Editing Heat index (section)

==Definition==
[[File:Heat index perspective plot.png|thumb|A generalized view of the heat index showing how the perception of heat by the human body increases with temperature but more rapidly at higher humidity levels.]]

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&nbsp;[[kilopascal|kPa]]. Quoting Steadman, "Thus, for instance, an apparent temperature of {{cvt|24|C}} refers to the same level of sultriness, and the same clothing requirements, as a dry-bulb temperature of {{cvt|24|C}} with a vapor pressure of 1.6 kPa."<ref name=SteadmanI/>

This vapor pressure corresponds for example to an air temperature of {{cvt|29|°C}} 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 {{cvt|26|-|31|°C}}. At [[standard atmospheric pressure]] (101.325 kPa), this baseline also corresponds to a [[dew point]] of {{cvt|14|°C}} and a [[mixing ratio]] of 0.01 (10 g of water vapor per kilogram of dry air).<ref name=SteadmanI/>

A given value of relative humidity causes larger increases in the heat index at higher temperatures. For example, at approximately {{cvt|27|°C}}, the heat index will agree with the actual temperature if the relative humidity is 45%, but at {{cvt|43|°C}}, any relative-humidity reading above 18% will make the heat index higher than {{nowrap|43 °C}}.<ref name=iweathernet>[http://www.iweathernet.com/educational/heat-index-calculator-and-conversion-table Heat index calculator and conversion table] from iWeatherNet</ref>

It has been suggested that the equation described is valid only if the temperature is {{cvt|27|°C}} or more.<ref>[http://www.campbellsci.com/documents/technical-papers/heatindx.pdf Heat Index Campbell Scientific Inc.] {{webarchive|url=https://web.archive.org/web/20100525004438/http://www.campbellsci.com/documents/technical-papers/heatindx.pdf |date=2010-05-25 }} (PDF file), CampbellSci.com.</ref> 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%.<ref name=iweathernet/>

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 [[Parametrization (atmospheric modeling)|parameterized as constants]] in the heat index formula. Wind, for example, is assumed to be {{convert|5|knots|km/h}}.<ref name=iweathernet/> 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 {{convert|15|°F-change|°C-change}} to the apparent heat compared to shade.<ref>[https://www.weather.gov/safety/heat-index Heat Index] from the National Weather Service. "exposure to full sunshine can increase heat index values by up to 15°F."</ref> 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 "[[AccuWeather|RealFeel]]".