National Weather Service, Lincoln IL -- Heat Safety

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.

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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)

Relative Humidity (%)
A i r T e m p (F)		10	15	20	25	30	35	40	45	50	55	60	65	70	75	80	85	90	95	100
	130	131
	125	123	131	141
	120	116	123	130	139	148
	115	111	115	120	127	135	143	151
	110	105	106	112	117	123	130	137	143	150
	105	100	102	105	109	113	118	123	129	135	142	149
	100	95	97	99	101	104	107	110	115	120	126	132	138	144
	95	90	91	93	94	96	98	101	104	107	110	114	119	124	130	136
	90	85	86	87	88	90	91	93	95	96	98	100	102	106	109	113	117	122
	85	80	81	82	83	84	85	86	87	88	89	90	91	93	95	97	99	102	105	106
	80	75	76	77	77	78	79	79	80	81	81	82	83	85	86	86	87	88	89	91

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. For most of Illinois, the criteria is as follows:

Heat Advisories are issued when the maximum heat index is expected to be at least 100 degrees, and/or the air temperature is expected to reach at least 95 degrees.
Excessive Heat Warnings are issued when the maximum heat index is expected to be at least 105 degrees, and the minimum heat index will be at least 75 degrees for a 48-hour period.

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.

National Weather Service Weather Forecast Office