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SunnyDays

ERA5 almanac — 8 years of real data

Napa in August

Historically one of the better months to visit Napa for sun: 96% of days are clear on average (n=248, avg high 88°F). Every percentage here is a historical average from 8 years of ERA5 data — 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 Napa 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 Napa 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		 97% n=120 88°F
Late August
16th–end		 95% n=128 88°F
Full month average 96% n=248 88°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 Napa 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		 100% n=120
Late August
16th–end		 100% n=128
Full month average 100% n=248
12.8 h median sunshine per day 100% of days exceed 8 h sunshine 1% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 96%. Sunshine-based “warm & sunny”: 100%.

Bottom line

Napa is historically one of the best US destinations for sun in August: 96% of days meet the clear bar (avg high 88°F, n=248 days across 8 years). Pack for outdoor activity — this month consistently delivers.

Every month in Napa

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

Historical clear-sky % for Napa, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 1% n=248 All destinations →
February 10% n=226 All destinations →
March 9% n=248 All destinations →
April 42% n=240 All destinations →
May 70% n=248 All destinations →
June 94% n=240 All destinations →
July 98% n=248 All destinations →
August ← this page 96% n=248 All destinations →
September 86% n=240 All destinations →
October 73% n=248 All destinations →
November 18% n=240 All destinations →
December 0% n=248 All destinations →

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.