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

World Cup matches will be played with the stadium's roof closed, but outside the climate-controlled venue, Houston's streets will likely see scorching summer temperatures with a heat index hovering around 40 Celsius.

From Barron's ● May 12, 2026

A heat index of 90 degrees would require mandatory rest breaks of 15 minutes every two hours and other heightened measures.

From Los Angeles Times ● Jul. 17, 2025

Temperatures have soared to 39C in New York and humidity has made it feel even worse - more than 50C on the heat index.

From BBC ● Jul. 14, 2025

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