — 8 years of real data

Aspen in September

Aspen in September is historically a poor month for sun: only 39% of days clear across 8 years (n=240, avg high 72°F). 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 Aspen 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 Aspen 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		 52% n=120 72°F
Late September
16th–end		 26% n=120 72°F
Full month average 39% n=240 72°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 Aspen 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		 68% n=120
Late September
16th–end		 31% n=120
Full month average 50% n=240
11.9 h median sunshine per day 96% of days exceed 8 h sunshine 23% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 39%. Sunshine-based “warm & sunny”: 50%. The 11-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

Aspen is mostly overcast in September historically: only 39% of days are clear across 8 years (n=240), avg high 72°F. The two halves differ materially (52% early, 26% late) — favour the early half of the month. Most visitors this month will see more clouds than sun.

Every month in Aspen

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

Historical clear-sky % for Aspen, 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 0% n=240 All destinations →
May 3% n=248 All destinations →
June 34% n=240 All destinations →
July 43% n=248 All destinations →
August 47% n=248 All destinations →
September ← this page 39% n=240 All destinations →
October 2% n=248 All destinations →
November 0% n=240 All destinations →
December 0% n=248 All destinations →

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.