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SunnyDays

ERA5 almanac — 8 years of real data

Honolulu in September

Honolulu in September: 26% of days see measurable rain, 85°F average high, 24% of days historically clear (n=240, ERA5 2018–2025).

Honolulu in September is historically a poor month for sun: only 24% of days clear across 8 years (n=240, avg high 85°F). January is the clearest month for Honolulu 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 Honolulu 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 Honolulu 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		 28% n=120 85°F
Late September
16th–end		 20% n=120 85°F
Full month average 24% n=240 85°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 Honolulu 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		 99% n=120
Late September
16th–end		 100% n=120
Full month average 100% n=240
11.8 h median sunshine per day 98% of days exceed 8 h sunshine 26% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 24%. Sunshine-based “warm & sunny”: 100%. The 76-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

Honolulu is mostly overcast in September historically: only 24% of days are clear across 8 years (n=240), avg high 85°F. If you want sun in Honolulu, January is historically much better. Most visitors this month will see more clouds than sun.

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

Every month in Honolulu

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

Historical clear-sky % for Honolulu, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 36% n=248 All destinations →
February 24% n=226 All destinations →
March 22% n=248 All destinations →
April 14% n=240 All destinations →
May 21% n=248 All destinations →
June 30% n=240 All destinations →
July 20% n=248 All destinations →
August 31% n=248 All destinations →
September ← this page 24% n=240 All destinations →
October 24% n=248 All destinations →
November 26% n=240 All destinations →
December 33% n=248 All destinations →

Frequently asked

Does it rain in Honolulu in September?

Sometimes. Honolulu sees measurable rain on 26% of days in September historically (n=240 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Honolulu good for sun in September?

Not usually. Honolulu is historically overcast in September: only 24% of days are clear (avg high 85°F, n=240 days, ERA5 2018–2025). January is historically far better for sun here.

How Honolulu compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Honolulu 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.