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
Julie E. GreeneThe 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
Neil MacFarquharExamples of heat index in a Sentence
Recent Examples on the Web
Daily highs will be in the mid-90s, with heat index values only a few degrees higher than actual temperatures.
—Brayden Garcia, Fort Worth Star-Telegram, 12 Sep. 2025
The heat wave began on June 20 and lasted for about a week, with outdoor air temperatures reaching 96 degrees Fahrenheit and a heat index of up to 105 degrees in eastern Missouri, according to the National Weather Service.
—Ivy Scott, Kansas City Star, 11 Sep. 2025
Use fan if heat index temperatures are below the high 90s.
—Anna Skinner, MSNBC Newsweek, 28 Aug. 2025
Amid the triple-digit heat index outside, fierce gameplay in the grand finals of titles like Crossfire, Street Fighter 6, and Counter-Strike 2 reached a fever pitch within Boulevard City’s SEF Arena.
—Christopher Cruz, Rolling Stone, 27 Aug. 2025
Afternoon temperatures will be in the mid-90s, with heat indices from 102 to 107 degrees, according to NWS Melbourne.
—Garfield Hylton, The Orlando Sentinel, 21 Aug. 2025
The National Weather Service issued a high-heat advisory from Homestead to West Palm Beach and warned of heat index levels up to 110 degrees.
—Christopher Cann, USA Today, 20 Aug. 2025
A day after some school activities were canceled due to high temperatures and heat index values, Louisville is starting to see some cooler temperatures.
—Leo Bertucci, The Courier-Journal, 20 Aug. 2025
These hardworking people are outside in over 90-degree weather (with a plus-100 degrees heat index) doing the jobs of people who undoubtedly would not want to be doing manual labor in this Florida heat.
—Sun Sentinel Editorial Board, Sun Sentinel, 20 Aug. 2025
Examples are automatically compiled from online sources to
show current usage.
Read More
Opinions expressed in the examples do not represent those of Merriam-Webster or its editors.
Send us feedback.
Brayden Garcia, Fort Worth Star-Telegram, 12 Sep. 2025
The heat wave began on June 20 and lasted for about a week, with outdoor air temperatures reaching 96 degrees Fahrenheit and a heat index of up to 105 degrees in eastern Missouri, according to the National Weather Service.
—Ivy Scott, Kansas City Star, 11 Sep. 2025
Use fan if heat index temperatures are below the high 90s.
—Anna Skinner, MSNBC Newsweek, 28 Aug. 2025
Amid the triple-digit heat index outside, fierce gameplay in the grand finals of titles like Crossfire, Street Fighter 6, and Counter-Strike 2 reached a fever pitch within Boulevard City’s SEF Arena.
—Christopher Cruz, Rolling Stone, 27 Aug. 2025
Afternoon temperatures will be in the mid-90s, with heat indices from 102 to 107 degrees, according to NWS Melbourne.
—Garfield Hylton, The Orlando Sentinel, 21 Aug. 2025
The National Weather Service issued a high-heat advisory from Homestead to West Palm Beach and warned of heat index levels up to 110 degrees.
—Christopher Cann, USA Today, 20 Aug. 2025
A day after some school activities were canceled due to high temperatures and heat index values, Louisville is starting to see some cooler temperatures.
—Leo Bertucci, The Courier-Journal, 20 Aug. 2025
These hardworking people are outside in over 90-degree weather (with a plus-100 degrees heat index) doing the jobs of people who undoubtedly would not want to be doing manual labor in this Florida heat.
—Sun Sentinel Editorial Board, Sun Sentinel, 20 Aug. 2025
Cite this Entry
“Heat index.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/heat%20index. Accessed 30 Sep. 2025.
Copy CitationLove words? Need even more definitions?
Merriam-Webster unabridged
Share