Heat Index
The apparent temperature combining air temperature and relative humidity, as published by the NWS using the Steadman formula.
The heat index is a single number that combines air temperature and relative humidity into an apparent temperature. It was developed by Robert Steadman in a 1979 paper that modeled how a healthy adult (5'7", 147 lb, walking 3 mph in shade) would experience various combinations of temperature and humidity.
The formula is a multivariate regression. Below 80 F or below 40% humidity it reduces to roughly the actual air temperature. Above those thresholds, the heat index rises faster than temperature because humidity slows evaporative cooling from sweat.
The NWS chart converts air temperature and relative humidity into one of four risk bands: Caution (80 to 90), Extreme Caution (90 to 103), Danger (103 to 125), and Extreme Danger (above 125). Heat advisories and warnings are issued based on forecast HI values.
The heat index assumes shade and light wind. Direct sun adds about 15 F. This is the most common reason people feel hotter than the heat index suggests.
Related terms
Wet-Bulb Temperature
The lowest temperature evaporative cooling can achieve under current humidity, measured by a thermometer wrapped in a wet cloth.
Feels-Like Temperature
A general-purpose apparent temperature, formula varies by weather provider.
Heat Advisory
An NWS bulletin issued when forecast heat index will reach 100 to 105 F (regional threshold) for 2+ hours within 36 hours.