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SunnyDays

ERA5 almanac — 8 years of real data

St. Barthélemy in September

St. Barthélemy in September: 56% of days see measurable rain, 84°F average high, 13% of days historically clear (n=240, ERA5 2018–2025).

St. Barthélemy in September is historically a poor month for sun: only 13% of days clear across 8 years (n=240, avg high 84°F). March is the clearest month for St. Barthélemy 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 St. Barthélemy 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 St. Barthélemy 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 84°F
Late September
16th–end		 12% n=120 84°F
Full month average 13% n=240 84°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 St. Barthélemy 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		 97% n=120
Late September
16th–end		 97% n=120
Full month average 97% n=240
11.9 h median sunshine per day 92% of days exceed 8 h sunshine 56% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 13%. Sunshine-based “warm & sunny”: 97%. The 84-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

St. Barthélemy is a poor choice for sun in September: just 13% 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 St. Barthélemy.

Every month in St. Barthélemy

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

Historical clear-sky % for St. Barthélemy, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 18% n=248 All destinations →
February 12% n=226 All destinations →
March 29% n=248 All destinations →
April 22% n=240 All destinations →
May 21% n=248 All destinations →
June 19% n=240 All destinations →
July 10% n=248 All destinations →
August 14% n=248 All destinations →
September ← this page 13% n=240 All destinations →
October 20% n=248 All destinations →
November 19% n=240 All destinations →
December 18% n=248 All destinations →

Frequently asked

Does it rain in St. Barthélemy in September?

Yes, frequently. In St. Barthélemy, 56% of days in September see measurable rain, based on 8 years of ERA5 historical data (n=240 days, 2018–2025). This is a historical average, not a forecast.

Is St. Barthélemy good for sun in September?

Not usually. St. Barthélemy is historically overcast in September: only 13% of days are clear (avg high 84°F, n=240 days, ERA5 2018–2025). March is historically far better for sun here.

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.