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SunnyDays

ERA5 almanac — 8 years of real data

Kauai in September

Kauai in September: 96% of days see measurable rain, 75°F average high, 0% of days historically clear (n=240, ERA5 2018–2025).

Kauai in September is historically a poor month for sun: only 0% of days clear across 8 years (n=240, avg high 75°F). January is the clearest month for Kauai 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 Kauai 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 Kauai 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		 1% n=120 75°F
Late September
16th–end		 0% n=120 75°F
Full month average 0% n=240 75°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 Kauai 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		 92% n=120
Late September
16th–end		 88% n=120
Full month average 90% n=240
11.5 h median sunshine per day 97% of days exceed 8 h sunshine 96% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 0%. Sunshine-based “warm & sunny”: 90%. The 90-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

Kauai is historically almost never clear in September: only 0% 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 Kauai.

Every month in Kauai

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

Historical clear-sky % for Kauai, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 5% n=248 All destinations →
February 2% n=226 All destinations →
March 2% n=248 All destinations →
April 1% n=240 All destinations →
May 0% n=248 All destinations →
June 2% n=240 All destinations →
July 0% n=248 All destinations →
August 0% n=248 All destinations →
September ← this page 0% n=240 All destinations →
October 1% n=248 All destinations →
November 2% n=240 All destinations →
December 4% n=248 All destinations →

Frequently asked

Does it rain in Kauai in September?

Yes, frequently. In Kauai, 96% 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 Kauai good for sun in September?

Not usually. Kauai is historically overcast in September: only 0% of days are clear (avg high 75°F, n=240 days, ERA5 2018–2025). January 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.