Here are the 1981-2010 averages, known as climate normals, for Illinois locations. Stations were chosen based on length of record and completeness of data. Products covered here are monthly and annual high, low, and mean temperatures, heating and cooling degree days, precipitation, and snowfall.
| Four Ways to Obtain the Data | |
|---|---|
| Station List Below | Monthly Maps for Illinois |
| Excel Spreadsheet | Google Map Interface |
The following is from the NOAA web site. In the strictest sense, a "normal" of a particular variable (e.g., temperature) is defined as the 30-year average. For example, the minimum temperature normal in January for a station in Chicago, Illinois, would be computed by taking the average of the 30 January values of monthly-averaged minimum temperatures from 1981 to 2010. Each of the 30 monthly values was in turn derived from averaging the daily observations of minimum temperature for the station. In practice, however, much more goes into NCDC's Normals product than simple 30-year averages. Procedures are put in place to deal with missing and suspect data values. Normals are a large suite of data products that provide users with many tools to understand typical climate conditions for thousands of locations across the United States. Normals in the U.S. are for 30 years and updated every decade. You can find more information about climate normals on the NOAA Climate Normals page, including frequently asked questions.
The data used in these products come from the National Weather Service (NWS) Cooperative Observer Program (COOP). Most of these sites are from a network of volunteer observers nationwide, using approved instruments and observing techniques to provide high-quality climate data to the nation. The data are archived at the National Climatic Data Center (NCDC).