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)