ERA5 almanac comparison — 8 years of real data
Hartford vs Yellowstone: weather in September
Hartford is the clear winner for sun in September: 38% of days historically clear versus just 8% in Yellowstone — a 30-point gap across 8 years of ERA5 data. This isn't close.
What you are reading: 8-year historical averages, not a forecast.
Either destination can still surprise you this specific September. For this
week’s actual forecast, each destination links to its live page below.
Hartford
CT
- Clear days, September
- 38%
- Sample size
- n=240 days
- Avg high
- 75°F
- Early September (1st–15th)
- 40%
- Late September (16th–end)
- 36%
Yellowstone
WY
- Clear days, September
- 8%
- Sample size
- n=240 days
- Avg high
- 62°F
- Early September (1st–15th)
- 16%
- Late September (16th–end)
- 0%
Compare Hartford vs Yellowstone 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.