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SunnyDays

ERA5 almanac — 8 years of real data

Scottsdale in September

Historically one of the better months to visit Scottsdale for sun: 82% of days are clear on average (n=240, avg high 100°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 Scottsdale 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 Scottsdale 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		 83% n=120 100°F
Late September
16th–end		 82% n=120 100°F
Full month average 82% n=240 100°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 Scottsdale 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		 100% n=120
Late September
16th–end		 98% n=120
Full month average 99% n=240
11.8 h median sunshine per day 98% of days exceed 8 h sunshine 10% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 82%. Sunshine-based “warm & sunny”: 99%. The 17-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

Scottsdale is historically one of the best US destinations for sun in September: 82% of days meet the clear bar (avg high 100°F, n=240 days across 8 years). Pack for outdoor activity — this month consistently delivers.

Every month in Scottsdale

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

Historical clear-sky % for Scottsdale, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 18% n=248 All destinations →
February 42% n=226 All destinations →
March 57% n=248 All destinations →
April 92% n=240 All destinations →
May 93% n=248 All destinations →
June 95% n=240 All destinations →
July 76% n=248 All destinations →
August 75% n=248 All destinations →
September ← this page 82% n=240 All destinations →
October 83% n=248 All destinations →
November 65% n=240 All destinations →
December 25% 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.