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SunnyDays

ERA5 almanac — 8 years of real data

Tucson in September

Historically one of the better months to visit Tucson for sun: 70% of days are clear on average (n=240, avg high 96°F). Every percentage here is a historical average from 8 years of ERA5 data — 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 Tucson 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 Tucson 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		 72% n=120 96°F
Late September
16th–end		 69% n=120 96°F
Full month average 70% n=240 96°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 Tucson 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		 97% n=120
Full month average 98% n=240
11.8 h median sunshine per day 96% of days exceed 8 h sunshine 20% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 70%. Sunshine-based “warm & sunny”: 98%. The 28-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

Tucson clears solidly in September: 70% of days historically (n=240), avg high 96°F. Most trips here in September will find more sun than cloud.

Every month in Tucson

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

Historical clear-sky % for Tucson, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 13% n=248 All destinations →
February 38% n=226 All destinations →
March 57% n=248 All destinations →
April 88% n=240 All destinations →
May 94% n=248 All destinations →
June 89% n=240 All destinations →
July 57% n=248 All destinations →
August 55% n=248 All destinations →
September ← this page 70% n=240 All destinations →
October 82% n=248 All destinations →
November 56% n=240 All destinations →
December 27% n=248 All destinations →

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.