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SunnyDays

ERA5 almanac — 8 years of real data

Charleston in September

Charleston in September is historically a poor month for sun: only 32% of days clear across 8 years (n=240, avg high 83°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 Charleston 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 Charleston 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		 28% n=120 83°F
Late September
16th–end		 37% n=120 83°F
Full month average 32% n=240 83°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 Charleston 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		 89% n=120
Late September
16th–end		 87% n=120
Full month average 88% n=240
11.8 h median sunshine per day 81% 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: 32%. Sunshine-based “warm & sunny”: 88%. The 56-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

Charleston is mostly overcast in September historically: only 32% of days are clear across 8 years (n=240), avg high 83°F. Most visitors this month will see more clouds than sun.

Every month in Charleston

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

Historical clear-sky % for Charleston, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 1% n=248 All destinations →
February 5% n=226 All destinations →
March 12% n=248 All destinations →
April 38% n=240 All destinations →
May 37% n=248 All destinations →
June 22% n=240 All destinations →
July 12% n=248 All destinations →
August 12% n=248 All destinations →
September ← this page 32% n=240 All destinations →
October 45% n=248 All destinations →
November 12% n=240 All destinations →
December 2% 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.