In 1984, the National Weather Service began reporting the heat index developed by R.G. Steadman in 1979. (1)

We give the heat index, or "feels like" temperature during the weather forecast to give people an idea of the potential danger from heat stress on their body. The body cools itself by sweating. If air is dry, sweat will evaporate from the skin, and cool the air near the skin. On a hot, humid day, it's more difficult for the body to lose heat through sweat.

Here are some tips to avoid fatigue and/or heat stress when the heat index is above 100 degrees: avoid extended work in the sun, wear light colored, loose fitting clothing, wear a hat and use sunscreen outside, drink plenty of water or non-alcoholic liquids, if possible, remain in air-conditioned buildings.

Source:
1. Michael D. Morgan, Joseph M. Moran, Weather and People (Upper Saddle River, New Jersey: Prentice-Hall, Inc.,1997), 30-35.