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SunnyDays

ERA5 almanac comparison — 8 years of real data

Costa Rica — Guanacaste vs St. Barthélemy: weather in September

St. Barthélemy edges out Costa Rica — Guanacaste for sun in September: 13% clear historically versus 2% (8-year ERA5 average, a 11-point gap). Costa Rica — Guanacaste also sees measurably more rain than St. Barthélemy this month (92% vs 56% of days). Both are viable — St. Barthélemy is the safer bet.

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.

Costa Rica — Guanacaste

CR

Clear days, September
2%
Sample size
n=240 days
Avg high
88°F
Early September (1st–15th)
3%
Late September (16th–end)
2%
Rain frequency
92% of days

St. Barthélemy

BL

Clear days, September
13%
Sample size
n=240 days
Avg high
84°F
Early September (1st–15th)
14%
Late September (16th–end)
12%
Rain frequency
56% of days

Compare Costa Rica — Guanacaste vs St. Barthélemy 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.