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SunnyDays

ERA5 almanac — 8 years of real data

Marfa in September

Marfa in September is a mixed month for sun: 64% of days historically clear (n=240, avg high 84°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 Marfa 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 Marfa 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		 58% n=120 84°F
Late September
16th–end		 71% n=120 84°F
Full month average 64% n=240 84°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 Marfa 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		 98% n=120
Full month average 95% n=240
11.8 h median sunshine per day 94% 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: 64%. Sunshine-based “warm & sunny”: 95%. The 31-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

Marfa clears solidly in September: 64% of days historically (n=240), avg high 84°F. Most trips here in September will find more sun than cloud.

Every month in Marfa

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

Historical clear-sky % for Marfa, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 1% n=248 All destinations →
February 19% n=226 All destinations →
March 48% n=248 All destinations →
April 78% n=240 All destinations →
May 73% n=248 All destinations →
June 70% n=240 All destinations →
July 61% n=248 All destinations →
August 60% n=248 All destinations →
September ← this page 64% n=240 All destinations →
October 69% n=248 All destinations →
November 34% 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.