Illinois weather in September 2025

Expect 24°C daytime maximum temperatures in the shade with on average 8 hours of sunshine per day in Illinois in September. Check more long-term weather averages for Illinois in September before you book your next holiday to Illinois in 2025/2026.

  • 24°C maximum daytime temperature in September in Illinois (Chicago)24
    24°C max day temperature
  • 8 hours of sunshine per day
  • 9 days with some rainfall in September in Illinois (Chicago)9
    9 days with some rainfall
  • 12°C minimum night-time temperature in September in Illinois (Chicago)12
    12°C min night temperature
  • 12 hours of daylight per day in September in Illinois (Chicago)12
    12 hours of daylight per day
  • No heat & humidity in September in Illinois (Chicago)0
    No heat & humidity
  • 87 mm monthly rainfall in September in Illinois (Chicago)87
    87 mm of monthly rainfall
  • UV (maximum) index 6 (High) in September in Illinois (Chicago)6
    6 (High) UV index
  • 19°C sea temperature in September in Illinois (Chicago)19
    19°C sea temperature

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The September weather guide for Illinois (Chicago) 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.

Top Illinois destinations

Below are average maximum temperatures at popular countries, regions and places in Illinois in September. Select a holiday destination to see more weather parameters.

All Illinois destinations

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

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More about the USA

How hot is it in Illinois in September?

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

How sunny is it in Illinois in September?

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

How warm is the sea around Illinois in September?

The average sea temperature around Chicago, Illinois in September is 19°C.

Does it rain in Illinois in September?

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

Illinois September sunrise & sunset times

Browse the sunrise and sunset times for Illinois in September 2025. Select a month to view Illinois sunrise and sunset times for the next 12 months. The Illinois sunrise and sunset times shown below are for Chicago.

JanFebMarAprMayJunJulAugSepOctNovDec

Date Sunrise times Sunset times
Monday, 1st September 2025 05:17 18:23
Monday, 15th September 2025 05:31 17:59
Tuesday, 30th September 2025 05:47 17:33

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