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SunnyDays

ERA5 almanac — 8 years of real data

Costa Rica — Guanacaste in December

Costa Rica — Guanacaste in December: 15% of days see measurable rain, 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

Sunshine & warmth in December

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 December. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period Warm & sunny (≥72°F, >6 h sun) Sample (n)
Early December
1st–15th		 98% n=120
Late December
16th–end		 98% n=128
Full month average 98% n=248
11.0 h median sunshine per day 97% of days exceed 8 h sunshine 15% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 62%. Sunshine-based “warm & sunny”: 98%. The 36-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 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

Does it rain in Costa Rica — Guanacaste in December?

Rarely. Only 15% of days in Costa Rica — Guanacaste see measurable rain in December, based on 8 years of ERA5 historical data (n=248 days, 2018–2025). This is a historical average, not a forecast.

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).

How Costa Rica — Guanacaste compares in December

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Costa Rica — Guanacaste this month — same passport tier, similar flight distance, both sides from the same 8-year ERA5 record.

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.