Heat related deaths outpace fatalities in several other
severe weather categories. Based on a 10 year average from 1992-2001,
excessive heat claimed 219 lives each year. By contrast, floods killed
88, tornadoes 57, lightning 52 and hurricanes 15.
On average, about 175 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 above shows the HI
that corresponds to the actual air temperature and relative humidity.
(NOTE: 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 of +/- 1.3F.)
Heat Index Chart
Click on the chart below to view a text table of values.
www.weather.gov 
