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| The '''heat index''' ('''HI''') or '''humiture''' or '''humidex''' (not to be confused with the [[humidex|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 feels. The result is also known as the "felt air temperature" or "[[apparent temperature]]". For example, when the temperature is {{j|90 °F}} {{j|(32 °C)}} with very high humidity, the heat index can be about {{j|105 °F (41 °C).}}
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| The human body normally cools itself by [[perspiration]], or sweating. [[Heat]] is removed from the body by [[evaporation]] of that sweat. However, 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 one at higher temperature and lower humidity.
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| == History ==
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| 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.<ref>[http://www.news4jax.com/news/19262258/detail.html George Winterling: A Lifelong Passion For Weather] [[WJXT]], April 23, 2009</ref> It is derived from work carried out by Robert G. Steadman.<ref name=SteadmanI>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 {{doi|10.1175/1520-0450(1979)018<0861:TAOSPI>2.0.CO;2}} [http://journals.ametsoc.org/doi/pdf/10.1175/1520-0450%281979%29018%3C0861%3ATAOSPI%3E2.0.CO%3B2]</ref><ref>The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature Journal of Applied Meteorology, R. G. Steadman, July 1979, Vol 18 No7, pp874-885</ref> 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.<ref>[http://www.slate.com/id/2123486/fr/rss/ How do they figure the heat index? - By Daniel Engber - Slate Magazine<!-- Bot generated title -->]</ref>
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| 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 {{convert|45|°F|°C}} as a base, whereas the heat index uses a dew point base of {{convert|57|°F|°C}}. 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.
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| 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 [[Pascal (unit)|kPa]]. At [[standard atmospheric pressure]] (101.325 kPa), this baseline corresponds to a [[dew point]] of {{j|14 °C}} {{j|(57 °F)}} and a [[mixing ratio]] of 0.01 (10 g of water vapor per kilogram of dry air).<ref name=SteadmanI/> This corresponds to an air temperature of {{j|25 °C}} {{j|(77 °F)}} and relative humidity of 50% in the sea-level [[psychrometric chart]].
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| At high temperatures, the level of ''relative'' humidity needed to make the heat index higher, than the actual temperature, is lower than at cooler temperatures. For example, at approximately {{j|27 °C}} {{j|(80 °F)}}, the heat index will agree with the actual temperature if the relative humidity is 45%, but at about {{j|43 °C (110 °F),}} any relative-humidity reading above 17% will make the heat index higher than {{j|43 °C}}.
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| The formula described is considered valid only if the actual temperature is above {{j|27 °C}} {{j|(80 °F)}}, dew point temperatures greater than {{j|12 °C (54 °F),}} and relative humidities higher than 40%.<ref>[http://www.campbellsci.com/documents/technical-papers/heatindx.pdf Heat Index Campbell Scientific Inc.] (PDF file), CampbellSci.com.</ref> 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.
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| Sometimes the heat index and the [[wind chill]] are denoted collectively by the single term "apparent temperature", "relative outdoor temperature", or "feels like".
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| ==Meteorological considerations==
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| Outdoors in open conditions, as the relative humidity increases, first haze and ultimately a thicker cloud cover develops, reducing the amount of direct sunlight reaching the surface. Thus, there is an inverse relationship between maximum potential temperature and maximum potential relative humidity. Because of this factor, it was once believed that the highest heat index reading actually attainable anywhere on Earth is approximately {{convert|71|°C|°F|abbr=on}}. However, in [[Dhahran]], [[Saudi Arabia]] on July 8, 2003, the [[dew point]] was {{convert|35|°C|°F|abbr=on}} while the temperature was {{convert|42|°C|°F|abbr=on}}, resulting in a heat index of {{convert|78|°C|°F|abbr=on}}. This is comparable to the temperatures that are recommended to kill bacteria in many meat products, and it is common in a [[sauna]]. High heat-index values also indicate that intense thunderstorms are approaching, depending on the intensity of the cold front, causing more violent storms.<ref>
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| {{cite book
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| | last = Burt | first = Christopher C.
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| | authorlink =
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| | title = Extreme Weather: A Guide & Record Book
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| | url = http://books.google.com/books?id=NuP7ATq9nWgC&dq=extreme+weather+a+guide+%26+record+book&printsec=frontcover#PPA28,M1
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| | publisher = W. W. Norton & Company
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| | year = 2004 | pages = 28
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| | doi =
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| | isbn = 0-393-32658-6 }}</ref>
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| ==Table of Heat Index values==
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| This table is from the U.S. [[National Oceanic and Atmospheric Administration]].
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| {{HeatTable}}
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| To find the Heat Index temperature, look at the Heat Index chart above. For example, if the air temperature is 96°F and the relative humidity is 65%, the heat index—how hot it feels—is 121°F.
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| This table is an approximation of the Heat Index, using the formula and first set of constants below, converted to Celsius.
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| {{HeatTableC}}
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| ==Effects of the heat index (shade values)==
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| {| class="wikitable"
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| |-
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| ! Celsius || Fahrenheit || Notes
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| |-
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| | 27–32 °C || 80–90 °F
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| | Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in [[heat cramp]]s.
