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SunnyDays

ERA5 almanac — 8 years of real data

Tokyo in September

Tokyo in September: 48% of days see measurable rain, 83°F average high, 26% of days historically clear (n=240, ERA5 2018–2025).

Tokyo in September is historically a poor month for sun: only 26% of days clear across 8 years (n=240, avg high 83°F). May is the clearest month for Tokyo 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 Tokyo 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 Tokyo 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		 25% n=120 83°F
Late September
16th–end		 26% n=120 83°F
Full month average 26% n=240 83°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 Tokyo 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		 71% n=120
Late September
16th–end		 69% n=120
Full month average 70% n=240
9.2 h median sunshine per day 60% of days exceed 8 h sunshine 48% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 26%. Sunshine-based “warm & sunny”: 70%. The 44-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

Tokyo is mostly overcast in September historically: only 26% of days are clear across 8 years (n=240), avg high 83°F. If you want sun in Tokyo, May is historically much better. Most visitors this month will see more clouds than sun.

→ See all destinations in May — historically the best month for sun in Tokyo.

Every month in Tokyo

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

Historical clear-sky % for Tokyo, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 3% n=248 All destinations →
April 12% n=240 All destinations →
May 32% n=248 All destinations →
June 27% n=240 All destinations →
July 27% n=248 All destinations →
August 29% n=248 All destinations →
September ← this page 26% n=240 All destinations →
October 21% n=248 All destinations →
November 2% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Tokyo in September?

Sometimes. Tokyo sees measurable rain on 48% of days in September historically (n=240 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Tokyo good for sun in September?

Not usually. Tokyo is historically overcast in September: only 26% of days are clear (avg high 83°F, n=240 days, ERA5 2018–2025). May is historically far better for sun here.

How Tokyo compares in September

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