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SunnyDays

ERA5 almanac — 8 years of real data

San Diego in September

Historically one of the better months to visit San Diego for sun: 79% of days are clear on average (n=240, avg high 78°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 San Diego 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 San Diego 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		 88% n=120 78°F
Late September
16th–end		 70% n=120 78°F
Full month average 79% n=240 78°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 San Diego 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		 98% n=120
Late September
16th–end		 82% n=120
Full month average 90% n=240
11.3 h median sunshine per day 92% of days exceed 8 h sunshine 4% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 79%. Sunshine-based “warm & sunny”: 90%. The 11-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

San Diego clears solidly in September: 79% of days historically (n=240), avg high 78°F. Most trips here in September will find more sun than cloud.

Every month in San Diego

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

Historical clear-sky % for San Diego, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 6% n=248 All destinations →
February 12% n=226 All destinations →
March 6% n=248 All destinations →
April 13% n=240 All destinations →
May 13% n=248 All destinations →
June 38% n=240 All destinations →
July 88% n=248 All destinations →
August 90% n=248 All destinations →
September ← this page 79% n=240 All destinations →
October 60% n=248 All destinations →
November 28% n=240 All destinations →
December 6% 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.