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.
Read more about Heat Index: History, Meteorological Considerations, Table of Fahrenheit Heat Index Values, Effects of The Heat Index (shade Values), Formula
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