Heat index is a measure of how hot the weather feels when air temperature and humidity are combined. High humidity can make heat feel more intense because sweat evaporates more slowly.
Why heat index matters
Heat index helps estimate heat stress risk. The body cools itself partly by sweating, but humid air makes that cooling process less effective.
Heat index versus temperature
The actual temperature is measured by a thermometer. Heat index describes what the combination of temperature and humidity feels like to the body.
For cold weather, a similar feels-like idea is wind chill.