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SunnyDays

ERA5 almanac — 8 years of real data

Hartford in September

Hartford in September: 30% of days see measurable rain, 75°F average high, 38% of days historically clear (n=240, ERA5 2018–2025).

Hartford in September is historically a poor month for sun: only 38% of days clear across 8 years (n=240, avg high 75°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 Hartford 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 Hartford 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		 40% n=120 75°F
Late September
16th–end		 36% n=120 75°F
Full month average 38% n=240 75°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 Hartford 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		 65% n=120
Late September
16th–end		 48% n=120
Full month average 56% n=240
11.2 h median sunshine per day 68% of days exceed 8 h sunshine 30% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 38%. Sunshine-based “warm & sunny”: 56%. The 18-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

Hartford is mostly overcast in September historically: only 38% of days are clear across 8 years (n=240), avg high 75°F. Most visitors this month will see more clouds than sun.

Every month in Hartford

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

Historical clear-sky % for Hartford, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 1% n=248 All destinations →
April 3% n=240 All destinations →
May 22% n=248 All destinations →
June 39% n=240 All destinations →
July 34% n=248 All destinations →
August 40% n=248 All destinations →
September ← this page 38% n=240 All destinations →
October 11% n=248 All destinations →
November 2% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Hartford in September?

Sometimes. Hartford sees measurable rain on 30% of days in September historically (n=240 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Hartford good for sun in September?

It's mixed. Hartford in September is clear on 38% of days historically (avg high 75°F, n=240 days, ERA5 2018–2025) — the outcome varies year to year.

How Hartford compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Hartford this month — same passport tier, similar flight distance, both sides from the same 8-year ERA5 record.

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.