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

Similar to the last several days, highs will top out in the low to mid 90s, with heat indices ranging from 95-100 degrees. — Matthew Villafane, CBS News, 10 Aug. 2026

The weather service forecasts highs in the 90s again on Tuesday with a heat index of 108, and Gonzalez expects another heat advisory. — Miami Herald Staff, Miami Herald, 10 Aug. 2026

Temperatures are expected to be around 98 degrees with a heat index between 105 and 110 degrees. — Kansas City Star, 10 Aug. 2026

Sticky, thick humidity is blanketing the Philadelphia region and causing heat indexes that could reach the low 100s through early next week. — Kaitlyn McCormick, USA Today, 7 Aug. 2026

Word History

First Known Use

1984, in the meaning defined above