ERA5 almanac — 8 years of real data

Cayman Islands in September

Cayman Islands in September: rain frequency data not yet backfilled for this month, 88°F average high, 4% of days historically clear (n=240, ERA5 2018–2025).

Cayman Islands in September is historically a poor month for sun: only 4% of days clear across 8 years (n=240, avg high 88°F). January is the clearest month for Cayman Islands 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 Cayman Islands 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 Cayman Islands 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	5%	n=120	88°F
Late September 16th–end	2%	n=120	88°F
Full month average	4%	n=240	88°F

Bottom line

Cayman Islands is historically almost never clear in September: only 4% of days meet the clear bar across 8 years (n=240). Come back in January if you want clear skies here. Plan entirely around indoor or overcast-friendly activities.

→ See all destinations in January — historically the best month for sun in Cayman Islands.

Every month in Cayman Islands

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

Historical clear-sky % for Cayman Islands, all months, ERA5 2018–2025.
Month	% Clear	Sample (n)
January	19%	n=248	All destinations →
February	11%	n=226	All destinations →
March	17%	n=248	All destinations →
April	10%	n=240	All destinations →
May	5%	n=248	All destinations →
June	4%	n=240	All destinations →
July	2%	n=248	All destinations →
August	4%	n=248	All destinations →
September ← this page	4%	n=240	All destinations →
October	5%	n=248	All destinations →
November	12%	n=240	All destinations →
December	12%	n=248	All destinations →

Frequently asked

Is Cayman Islands good for sun in September?

Not usually. Cayman Islands is historically overcast in September: only 4% of days are clear (avg high 88°F, n=240 days, ERA5 2018–2025). January is historically far better for sun here.

→ Cayman Islands live forecast — this week’s actual 7-day weather, not historical
→ Best destinations in September — all destinations ranked by September 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.