The heat index (HI) or humiture is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels. The result is also known as the "felt air temperature" or "apparent temperature". For example, when the temperature is 90 °F (32 °C) with very high humidity, the heat index can be about 105 °F (41 °C).
The human body normally cools itself by perspiration, or sweating. The evaporation of water needs heat that is absorbed from the body to yield water vapor. However, a higher relative humidity reduces the evaporation rate because of the higher vapor content. This results in a slower heat removal from the body, hence the overheat sensation. this effect being subjective, its measurement has been based on subjective descriptions of how hot subjects feel for a given temperature and humidity. This results in the heat index that relates one combination of temperature and humidity to another one at higher temperature and lower humidity.