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

This triggers a heat index which governing body the FIA uses to define potentially dangerous conditions for drivers.

From BBC • Oct. 16, 2025

Through Monday’s games, the median heat index was 87 degrees.

From The Wall Street Journal • Jun. 27, 2025

This past week, the heat index reached 115 degrees in some places, even though the actual temperature was lower.

From Slate • Jun. 25, 2025

“The heat index was 106 degrees,” he recalled.

From Los Angeles Times • May 23, 2024

In metropolitan Manila, where the heat index is forecast to hit 45 degrees Celsius early this week, residents in overcrowded slums have been cooling off by setting up colorful inflatable pools on busy roads.

From New York Times • Apr. 29, 2024