A joint committee formed by the United States and Canada to resolve differences has since been disbanded. Further, the heat index uses heat balance equations which account for many variables other than vapor pressure, which is used exclusively in the humidex calculation. While both the humidex and the heat index are calculated using dew point, the humidex uses a dew point of 7 ☌ (45 ☏) as a base, whereas the heat index uses a dew point base of 14 ☌ (57 ☏). In Canada, the similar humidex (a Canadian innovation introduced in 1965) is used in place of the heat index. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature. Like the wind chill index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, individual heat tolerance, sunlight and ultraviolet radiation exposure, and the wind speed. The heat index was developed in 1979 by Robert G. Also, for people exercising or active, at the time, then the heat index could give a temperature lower than the felt conditions. This results in a heat index that relates one combination of temperature and humidity to another.īecause the heat index is based on temperatures in the shade, while people often move across sunny areas, the heat index can give a much lower temperature than actual conditions of typical outdoor activities. This effect is subjective, with different individuals perceiving heat differently for various reasons (such as differences in body shape, metabolic differences, differences in hydration, pregnancy, menopause, effects of drugs or drug withdrawal) its measurement has been based on subjective descriptions of how hot subjects feel for a given temperature and humidity. This results in a lower rate of heat removal from the body, hence the sensation of being overheated. However, high relative humidity reduces the evaporation rate. Heat is removed from the body by evaporation of that sweat. The human body normally cools itself by perspiration, or sweating. For example, when the temperature is 32 ☌ (90 ☏) with 70% relative humidity, the heat index is 41 ☌ (106 ☏) (see table below). The result is also known as the "felt air temperature", " apparent temperature", "real feel" or "feels like". The heat index ( HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade.
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