Tracking Dangerous Heat in the U.S.
About 55.8 million people — 17 percent of the population of the contiguous United States — live in the areas expected to have dangerous levels of heat.
The heat index is a measure of how hot it really feels outside, taking into account humidity along with temperature. The measurement is used to indicate when the level of heat is dangerous for the human body while in the shade. When out in the sun, a person could perceive that temperature as being up to 15 degrees Fahrenheit higher.