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SunnyDays

ERA5 almanac — 8 years of real data

Bozeman in September

Bozeman in September: 18% of days see measurable rain, 72°F average high, 48% of days historically clear (n=240, ERA5 2018–2025).

Bozeman in September is a mixed month for sun: 48% of days historically clear (n=240, avg high 72°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 Bozeman 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 Bozeman 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		 60% n=120 72°F
Late September
16th–end		 35% n=120 72°F
Full month average 48% 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 Bozeman 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		 68% n=120
Late September
16th–end		 38% n=120
Full month average 53% n=240
11.9 h median sunshine per day 87% of days exceed 8 h sunshine 18% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 48%. Sunshine-based “warm & sunny”: 53%.

Bottom line

Bozeman in September is genuinely mixed: 48% of days historically clear (n=240), avg high 72°F — roughly even odds. The two halves differ materially (60% early, 35% late) — favour the early half of the month. The outcome varies year to year; don't book if you need guaranteed sun.

Every month in Bozeman

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

Historical clear-sky % for Bozeman, 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 8% n=248 All destinations →
June 38% n=240 All destinations →
July 72% n=248 All destinations →
August 67% n=248 All destinations →
September ← this page 48% n=240 All destinations →
October 5% n=248 All destinations →
November 0% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Bozeman in September?

Rarely. Only 18% of days in Bozeman see measurable rain in September, based on 8 years of ERA5 historical data (n=240 days, 2018–2025). This is a historical average, not a forecast.

Is Bozeman good for sun in September?

It's mixed. Bozeman in September is clear on 48% of days historically (avg high 72°F, n=240 days, ERA5 2018–2025) — the outcome varies year to year.

How Bozeman compares in September

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

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.