What is the Heat Index and Why Is It Used?

Brian Donegan
Published: July 8,2016

Heat Index Chart

You've all heard the term "heat index" used in a weather forecast on those hot, humid summer days, but do you know what it actually measures and why it's important to our health?
In simplest terms, the heat index is the "feels-like" temperature, or how hot it really feels when the relative humidity is factored in with the actual air temperature.
Your body cools itself by the evaporation of persperation from your skin. On a hot, humid day, less evaporation of sweat occurs, diminishing the body's ability to cool itself.
By looking at the chart above, you can find the heat index temperature by matching the air temperature to the relative humidity. For example, if the air temperature is 94 degrees and the relative humidity is 70 percent, the heat index is 119 degrees.
Since official surface temperatures are taken using thermometers shaded from the sun, the NWS notes that actual heat indices can be up to 15 degrees higher in full sunshine than the shade.
The red areas on the chart indicate dangerous or extremely dangerous conditions, meaning you should not spend any significant time outdoors because it places you at risk for heat exhaustion and heat stroke. This is why it's very important to pay close attention not just to the air temperature, but also to the heat index on the hottest days of summer.
(MORE: Four Things Extreme Heat Does to Your Body)
The National Weather Service (NWS) issues excessive heat watches, heat advisories or excessive heat warnings when the heat index is expected to reach or exceed 100 to 110 degrees, depending on the local climate, for at least two consecutive days.
Another way to figure out the heat index without using the chart above is by using the NWS's heat index calculator.