heat index

[heet in-deks]

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.

heat index

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

Word History and Origins

Origin of heat index¹

First recorded in 1975–80

Example Sentences

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

Read more on The Wall Street Journal

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

Read more on Los Angeles Times

The proposal, which would apply to almost all employers, would be triggered whenever employees were exposed to a heat index — a measure taking into account heat and humidity — of 80 degrees or higher for more than 15 minutes in an hour-long period.

Read more on Los Angeles Times

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

Read more on BBC

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

Read more on Slate