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SunnyDays

ERA5 almanac — 8 years of real data

Big Sky in August

Big Sky in August: 46% of days see measurable rain, 70°F average high, 18% of days historically clear (n=248, ERA5 2018–2025).

Big Sky in August is historically a poor month for sun: only 18% of days clear across 8 years (n=248, avg high 70°F). July is the clearest month for Big Sky 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 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 August

Clear-sky percentage for Big Sky in August, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period % Clear days Sample (n) Avg high
Early August
1st–15th		 20% n=120 70°F
Late August
16th–end		 16% n=128 70°F
Full month average 18% n=248 70°F

Sunshine & warmth in August

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 August. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period Warm & sunny (≥72°F, >6 h sun) Sample (n)
Early August
1st–15th		 48% n=120
Late August
16th–end		 35% n=128
Full month average 41% n=248
13.2 h median sunshine per day 93% of days exceed 8 h sunshine 46% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 18%. Sunshine-based “warm & sunny”: 41%. The 23-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 August: just 18% of days clear historically (n=248). July is the historically strongest month for sun here. The 8-year record is consistent — this is not a fluke bad stretch.

→ See all destinations in July — historically the best month for sun in Big Sky.

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 18% n=248 All destinations →
August ← this page 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 August?

Sometimes. Big Sky sees measurable rain on 46% of days in August historically (n=248 days, ERA5 2018–2025) — not frequent enough to plan around, but not negligible either.

Is Big Sky good for sun in August?

Not usually. Big Sky is historically overcast in August: only 18% of days are clear (avg high 70°F, n=248 days, ERA5 2018–2025). July is historically far better for sun here.

How Big Sky compares in August

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.

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.