ERA5 almanac — 8 years of real data

Costa Rica — Guanacaste in December

Costa Rica — Guanacaste in December: rain frequency data not yet backfilled for this month, 89°F average high, 62% of days historically clear (n=248, ERA5 2018–2025).

Costa Rica — Guanacaste in December is a mixed month for sun: 62% of days historically clear (n=248, avg high 89°F). The outcome varies year to year — check the early/late split below. 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 December

Clear-sky percentage for Costa Rica — Guanacaste in December, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period	% Clear days	Sample (n)	Avg high
Early December 1st–15th	64%	n=120	89°F
Late December 16th–end	61%	n=128	89°F
Full month average	62%	n=248	89°F

Bottom line

Costa Rica — Guanacaste clears solidly in December: 62% of days historically (n=248), avg high 89°F. Most trips here in December will find more sun than cloud.

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	2%	n=240	All destinations →
October	2%	n=248	All destinations →
November	18%	n=240	All destinations →
December ← this page	62%	n=248	All destinations →

Frequently asked

Is Costa Rica — Guanacaste good for sun in December?

Yes. Costa Rica — Guanacaste is historically strong for sun in December: 62% of days are clear on average (avg high 89°F, n=248 days, ERA5 2018–2025).

→ Costa Rica — Guanacaste live forecast — this week’s actual 7-day weather, not historical
→ Best destinations in December — all destinations ranked by December clear-sky %
→ All destinations
→ Raw data (JSON) — buckets, n, source

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.