ERA5 almanac — 8 years of real data

Martha's Vineyard in September

Martha's Vineyard in September: 34% of days see measurable rain, 72°F average high, 27% of days historically clear (n=240, ERA5 2018–2025).

Martha's Vineyard in September is historically a poor month for sun: only 27% of days clear across 8 years (n=240, avg high 72°F). August is the clearest month for Martha's Vineyard 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 Martha's Vineyard 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 Martha's Vineyard 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	39%	n=120	72°F
Late September 16th–end	15%	n=120	72°F
Full month average	27%	n=240	72°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 Martha's Vineyard 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	60%	n=120
Late September 16th–end	26%	n=120
Full month average	43%	n=240

11.4 h median sunshine per day 73% 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: 27%. Sunshine-based “warm & sunny”: 43%. The 16-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

Martha's Vineyard is mostly overcast in September historically: only 27% of days are clear across 8 years (n=240), avg high 72°F. The two halves differ materially (39% early, 15% late) — favour the early half of the month. If you want sun in Martha's Vineyard, August is historically much better. Most visitors this month will see more clouds than sun.

→ See all destinations in August — historically the best month for sun in Martha's Vineyard.

Every month in Martha's Vineyard

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

Historical clear-sky % for Martha's Vineyard, all months, ERA5 2018–2025.
Month	% Clear	Sample (n)
January	0%	n=248	All destinations →
February	0%	n=226	All destinations →
March	0%	n=248	All destinations →
April	0%	n=240	All destinations →
May	3%	n=248	All destinations →
June	25%	n=240	All destinations →
July	45%	n=248	All destinations →
August	46%	n=248	All destinations →
September ← this page	27%	n=240	All destinations →
October	3%	n=248	All destinations →
November	0%	n=240	All destinations →
December	0%	n=248	All destinations →

Frequently asked

Does it rain in Martha's Vineyard in September?

Sometimes. Martha's Vineyard sees measurable rain on 34% of days in September historically (n=240 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Martha's Vineyard good for sun in September?

Not usually. Martha's Vineyard is historically overcast in September: only 27% of days are clear (avg high 72°F, n=240 days, ERA5 2018–2025). August is historically far better for sun here.

How Martha's Vineyard compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Martha's Vineyard this month — same passport tier, similar flight distance, both sides from the same 8-year ERA5 record.

→ Martha's Vineyard vs Cape Cod in September — Cape Cod is 27% clear historically
→ Martha's Vineyard vs Burlington in September — Burlington is 26% clear historically
→ Martha's Vineyard vs New Orleans in September — New Orleans is 28% clear historically

→ Martha's Vineyard live forecast — this week’s actual 7-day weather, not historical
→ Best destinations in September — all destinations ranked by September clear-sky %
→ All destinations
→ Raw data (JSON) — buckets, n, source

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.