SUMMER HEAT AND HUMIDITY

Summertime often brings excessive heat and humidity to central Illinois.  With people frequently involved in outdoor activities this time of year, it can be easy to quickly be overcome by the heat.  During the last 10 years, an average of 237 people nationwide have been killed each year due to the effects of high heat.  Learning what to do ahead of time can help keep you safe.

The Heat Index

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.

HEAT INDEX CHART (Apparent Temperature)

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.).

 

Heat Index Value	Possible Heat Disorder
80 to 90	Fatigue possible with prolonged exposure and physical activity.
91 to 105	Sunstroke, heat cramps and heat exhaustion possible.
106 to 129	Sunstroke, heat cramps, and heat exhaustion likely; heat stroke possible.
130 or higher	Heat stroke highly likely with continued exposure.

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.

Heat Forecasts

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.

During the summer of 2004, the Lincoln NWS is participating in an experiment with NWS offices in Indianapolis, Paducah, and Louisville.   These offices will issue Excessive Heat Warnings if the maximum heat index for the day is expected to be 105 or higher for at least 3 hours, and if nighttime low temperatures are expected to be 75 or above.  Previously, the two-tier concept of Heat Advisories and Excessive Heat Warnings were issued, and sometimes caused confusion among forecasters and the public alike.

Other offices that cover Illinois will remain with existing policies for the 2004 season.  In general, these are as follows:

• Heat Advisory:  Maximum heat index of 105 or higher for 3+ hours, minumum low temperature in the upper 70s for a 24+ hour period.
• Excessive Heat Warning:  Maximum heat index of 115 or higher for 3+ hours, minimum low temperature in the upper 70s for a 24+ hour period.

Criteria are lower for the immediate Chicago and St. Louis areas, due to the urban "heat island" effect.

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.