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SunnyDays

ERA5 almanac — 8 years of real data

Napa in March

Napa in March is historically a poor month for sun: only 9% of days clear across 8 years (n=248, avg high 63°F). July is the clearest month for Napa 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 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 March

Clear-sky percentage for Napa in March, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period % Clear days Sample (n) Avg high
Early March
1st–15th		 4% n=120 63°F
Late March
16th–end		 14% n=128 63°F
Full month average 9% n=248 63°F

Sunshine & warmth in March

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 March. Primary: % of days ≥ 72°F with > 6 h of sunshine.
Period Warm & sunny (≥72°F, >6 h sun) Sample (n)
Early March
1st–15th		 4% n=120
Late March
16th–end		 14% n=128
Full month average 9% n=248
10.5 h median sunshine per day 69% of days exceed 8 h sunshine 37% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 9%. Sunshine-based “warm & sunny”: 9%. The two measures are nearly identical here — no convective-storm bias for this climate and month.

Bottom line

Napa is a poor choice for sun in March: just 9% 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 Napa.

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 ← this page 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 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.