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The heat index (HI) or humiture or humidex (not to be confused with the Canadian humidex) is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature—how hot it would feel if the humidity were some other value. The result is also known as the "felt air temperature" or "apparent temperature". For example, when the temperature is with very high humidity, the heat index can be about . The human body normally cools itself by perspiration, or sweating. Heat is removed from the body by evaporation of that sweat. However, high relative humidity reduces the evaporation rate because the higher vapor content of the surrounding air does not allow the maximum amount of evaporation from the body to occur. This results in a lower rate of heat removal from the body, hence the sensation of being overheated. This effect is subjective; its measurement has been based on subjective descriptions of how hot subjects feel for a given temperature and humidity. This results in a heat index that relates one combination of temperature and humidity to another. == History == The heat index was developed in 1978 by George Winterling as the "humiture" and was adopted by the USA's National Weather Service a year later.〔(George Winterling: A Lifelong Passion For Weather ) WJXT, April 23, 2009〕 It is derived from work carried out by Robert G. Steadman.〔The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science, R. G. Steadman, Journal of Applied Meteorology, July 1979, Vol 18 No7, pp861–873 ()〕〔The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature, R. G. Steadman, Journal of Applied Meteorology, July 1979, Vol 18 No7, pp874–885〕 Like the wind chill index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, thickness of blood, sunlight and ultraviolet radiation exposure, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature.〔(How do they figure the heat index? - By Daniel Engber - Slate Magazine )〕 In Canada, the similar humidex is used in place of the heat index. While both the humidex and the heat index are calculated using dew point, the humidex uses a dew point of as a base, whereas the heat index uses a dew point base of . 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. A joint committee formed by the United States and Canada to resolve differences has since been disbanded . The heat index is defined so as to equal the actual air temperature when the partial pressure of water vapor is equal to a baseline value of 1.6 kPa. At standard atmospheric pressure (101.325 kPa), this baseline corresponds to a dew point of and a mixing ratio of 0.01 (10 g of water vapor per kilogram of dry air).〔 This corresponds to an air temperature of and relative humidity of 50% in the sea-level psychrometric chart. A given value of relative humidity causes larger increases in the heat index at higher temperatures. For example, at approximately , the heat index will agree with the actual temperature if the relative humidity is 45%, but at about , any relative-humidity reading above 17% will make the heat index higher than . It has been suggested that the equation described is valid only if the temperature is or more, and the relative humidity is 40% or more.〔(Heat Index Campbell Scientific Inc. ) (PDF file), CampbellSci.com.〕 However, a recent analysis by (iWeatherNet ) found the assumption to be erroneous given that the heat index/relative humidity relationship and the corresponding equilibrium temperature (the point at which the air temperature and the heat index are equal) are nonlinear. The heat index and humidex figures are based on temperature measurements taken in the shade and not the sun, so extra care must be taken while in the sun. The heat index also does not factor in the effects of wind, which lowers the apparent temperature, unless the air is above body temperature. Sometimes the heat index and the wind chill are denoted collectively by the single term "apparent temperature", "relative outdoor temperature", or "feels like". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「heat index」の詳細全文を読む スポンサード リンク
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