What is the Heat Index and Why is it Used?

Brian Donegan
Published: July 16, 2019

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 your 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.

The heat index is forecast to rise to dangerous levels into this weekend, above 100 degrees in many cities from the Plains and Midwest to the East Coast.

(MORE: Heat Wave to Expand This Week

Fortunately, schools are not in session in mid-July – with the exception of summer schools – so most teachers and students will not be sweltering in non-air-conditioned classrooms.

Some sports such as football are already holding practices in preparation for the upcoming fall season, however.

One high-school football coach in central Ohio is scheduling his team's practices early in the morning this week and giving his players water breaks every 15 minutes, WSYX-TV in Columbus reported. Early morning is generally the coolest time of day, even though it may not feel all that cool during this heat wave.

Your body cools itself by the evaporation of perspiration 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 below, 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.

The heat-index temperature can be found using this chart by matching the air temperature to the relative humidity.
(NOAA/NWS)

Since official surface temperatures are taken using thermometers shaded from the sun, the National Weather Service noted that actual heat indices can be up to 15 degrees higher in full sunshine than in 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: 6 Things You Should Know About Heat Waves

The 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.


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