What Is the Heat Index? Is it the Heat, the Humidity, or Both?

A heat wave this week is bringing searing conditions to tens of millions of Americans in a sticky, sweaty swath of the country stretching from Texas up through the Midwest and across to the East Coast.

Floating along the cool Comal River in New Braunfels, Tex., on Thursday.

In an effort to keep people safe, the National Weather Service has issued an excessive heat warning that spans 12 states in the middle of the country, as well as Philadelphia. Cleveland, Pittsburgh and New York are under a heat watch. 

But while temperatures are expected to top 100 in several states, forecasters mostly rely on the heat index — not the old-fashioned thermometer reading — to issue warnings. So what does the number mean?

Read the story from The New York Times by  Nicholas Bogel-Burroughs - “What Is the Heat Index? Is it the Heat, the Humidity, or Both?”