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SunnyDays

ERA5 almanac — 8 years of real data

Vail in September

Vail in September is historically a poor month for sun: only 23% of days clear across 8 years (n=240, avg high 70°F). August is the clearest month for Vail 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 Vail 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 Vail 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		 38% n=120 70°F
Late September
16th–end		 8% n=120 70°F
Full month average 23% 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 Vail 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		 48% n=120
Late September
16th–end		 10% n=120
Full month average 29% n=240
11.9 h median sunshine per day 97% of days exceed 8 h sunshine 22% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 23%. Sunshine-based “warm & sunny”: 29%.

Bottom line

Vail is mostly overcast in September historically: only 23% of days are clear across 8 years (n=240), avg high 70°F. The two halves differ materially (38% early, 8% late) — favour the early half of the month. If you want sun in Vail, August is historically much better. Most visitors this month will see more clouds than sun.

→ See all destinations in August — historically the best month for sun in Vail.

Every month in Vail

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

Historical clear-sky % for Vail, 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 0% n=248 All destinations →
June 30% n=240 All destinations →
July 36% n=248 All destinations →
August 46% n=248 All destinations →
September ← this page 23% n=240 All destinations →
October 0% 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.