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SunnyDays

ERA5 almanac — 8 years of real data

Costa Rica — Guanacaste in September

Costa Rica — Guanacaste in September: 92% of days see measurable rain, 88°F average high, 2% of days historically clear (n=240, ERA5 2018–2025).

Costa Rica — Guanacaste in September is historically a poor month for sun: only 2% of days clear across 8 years (n=240, avg high 88°F). February is the clearest month for Costa Rica — Guanacaste 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 Costa Rica — Guanacaste 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 Costa Rica — Guanacaste 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		 3% n=120 88°F
Late September
16th–end		 2% n=120 88°F
Full month average 2% n=240 88°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 Costa Rica — Guanacaste 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		 93% n=120
Late September
16th–end		 83% n=120
Full month average 88% n=240
8.8 h median sunshine per day 66% of days exceed 8 h sunshine 92% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 2%. Sunshine-based “warm & sunny”: 88%. The 86-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

Costa Rica — Guanacaste is historically almost never clear in September: only 2% of days meet the clear bar across 8 years (n=240). Come back in February if you want clear skies here. Plan entirely around indoor or overcast-friendly activities.

→ See all destinations in February — historically the best month for sun in Costa Rica — Guanacaste.

Every month in Costa Rica — Guanacaste

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

Historical clear-sky % for Costa Rica — Guanacaste, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 79% n=248 All destinations →
February 84% n=226 All destinations →
March 70% n=248 All destinations →
April 33% n=240 All destinations →
May 9% n=248 All destinations →
June 5% n=240 All destinations →
July 16% n=248 All destinations →
August 6% n=248 All destinations →
September ← this page 2% n=240 All destinations →
October 2% n=248 All destinations →
November 18% n=240 All destinations →
December 62% n=248 All destinations →

Frequently asked

Does it rain in Costa Rica — Guanacaste in September?

Yes, frequently. In Costa Rica — Guanacaste, 92% 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 Costa Rica — Guanacaste good for sun in September?

Not usually. Costa Rica — Guanacaste is historically overcast in September: only 2% of days are clear (avg high 88°F, n=240 days, ERA5 2018–2025). February 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.