ERA5 almanac comparison — 8 years of real data
Aspen vs Big Sur: weather in September
Essentially a tie in September: Aspen is 39% clear historically, Big Sur is 35% clear (8-year ERA5 average) — within the noise. Avg high 72°F vs 72°F. Aspen also sees measurably more rain than Big Sur this month (23% vs 4% of days). Pick based on flight time or price, not weather — the weather won't decide this one for you.
Aspen
CO
- Clear days, September
- 39%
- Sample size
- n=240 days
- Avg high
- 72°F
- Early September (1st–15th)
- 52%
- Late September (16th–end)
- 26%
- Rain frequency
- 23% of days
Big Sur
CA
- Clear days, September
- 35%
- Sample size
- n=240 days
- Avg high
- 72°F
- Early September (1st–15th)
- 40%
- Late September (16th–end)
- 30%
- Rain frequency
- 4% of days
Compare Aspen vs Big Sur in another month
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.