Heat Index (HI) is an index that combines airtemperature 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.
As relative humidity increases, however, first haze and ultimately thicker cloud cover must develop, reducing the amount of direct sunlight reaching the surface; thus there is an inverse relationship between maximum potential temperature and maximum potential relative humidity, therefore making, say, a simultaneous temperature of 50 °C (122 °F) and 90 per cent relative humidity physically impossible. Because of this factor, it is believed that the highest heat-index reading actually attainable anywhere on Earth is approximately 70 °C (158 °F).
A good example of the difference between heat index and true temperature would be comparing the climates of New Orleans, Louisiana and Bakersfield, California. New Orleans 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 Bakersfield actually has hotter daytime temperatures, Bakersfield has a dry heat, so it doesn't feel as hot.