ERA5 almanac — 8 years of real data

New York City in September

New York City in September: 34% of days see measurable rain, 76°F average high, 40% of days historically clear (n=240, ERA5 2018–2025).

New York City in September is a mixed month for sun: 40% of days historically clear (n=240, avg high 76°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 New York City 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 New York City 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	41%	n=120	76°F
Late September 16th–end	38%	n=120	76°F
Full month average	40%	n=240	76°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 New York City 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	77%	n=120
Late September 16th–end	56%	n=120
Full month average	66%	n=240

11.4 h median sunshine per day 70% 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: 40%. Sunshine-based “warm & sunny”: 66%. The 26-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

New York City in September is genuinely mixed: 40% of days historically clear (n=240), avg high 76°F — more cloudy than clear. The outcome varies year to year; don't book if you need guaranteed sun.

Every month in New York City

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

Historical clear-sky % for New York City, 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	6%	n=240	All destinations →
May	24%	n=248	All destinations →
June	39%	n=240	All destinations →
July	31%	n=248	All destinations →
August	35%	n=248	All destinations →
September ← this page	40%	n=240	All destinations →
October	13%	n=248	All destinations →
November	2%	n=240	All destinations →
December	0%	n=248	All destinations →

Frequently asked

Does it rain in New York City in September?

Sometimes. New York City 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 New York City good for sun in September?

It's mixed. New York City in September is clear on 40% of days historically (avg high 76°F, n=240 days, ERA5 2018–2025) — the outcome varies year to year.

How New York City compares in September

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

→ New York City vs Asheville in September — Asheville is 40% clear historically
→ New York City vs Atlanta in September — Atlanta is 40% clear historically
→ New York City vs Outer Banks in September — Outer Banks is 40% clear historically

→ New York City 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.