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SunnyDays

ERA5 almanac — 8 years of real data

Lisbon in September

Lisbon in September: 16% of days see measurable rain, 80°F average high, 66% of days historically clear (n=240, ERA5 2018–2025).

Lisbon in September is a mixed month for sun: 66% of days historically clear (n=240, avg high 80°F). The outcome varies year to year — check the early/late split below. 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 Lisbon 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 Lisbon 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		 69% n=120 80°F
Late September
16th–end		 63% n=120 80°F
Full month average 66% n=240 80°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 Lisbon 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		 97% n=120
Late September
16th–end		 84% n=120
Full month average 90% n=240
11.5 h median sunshine per day 92% of days exceed 8 h sunshine 16% of days see ≥ 1 mm rain — rain is not hidden
Comparison (old vs new): WMO-code “clear” bar: 66%. Sunshine-based “warm & sunny”: 90%. The 24-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

Lisbon clears solidly in September: 66% of days historically (n=240), avg high 80°F. Most trips here in September will find more sun than cloud.

Every month in Lisbon

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

Historical clear-sky % for Lisbon, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 7% n=248 All destinations →
April 19% n=240 All destinations →
May 49% n=248 All destinations →
June 58% n=240 All destinations →
July 85% n=248 All destinations →
August 91% n=248 All destinations →
September ← this page 66% n=240 All destinations →
October 42% n=248 All destinations →
November 2% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Does it rain in Lisbon in September?

Rarely. Only 16% of days in Lisbon see measurable rain in September, based on 8 years of ERA5 historical data (n=240 days, 2018–2025). This is a historical average, not a forecast.

Is Lisbon good for sun in September?

Yes. Lisbon is historically strong for sun in September: 66% of days are clear on average (avg high 80°F, n=240 days, ERA5 2018–2025).

How Lisbon compares in September

Head-to-head clear-sky comparisons against destinations travelers plausibly weigh against Lisbon this month — same passport tier, similar flight distance, both sides from the same 8-year ERA5 record.

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.