Here are normal monthly and seasonal temperatures and precipitation as well as selected records for the state of Illinois. The normals are based on the 1981-2010 averaging period. A majority of the official climate data comes from a large network of volunteer observers known as the cooperative observer network with official training and equipment from the National Weather Service. For normals for individual stations, please refer to the Illinois climate normals page.
| Month/Season | Temperature (F) | Precipitation (in) |
|---|---|---|
| January | 26.3 | 2.12 |
| February | 30.8 | 2.11 |
| March | 41.1 | 2.98 |
| April | 52.4 | 3.80 |
| May | 62.5 | 4.62 |
| June | 71.8 | 4.20 |
| July | 75.3 | 4.05 |
| August | 73.5 | 3.60 |
| September | 65.9 | 3.24 |
| October | 54.1 | 3.26 |
| November | 42.5 | 3.47 |
| December | 29.9 | 2.74 |
| Winter | 29.0 | 6.97 |
| Spring | 52.0 | 11.40 |
| Summer | 73.5 | 11.85 |
| Fall | 54.2 | 9.97 |
| Annual | 52.2 | 40.20 |
The following records were collected from the available climate archives for Illinois.
The highest temperature reported in Illinois was 117°F in East St. Louis on July 14, 1954.
The lowest temperature reported in Illinois was -36°F in Congerville on January 5, 1999.
It is important to note that some sources, including Wikipedia, have list -37°F in Rochelle on January 15, 2009 as the lowest temperature in Illinois. However, that observation came from a small airport station designed for aviation purposes and was not part of the climate network in Illinois. As a result, the data were not always archived and no quality control procedures were applied to the data. In fact, sites in that network (Automated Weather Observing System or AWOS) do not record the minimum temperature, only the temperature every 20 minutes. In addition, temperatures below -30°F were very close to the design limits of the equipment. An examination of the site, the available 20-minute data (before, during, and afterwards), and the equipment, along with the consideration of the meteorological factors of that morning, raised questions about the validity of the one data point. As a result, it was never officially recognized as the state record.
The most rainfall from a single event was 16.94 inches in Aurora during a 24-hour period spanning July 17-18, 1996.
The most precipitation in a year was 86.36 inches in Rosiclare (southeastern IL) in 2011.
The most snow from a single event appears to be Astoria on February 27-28, 1900 with 37.8 inches.
The most snow for a single winter was 105.1 inches at Antioch during the winter of 1978-1979.