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Excessive heat results from a combination of high temperatures and high humidities. At certain levels, the human body cannot maintain proper internal temperatures and may experience heat stroke. The "Heat Index" (HI) is a measure of the effect of the combined elements on the body.
The colors in the chart represent various heat disorders that can occur at the given heat index value, given prolonged exposure to the heat and/or physical activity. Note that exposure to full sunshine can increase the apparent heat index by up to 15 degrees. Additionally, the effects can vary from person to person (due to age, medical health, etc.).
To learn how to protect yourself from the heat, please read the American Red Cross publication, " Are You Ready for a Heat Wave?". For more scientific information on how the heat index is calculated, read the NWS Technical Attachment SR90-23, "The Heat Index Equation (or, More Than You Ever Wanted to Know About the Heat Index)". This is in PDF format. The National Weather Service will issue advisories and warnings of excessive heat and humidity as necessary. Thresholds for issuances will vary on location, depending on how urban the area is, and the part of the country. For most of Illinois, the criteria is as follows:
For southwest and far west central Illinois, the criteria is slightly higher. Additionally, the cities of Chicago and St. Louis have separate criteria, due to the urban nature of those areas. Long-range outlooks (generally 3 days or more in advance) are generated by NWS's Hydrometeorological Prediction Center (HPC). These outlooks contain forecasts of maximum and minimum heat index, as well as the chances of these thresholds being reached. These are used as guidance in local NWS offices' forecasts and Hazardous Weather Outlooks. |