Heat index

From Glossary of Meteorology

heat index

The heat index (HI) or “apparent temperature” is an approximation of how hot it “feels” for a given combination of air temperature and relative humidity (RH). Generally, higher RH values at the same temperature feel warmer or more stressful because of less evaporative cooling when people perspire. The HI is the result of extensive biometeorological studies over a period of decades by various researchers, most notably Robert G. Steadman.

The U.S. National Weather Service (NWS) performed regression analysis to adapt Steadman’s data into an NWS HI equation that can use readily available variables as input. This equation requires only air temperature and RH. The NWS HI table is formulated from that equation.



Steadman, R. G., 1979: The assessment of sultriness. Part I: A temperature–humidity index based on human physiology and clothing science. J. Appl. Meteor., 18, 861–873.

(Image from NOAA/NWS)

Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact [email protected]. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.