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SunnyDays

ERA5 almanac — 8 years of real data

Aspen in May

Aspen in May is historically a poor month for sun: only 3% of days clear across 8 years (n=248, avg high 59°F). August is the clearest month for Aspen 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 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 May

Clear-sky percentage for Aspen in May, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period % Clear days Sample (n) Avg high
Early May
1st–15th		 0% n=120 59°F
Late May
16th–end		 5% n=128 59°F
Full month average 3% n=248 59°F

Sunshine & warmth in May

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 May. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period Warm & sunny (≥72°F, >6 h sun) Sample (n)
Early May
1st–15th		 0% n=120
Late May
16th–end		 9% n=128
Full month average 5% n=248
13.6 h median sunshine per day 93% of days exceed 8 h sunshine 32% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 3%. Sunshine-based “warm & sunny”: 5%. The two measures are nearly identical here — no convective-storm bias for this climate and month.

Bottom line

Aspen is historically almost never clear in May: only 3% of days meet the clear bar across 8 years (n=248). Come back in August if you want clear skies here. Plan entirely around indoor or overcast-friendly activities.

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

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 ← this page 3% n=248 All destinations →
June 34% n=240 All destinations →
July 43% n=248 All destinations →
August 47% n=248 All destinations →
September 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.