heat index

noun

plural heat indexes or heat indices

: a value that indicates what the ambient temperature feels like to the human body and that is derived from a calculation using air temperature and relative humidity

The most likely chances for a heat advisory are Thursday and Friday, when the heat index could approach at least 100 degrees, according to National Weather Service meteorologists …

—Julie E. Greene

The heat index is defined as a function of temperature and relative humidity. The combination of the two results in an apparent temperature that gives an idea of what the hot and humid air feels like to the average person.

—Neil MacFarquhar

Examples of heat index in a Sentence

Recent Examples on the Web

The upper Northeast will be hottest on Tuesday with a heat index of 100 to 104 from Rochester to Albany to Boston. — Kenton Gewecke, ABC News, 13 July 2026

This will be the second heat wave of the summer, with the most recent stretch taking place June 30 through July 2, when highs reached 94 to 95 degrees and heat indices were over 100. — Adriana Pérez, Chicago Tribune, 13 July 2026

Forecast highs are expected to approach 100 degrees both days, and heat index values could climb well above that mark. — The Baltimore Sun, Baltimore Sun, 13 July 2026

Expect highs in the mid-90s, but heat indices stay close to 100 with the humidity. — July 12, CBS News, 12 July 2026

Word History

First Known Use

1984, in the meaning defined above