"It's not the heat, it's the humidity." Actually, it's both, and that's where the National Weather Service's heat index comes in. Also known as the apparent temperature, the heat index is a number ...

Add Yahoo as a preferred source to see more of our stories on Google. The sticky combination of heat and high humidity can be more than uncomfortable -- it can be deadly. Mario Tama/Getty Images ...

The heat index is a measure of how hot it really feels on your body when relative humidity is factored in with the actual air temperature. Since heat index values were devised for shady, light wind ...