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)