ERA5 almanac — 8 years of real data

Big Sky in July

Big Sky in July: 42% of days see measurable rain, 72°F average high, 18% of days historically clear (n=248, ERA5 2018–2025).

Big Sky in July is historically a poor month for sun: only 18% of days clear across 8 years (n=248, avg high 72°F). 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 Big Sky 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 July

Clear-sky percentage for Big Sky in July, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period	% Clear days	Sample (n)	Avg high
Early July 1st–15th	13%	n=120	72°F
Late July 16th–end	23%	n=128	72°F
Full month average	18%	n=248	72°F

Sunshine & warmth in July

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 Big Sky in July. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period	Warm & sunny (≥72°F, >6 h sun)	Sample (n)
Early July 1st–15th	43%	n=120
Late July 16th–end	62%	n=128
Full month average	53%	n=248

14.8 h median sunshine per day 99% of days exceed 8 h sunshine 42% of days see ≥ 1 mm rain — rain is not hidden

Comparison (old vs new): WMO-code “clear” bar: 18%. Sunshine-based “warm & sunny”: 53%. The 35-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

Big Sky is a poor choice for sun in July: just 18% of days clear historically (n=248). The 8-year record is consistent — this is not a fluke bad stretch.

Every month in Big Sky

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

Historical clear-sky % for Big Sky, 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	0%	n=248	All destinations →
June	2%	n=240	All destinations →
July ← this page	18%	n=248	All destinations →
August	18%	n=248	All destinations →
September	8%	n=240	All destinations →
October	0%	n=248	All destinations →
November	0%	n=240	All destinations →
December	0%	n=248	All destinations →

Frequently asked

Does it rain in Big Sky in July?

Sometimes. Big Sky sees measurable rain on 42% of days in July historically (n=248 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Big Sky good for sun in July?

Not usually. Big Sky is historically overcast in July: only 18% of days are clear (avg high 72°F, n=248 days, ERA5 2018–2025).

How Big Sky compares in July

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

→ Big Sky vs Birmingham in July — Birmingham is 17% clear historically
→ Big Sky vs Nashville in July — Nashville is 19% clear historically
→ Big Sky vs Telluride in July — Telluride is 19% clear historically

→ Big Sky live forecast — this week’s actual 7-day weather, not historical
→ Best destinations in July — all destinations ranked by July 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.