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Heat index

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.

A good example of the difference between heat index and true temperature would be comparing the climates of Houston, Texas and Dallas, Texas. Houston has lower heat temperatures due to being closer to the Gulf of Mexico, yet the city has a higher heat index because it is a humid heat. Likewise, while Dallas actually has hotter temperatures, Dallas has a dry heat, so it doesn't feel as hot.

In Canada the term "humidex" is used for the heat index developed by Environment Canada.

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