ERA5 almanac comparison — 8 years of real data
Costa Rica — Guanacaste vs Sea Island: weather in September
Sea Island is the clear winner for sun in September: 32% of days historically clear versus just 2% in Costa Rica — Guanacaste — a 30-point gap across 8 years of ERA5 data. Costa Rica — Guanacaste also sees measurably more rain than Sea Island this month (92% vs 53% of days). This isn't close.
Costa Rica — Guanacaste
Costa Rica
- 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
Sea Island
GA
- Clear days, September
- 32%
- Sample size
- n=240 days
- Avg high
- 83°F
- Early September (1st–15th)
- 26%
- Late September (16th–end)
- 37%
- Rain frequency
- 53% of days
Frequently asked
Is Costa Rica — Guanacaste or Sea Island warmer in September?
Costa Rica — Guanacaste runs warmer by day in September. Costa Rica — Guanacaste averages highs of 88°F (highs n=240 days); Sea Island averages highs of 83°F (highs n=240 days) (ERA5 2018–2025). Historical averages, not a forecast.
Does it rain more in Costa Rica — Guanacaste or Sea Island in September?
Costa Rica — Guanacaste is measurably wetter than Sea Island. Days with measurable rain: Costa Rica — Guanacaste 92%, Sea Island 53% (n=240 and n=240 days). ERA5 2018–2025; historical, not a forecast.
Which is sunnier in September, Costa Rica — Guanacaste or Sea Island?
Sea Island is the clear winner for sun in September: 32% of days historically clear versus just 2% in Costa Rica — Guanacaste — a 30-point gap across 8 years of ERA5 data. Costa Rica — Guanacaste also sees measurably more rain than Sea Island this month (92% vs 53% of days). This isn't close.
Compare Costa Rica — Guanacaste vs Sea Island 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 forecast for any destination is on its own page and may differ from the long-run average. Data from Open-Meteo.