Decatur weather in September 2026

Q: How hot is it in Decatur in September?

Daytime temperatures usually reach 24°C in Decatur in September, falling to 12°C at night.

Q: How sunny is it in Decatur in September?

There are normally 8 hours of bright sunshine each day in Decatur in September - that's 61% of daylight hours.

Q: How warm is the sea around Decatur in September?

The average sea temperature around Decatur in September is 19°C.

Q: Does it rain in Decatur in September?

There are usually 9 days with some rain in Decatur in September and the average monthly rainfall is 87mm.

Expect daytime maximum temperatures of 26°C in Decatur, Illinois in September based on long-term weather averages. There are 8 hours of sunshine per day on average with typically 81 mm of rainfall in the month.

26
26°C max day temperature
8
8 hours of sunshine per day
8
8 days with some rainfall
13
13°C min night temperature
12
12 hours of daylight per day
L
Low heat & humidity
81
81 mm of monthly rainfall
6
6 (High) UV index

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The September weather guide for Illinois (Decatur) shows long term weather averages processed from data supplied by CRU (University of East Anglia), the Met Office & the Netherlands Meteorological Institute. Find out more about our data sources.

Metric (°C / mm) | Imperial (°F / inches)