The heat index (HI) is a measure of temperature as perceived by humans, combining actual air temperature and relative humidity, in shaded areas; it takes into account that elevated humidity makes hot ambient temperature feel higher than the same temperature with lower humidity, as evaporative cooling of sweat is less effective. For example, when the temperature is 32 °C (90 °F) but relative humidity is high at 70% it feels as hot as 41 °C (106 °F) (the heat index) at low humidity (see § Table of values). The heat index relates to sensed temperatures in the shade without taking into account heating by direct sunlight and physical activity, or cooling by wind. The human body normally cools itself largely by evaporation of sweat. High relative humidity reduces evaporation and cooling, increasing discomfort and potential heat stress. Different individuals perceive heat differently on the basis of body shape, metabolism, level of hydration, pregnancy, other physical conditions, acclimatisation, and even psychological factors. Measurement of perceived temperature has been based on reports of how hot subjects feel under controlled conditions of temperature and humidity. Besides the heat index, other measures of apparent temperature include the Canadian humidex, the wet-bulb globe temperature, the "relative outdoor temperature", and the proprietary "RealFeel". A term sometimes translated as "heat index" but not accounting for humidity is the national thermal indicator—indicateur thermique national (ITN)−of France, an average of temperature readings at 30 mainland weather stations used to track temperature trends and define heatwaves.
History The heat index theoretical structure and supporting equations were developed in 1979 by Robert G. Steadman. 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. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature. In Canada, the similar humidex (a Canadian innovation introduced in 1965) 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 7 °C (45 °F) as a base, whereas the heat index uses a dew point base of 14 °C (57 °F). 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. In the summer of 1979, while employed by National Oceanic and Atmospheric Association (NOAA) and between semesters at Saint Louis University, Daniel J Mosley co-developed the FORTRAN program that calculated the heat humidity index, first termed the "Humidex." He incorporated seven indices in the program based on Steadman's work. He did this while progressing towards his PhD in Biology.
Definition
The heat index of a given combination of (dry-bulb) temperature and humidity is defined as the dry-bulb temperature which would feel the same if the water vapor pressure were 1.6 kPa. Quoting Steadman, "Thus, for instance, an apparent temperature of 24 °C (75 °F) refers to the same level of sultriness, and the same clothing requirements, as a dry-bulb temperature of 24 °C (75 °F) with a vapor pressure of 1.6 kPa." This vapor pressure corresponds for example to an air temperature of 29 °C (84 °F) and relative humidity of 40% in the sea-level psychrometric chart, and in Steadman's table at 40% RH the apparent temperature is equal to the true temperature between 26–31 °C (79–88 °F). At standard atmospheric pressure (101.325 kPa), this baseline also corresponds to a dew point of 14 °C (57 °F) and a mixing ratio of 0.01 (10 g of water vapor per kilogram of dry air). A given value of relative humidity causes larger increases in the heat index at higher temperatures. For example, at approximately 27 °C (81 °F), the heat index will agree with the actual temperature if the relative humidity is 45%, but at 43 °C (109 °F), any relative-humidity reading above 18% will make the heat index higher than 43 °C. It has been suggested that the equation described is valid only if the temperature is 27 °C (81 °F) or more. The relative humidity threshold, below which a heat index calculation will return a number equal to or lower than the air temperature (a lower heat index is generally considered invalid), varies with temperature and is not linear. The threshold is commonly set at an arbitrary 40%. The heat index and its counterpart the humidex both take into account only two variables, shade temperature and atmospheric moisture (humidity), thus providing only a limited estimate of thermal comfort. Additional factors such as wind, sunshine and individual clothing choices also affect perceived temperature; these factors are parameterized as constants in the heat index formula. Wind, for example, is assumed to be 5 knots (9.3 km/h). Wind passing over wet or sweaty skin causes evaporation and a wind chill effect that the heat index does not measure. The other major factor is sunshine; standing in direct sunlight can add up to 15 °F (8.3 °C) to the apparent heat compared to shade. There have been attempts to create a universal apparent temperature, such as the wet-bulb globe temperature, "relative outdoor temperature", "feels like", or the proprietary "RealFeel".
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