Illinois State Water Survey - University of Illinois at Urbana-Champaign
INRS State Scientific Surveys
State Climatologist Office for Illinois
Illinois Climate Summaries (normals and extremes)
Jim Angel, state climatologist
Here are extensive climate statistics for individual sites in Illinois, including monthly normals and extremes. Stations are chosen based on length of record and completeness of data. Normal monthly temperature and precipitation data for all sites are available as a single Microsoft Excel file or separate text files containing the site information, temperature normals, and precipitation normals. Recently, it was discovered that the record low precipitation records stated here may not always be correct. Missing data can lead to monthly, seasonal, or annual totals that are too low. If determining the record low precipitation for a station is important to you, please contact me at jimangel@illinois.edu for more information. See Data Disclaimer.
The seasons mentioned in the summaries are defined as: Winter (December, January, February); Spring (March, April, May); Summer (June, July, August); and Fall (September, October, November).
The data used in these products come from the National Weather Service (NWS) Cooperative Observer Program (COOP). This network of more than 11,000 volunteer observers nationwide, using approved instruments and observing techniques, provides high-quality climate data to the nation. The data are archived at the National Climatic Data Center (NCDC).