ERA5 almanac comparison — 8 years of real data
Costa Rica — Guanacaste vs St. Martin: weather in September
St. Martin edges out Costa Rica — Guanacaste for sun in September: 15% clear historically versus 2% (8-year ERA5 average, a 13-point gap). Costa Rica — Guanacaste also sees measurably more rain than St. Martin this month (92% vs 54% of days). Both are viable — St. Martin 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. Martin
SX
- Clear days, September
- 15%
- Sample size
- n=240 days
- Avg high
- 80°F
- Early September (1st–15th)
- 18%
- Late September (16th–end)
- 12%
- Rain frequency
- 54% of days
Compare Costa Rica — Guanacaste vs St. Martin 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.