Heat Index (
HI) is an index that combines
air temperature and
relative humidity to determine an apparent temperature — how hot it actually feels. The human body normally cools itself by
perspiration, or
sweating, in which the
water in the sweat
evaporates and carries
heat away from the body. However, when the relative humidity is high, the evaporation rate of the water is reduced. This means heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which corresponds a temperature and humidity combination to a higher temperature in dry air.
In Canada the term "humidex" is used for the heat index developed by Environment Canada[?].