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SunnyDays

ERA5 almanac — 8 years of real data

Austin in September

Austin in September is a mixed month for sun: 41% of days historically clear (n=240, avg high 91°F). The outcome varies year to year — check the early/late split below. 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 Austin 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 Austin 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		 35% n=120 91°F
Late September
16th–end		 47% n=120 91°F
Full month average 41% n=240 91°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 Austin 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		 86% n=120
Late September
16th–end		 95% n=120
Full month average 90% n=240
11.3 h median sunshine per day 82% of days exceed 8 h sunshine 34% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 41%. Sunshine-based “warm & sunny”: 90%. The 49-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

Austin in September is genuinely mixed: 41% of days historically clear (n=240), avg high 91°F — more cloudy than clear. The outcome varies year to year; don't book if you need guaranteed sun.

Every month in Austin

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

Historical clear-sky % for Austin, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 4% n=248 All destinations →
February 15% n=226 All destinations →
March 26% n=248 All destinations →
April 35% n=240 All destinations →
May 27% n=248 All destinations →
June 32% n=240 All destinations →
July 47% n=248 All destinations →
August 46% n=248 All destinations →
September ← this page 41% n=240 All destinations →
October 48% n=248 All destinations →
November 25% n=240 All destinations →
December 11% 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.