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SunnyDays

ERA5 almanac — 8 years of real data

Key West in September

Key West in September is historically a poor month for sun: only 11% of days clear across 8 years (n=240, avg high 85°F). March is the clearest month for Key West 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 Key West 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 Key West 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		 14% n=120 85°F
Late September
16th–end		 8% n=120 85°F
Full month average 11% n=240 85°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 Key West 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		 94% n=120
Late September
16th–end		 93% n=120
Full month average 94% n=240
11.5 h median sunshine per day 88% of days exceed 8 h sunshine 64% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 11%. Sunshine-based “warm & sunny”: 94%. The 83-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

Key West is a poor choice for sun in September: just 11% of days clear historically (n=240). March is the historically strongest month for sun here. The 8-year record is consistent — this is not a fluke bad stretch.

→ See all destinations in March — historically the best month for sun in Key West.

Every month in Key West

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

Historical clear-sky % for Key West, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 27% n=248 All destinations →
February 37% n=226 All destinations →
March 45% n=248 All destinations →
April 44% n=240 All destinations →
May 33% n=248 All destinations →
June 14% n=240 All destinations →
July 17% n=248 All destinations →
August 13% n=248 All destinations →
September ← this page 11% n=240 All destinations →
October 13% n=248 All destinations →
November 30% n=240 All destinations →
December 41% 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.