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

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

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

From The Wall Street Journal • Jun. 27, 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

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

Weather experts say that the heat index - or the apparent or experienced temperature - has crossed 50C because of the high levels of humidity.

From BBC • May 24, 2024