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
Today's weather Expect hazy sunshine and high humidity, pushing heat index values into the mid and upper 90s.
—Justin Lewis, CBS News, 19 May 2026
Prolonged exposure to a heat index above 80 degrees can lead to fatigue, as previously reported by USA TODAY.
—Brandi D. Addison, USA Today, 15 May 2026
Very humid conditions are also expected, resulting in heat index values as high as 97 degrees.
—Anthony Franze, San Antonio Express-News, 10 May 2026
Afternoon heat index may reach 100 on Thursday afternoon along the I-95 corridor.
—Doris Alvarez Cea, Florida Times-Union, 6 May 2026
The demonstration on Friday, with a heat index projected to hit 100 degrees, kicked off Miami-Dade’s heat season, which now arrives earlier, lasts longer and is more intense because of climate change.
—Ashley Miznazi, Miami Herald, 4 May 2026
Highs in the mid 90s inland by Thursday, heat index near 100.
—Bill Kearney, Sun Sentinel, 4 May 2026
Moisture levels remain high for April, and a bit of sunshine will help highs climb into the mid-80s with heat index values near 90 degrees.
—Newsroom Meteorologist, Austin American Statesman, 13 Apr. 2026
The change will ensure these households can use fans or air conditioners when the NWS issues heat indexes of 95 degrees Fahrenheit or above.
—Sophie Hartley, IndyStar, 27 Feb. 2026
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.
Justin Lewis, CBS News, 19 May 2026
Prolonged exposure to a heat index above 80 degrees can lead to fatigue, as previously reported by USA TODAY.
—Brandi D. Addison, USA Today, 15 May 2026
Very humid conditions are also expected, resulting in heat index values as high as 97 degrees.
—Anthony Franze, San Antonio Express-News, 10 May 2026
Afternoon heat index may reach 100 on Thursday afternoon along the I-95 corridor.
—Doris Alvarez Cea, Florida Times-Union, 6 May 2026
The demonstration on Friday, with a heat index projected to hit 100 degrees, kicked off Miami-Dade’s heat season, which now arrives earlier, lasts longer and is more intense because of climate change.
—Ashley Miznazi, Miami Herald, 4 May 2026
Highs in the mid 90s inland by Thursday, heat index near 100.
—Bill Kearney, Sun Sentinel, 4 May 2026
Moisture levels remain high for April, and a bit of sunshine will help highs climb into the mid-80s with heat index values near 90 degrees.
—Newsroom Meteorologist, Austin American Statesman, 13 Apr. 2026
The change will ensure these households can use fans or air conditioners when the NWS issues heat indexes of 95 degrees Fahrenheit or above.
—Sophie Hartley, IndyStar, 27 Feb. 2026
Cite this Entry
“Heat index.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/heat%20index. Accessed 23 May. 2026.
Copy CitationLove words? Need even more definitions?
Merriam-Webster unabridged
