ERA5 almanac — 8 years of real data

Crater Lake in September

Crater Lake in September: rain frequency data not yet backfilled for this month, 66°F average high, 24% of days historically clear (n=240, ERA5 2018–2025).

Crater Lake in September is historically a poor month for sun: only 24% of days clear across 8 years (n=240, avg high 66°F). July is the clearest month for Crater Lake 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 Crater Lake 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 Crater Lake 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	37%	n=120	66°F
Late September 16th–end	11%	n=120	66°F
Full month average	24%	n=240	66°F

Bottom line

Crater Lake is mostly overcast in September historically: only 24% of days are clear across 8 years (n=240), avg high 66°F. The two halves differ materially (37% early, 11% late) — favour the early half of the month. If you want sun in Crater Lake, July is historically much better. Most visitors this month will see more clouds than sun.

→ See all destinations in July — historically the best month for sun in Crater Lake.

Every month in Crater Lake

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

Historical clear-sky % for Crater Lake, all months, ERA5 2018–2025.
Month	% Clear	Sample (n)
January	0%	n=248	All destinations →
February	0%	n=226	All destinations →
March	0%	n=248	All destinations →
April	0%	n=240	All destinations →
May	0%	n=248	All destinations →
June	13%	n=240	All destinations →
July	52%	n=248	All destinations →
August	42%	n=248	All destinations →
September ← this page	24%	n=240	All destinations →
October	1%	n=248	All destinations →
November	0%	n=240	All destinations →
December	0%	n=248	All destinations →

Frequently asked

Is Crater Lake good for sun in September?

Not usually. Crater Lake is historically overcast in September: only 24% of days are clear (avg high 66°F, n=240 days, ERA5 2018–2025). July is historically far better for sun here.

→ Crater Lake live forecast — this week’s actual 7-day weather, not historical
→ Best destinations in September — all destinations ranked by September clear-sky %
→ All destinations
→ Raw data (JSON) — buckets, n, source

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.