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SunnyDays

ERA5 almanac — 8 years of real data

Charleston in August

Charleston in August is historically a poor month for sun: only 12% of days clear across 8 years (n=248, avg high 85°F). October is the clearest month for Charleston 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 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 August

Clear-sky percentage for Charleston 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		 11% n=120 85°F
Late August
16th–end		 12% n=128 85°F
Full month average 12% n=248 85°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 Charleston 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		 86% n=120
Late August
16th–end		 90% n=128
Full month average 88% n=248
12.1 h median sunshine per day 81% of days exceed 8 h sunshine 62% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 12%. Sunshine-based “warm & sunny”: 88%. The 76-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 a poor choice for sun in August: just 12% of days clear historically (n=248). October 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 October — historically the best month for sun in Charleston.

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 ← this page 12% n=248 All destinations →
September 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.