About 237 Americans succumb to the taxing demands of heat every year*. Our bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and as a last resort, by panting, when blood is heated above 98.6°F. Sweating cools the body through evaporation. However, high relative humidity retards evaporation, robbing the body of its ability to cool itself.

When heat gain exceeds the level the body can remove, body temperature begins to rise, and heat related illnesses and disorders may develop.

The Heat Index (HI) is the temperature the body feels when heat and humidity are combined. The chart below shows the HI that corresponds to the actual air temperature and relative humidity. (This chart is based upon shady, light wind conditions. Exposure to direct sunlight can increase the HI by up to 15°F.)

(Due to the nature of the heat index calculation, the values in the tables below have an error +/- 1.3F.) 

• Heat Wave - A Major Summer Killer (.pdf)
• Heat Index Equation - NWS Birmingham, AL
• NWS Heat and Drought Awareness Page
• The Heat Index Equation - Technical Attachment (PDF)
  

* 10-year average of heat related fatalities from 1994-2003. U.S. Natural Hazard Statistics.