heat index

American  

[heet in-deks] / ˈhit ˌɪn dɛks /

noun

Meteorology.

1. a formulaically calculated approximation of how hot it feels given the temperature of the air combined with the relative humidity, based on the fact that an actual temperature will feel warmer as the relative humidity rises and causes a decrease in the natural evaporative cooling from perspiration. HI, H.I.

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heat index Scientific  

1. A measurement of the air temperature in relation to the relative humidity, used as an indicator of discomfort. The heat index is higher when high air temperatures occur with high humidity, and lower when they occur with low humidity. The heat index is based on studies of skin cooling caused by the evaporation of sweat.

2. Also called apparent temperature

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Etymology

Origin of heat index

First recorded in 1975–80

Example Sentences

See Examples For:

heat index

On a day when the heat index neared 100 degrees, local officials urged residents to minimize their water use to prevent the town’s limited supply from running dry.

From The Wall Street Journal ● Aug. 8, 2026

On Saturday, France and Paraguay will clash in Philadelphia where the heat index could reach 105F.

From Barron's ● Jul. 3, 2026

Forecasters say high humidity levels will make conditions feel even more oppressive, with heat index values set to reach 115F in the coming days.

From BBC ● Jul. 2, 2026

It’s obviously helpful to know the heat index, but the number on its own doesn’t provide insight into the breakdown of heat versus humidity.

From Slate ● Jun. 25, 2025

Running competition began at noon but was halted 90 minutes later while monitoring the heat index to make sure it was safe to resume.

From Los Angeles Times ● May 10, 2025