Illinois climate

World Climates covers 7 places in Illinois. By annual mean temperature, the warmest is Springfield (12 °C53 °F) and the coolest is Rockford (9 °C48 °F). The wettest is Springfield with 1,001 mm39 in of precipitation per year, the driest Rockford with 967 mm38 in.

The most common climate types (Köppen classification computed from these averages) are hot-summer humid continental (Dfa, 7 places).

The highest monthly average high is in Peoria (30 °C86 °F in July); the lowest monthly average low is in Rockford (−11 °C12 °F in January).

Across the places in Illinois, the months that most often rank as the most comfortable are July, August and September (2, 3 and 7 of 7 places respectively).

Temperature and rainfall across Illinois

Annual mean temperatures in Illinois range from 9 °C48 °F to 12 °C53 °F. The typical place (the median of all 7) has a year average of 10 °C50 °F. Annual precipitation ranges from 967 mm38 in to 1,001 mm39 in. The median place receives about 983 mm39 in in an average year.

In 7 places of 7, the coldest month has an average low below 0 °C (32 °F). In 0 places, the warmest month has an average high above 30 °C (86 °F). No place here gets less than 250 mm (10 in) of precipitation a year. None gets more than 2,000 mm (79 in) a year.

Averaged over all 7 places, July is the warmest month, with a mean of 24 °C75 °F. January is the coldest, with −5 °C24 °F. The wettest month on average is May (116 mm4.6 in). The driest is January (47 mm1.9 in). Single places can differ a lot from this average, so check the page of each city.

The table below lists the places sorted by population. The largest ones covered here are Chicago, Aurora and Rockford. Each city page has a chart and a table for every month. It also shows the best time to visit and what to pack. The figures are 25-year averages for 2001–2025 from NASA POWER. They describe grid cells of roughly 50–70 km, not single weather stations.

Climate of each place in Illinois

Chicago has a hot-summer humid continental climate (Köppen Dfa). Its year average is 10 °C50 °F. The warmest month is August, with an average high of 25 °C77 °F. The coldest is January, with an average low of −5 °C23 °F. About 982 mm39 in of precipitation falls in a year. May is the wettest month and January the driest. By our comfort score, the best months to visit are June and July and September.

Aurora has a hot-summer humid continental climate (Köppen Dfa). Its year average is 10 °C49 °F. The warmest month is July, with an average high of 29 °C85 °F. The coldest is January, with an average low of −10 °C15 °F. About 983 mm39 in of precipitation falls in a year. May is the wettest month and January the driest. By our comfort score, the best months to visit are August and September.

Rockford has a hot-summer humid continental climate (Köppen Dfa). Its year average is 9 °C48 °F. The warmest month is July, with an average high of 28 °C83 °F. The coldest is January, with an average low of −11 °C12 °F. About 967 mm38 in of precipitation falls in a year. June is the wettest month and January the driest. By our comfort score, the best months to visit are May and July–September.

Joliet has a hot-summer humid continental climate (Köppen Dfa). Its year average is 10 °C51 °F. The warmest month is July, with an average high of 29 °C84 °F. The coldest is January, with an average low of −9 °C16 °F. About 985 mm39 in of precipitation falls in a year. May is the wettest month and January the driest. By our comfort score, the best months to visit are September.

Peoria has a hot-summer humid continental climate (Köppen Dfa). Its year average is 11 °C52 °F. The warmest month is July, with an average high of 30 °C86 °F. The coldest is January, with an average low of −8 °C18 °F. About 983 mm39 in of precipitation falls in a year. May is the wettest month and February the driest. By our comfort score, the best months to visit are September.

Springfield has a hot-summer humid continental climate (Köppen Dfa). Its year average is 12 °C53 °F. The warmest month is July, with an average high of 30 °C86 °F. The coldest is January, with an average low of −8 °C18 °F. About 1,001 mm39 in of precipitation falls in a year. May is the wettest month and February the driest. By our comfort score, the best months to visit are September.

Elgin has a hot-summer humid continental climate (Köppen Dfa). Its year average is 10 °C49 °F. The warmest month is July, with an average high of 29 °C84 °F. The coldest is January, with an average low of −10 °C14 °F. About 983 mm39 in of precipitation falls in a year. May is the wettest month and January the driest. By our comfort score, the best months to visit are August and September.

Frequently asked questions

Which is the warmest place in Illinois?

By annual mean temperature, the warmest place in Illinois is Springfield at 12 °C53 °F. The coolest is Rockford at 9 °C48 °F.

Where does it rain the most in Illinois?

Springfield gets the most precipitation, 1,001 mm39 in in an average year. The driest place is Rockford with 967 mm38 in.

How many places in Illinois does this site cover?

World Climates has climate data for 7 places in Illinois. Each one has a page with monthly averages for temperature, rainfall, humidity and daylight.

When is the best time to visit Illinois?

It depends on the place. Across Illinois, the months that most often rank as the most comfortable are July, August and September. Check the page of each city for its own best months.