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| |-
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| | 32–41 °C || 90–105 °F
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| | Extreme caution: [[heat cramp]]s and [[heat exhaustion]] are possible. Continuing activity could result in [[heat stroke]].
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| |-
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| | 41–54 °C || 105–130 °F
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| | Danger: [[heat cramp]]s and [[heat exhaustion]] are likely; [[heat stroke]] is probable with continued activity.
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| |-
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| | over 54 °C || over 130 °F
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| | Extreme danger: [[heat stroke]] is imminent.
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| |-
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| |}
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| Exposure to full sunshine can increase heat index values by up to 8 °C (14 °F).<ref name=Pueblo>[http://web.archive.org/web/20110629041320/http://www.crh.noaa.gov/pub/heat.php Heat Index] on the website of the Pueblo, CO United States National Weather Service.</ref>
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| ==Formula==
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| The formula below approximates the heat index in degrees Fahrenheit, to within ±1.3 °F. It is the result of a multivariate fit (temperature equal to or greater than 80°F and relative humidity equal to or greater than 40%) to a model of the human body.<ref>Lans P. Rothfusz. "The Heat Index 'Equation' (or, More Than You Ever Wanted to Know About Heat Index)", Scientific Services Division (NWS Southern Region Headquarters), 1 July 1990 [http://www.srh.noaa.gov/images/ffc/pdf/ta_htindx.PDF]</ref><ref>R.G. Steadman, 1979. "The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science," J. Appl. Meteor., 18, 861-873</ref> This equation reproduces the above NOAA National Weather Service table (except the values at 90°F & 45%/70% relative humidity vary unrounded by less than -1/+1, respectively).
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| :<math>\mathrm{HI} = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2R + c_8 T R^2 + c_9 T^2 R^2\ \, </math>
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| where
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| :<math>\mathrm{HI}\,\!</math> = heat index (in degrees Fahrenheit)
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| :<math>T\,\!</math> = ambient [[dry-bulb temperature]] (in degrees Fahrenheit)
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| :<math>R\,\!</math> = relative humidity (percentage value between 0 and 100)
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| :<math>c_1 = -42.379, \,\!</math> <math>c_2 = 2.04901523, \,\!</math> <math>c_3 = 10.14333127,\,\!</math> <math>c_4 = -0.22475541, \,\!</math> <math>c_5 = -6.83783 \times 10^{-3},\,\!</math> <math>c_6 = -5.481717 \times 10^{-2},\,\!</math> <math>c_7 = 1.22874 \times 10^{-3}, \,\!</math> <math>c_8 = 8.5282 \times 10^{-4}, \,\!</math> <math>c_9 = -1.99 \times 10^{-6}.\,\!</math>
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| An alternative set of constants for this equation that is within 3 degrees of the NWS master table for all humidities from 0 to 80% and all temperatures between 70 and 115 °F and all heat indexes < 150 °F is
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| :<math>c_1 = 0.363445176, \,\!</math> <math> c_2 = 0.988622465, \,\!</math> <math> c_3 = 4.777114035, \,\!</math> <math> c_4 = -0.114037667, \,\!</math> <math> c_5 = -0.000850208, \,\!</math> <math> c_6 = -0.020716198, \,\!</math> <math> c_7 = 0.000687678, \,\!</math> <math> c_8 = 0.000274954, \,\!</math> <math> c_9 = 0 \,\!</math> <math>(c_9 \,\!</math> <math>unused).</math>
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| A further alternate is this:<ref>
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| {{cite book
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| | last = Stull | first = Richard
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| | authorlink =
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| | title = Meteorology for Scientists and Engineers, Second Edition
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| | url = http://books.google.com/books?ei=r_D5T9XTBIrOqAHe_c2LCQ&id=QrYRAQAAIAAJ&dq=Meteorology+for+Scientists+and+Engineers&q=5.37941#search_anchor
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| | publisher = Brooks/Cole
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| | year = 2000 | page = 60
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| | doi =
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| | isbn = 9780534372149 }}</ref>
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| :<math>\mathrm{HI} = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2 R + c_8 T R^2 + c_9 T^2 R^2 + c_{10} T^3 + c_{11} R^3 + c_{12} T^3 R + c_{13} T R^3 + c_{14} T^3 R^2 + c_{15} T^2 R^3 + c_{16} T^3 R^3\ \, </math>
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| where
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| :<math>c_1 = 16.923, \,\!</math> <math>c_2 = 0.185212, \,\!</math> <math>c_3 = 5.37941,\,\!</math> <math>c_4 = -0.100254, \,\!</math> <math>c_5 = 9.41695 \times 10^{-3},\,\!</math> <math>c_6 = 7.28898 \times 10^{-3},\,\!</math> <math>c_7 = 3.45372\times 10^{-4}, \,\!</math> <math>c_8 = -8.14971 \times 10^{-4}, \,\!</math> <math>c_9 = 1.02102 \times 10^{-5},\,\!</math> <math>c_{10} = -3.8646 \times 10^{-5},\,\!</math> <math>c_{11} = 2.91583 \times 10^{-5},\,\!</math> <math>c_{12} = 1.42721 \times 10^{-6},\,\!</math> <math>c_{13} = 1.97483 \times 10^{-7},\,\!</math> <math>c_{14} = -2.18429 \times 10^{-8},\,\!</math> <math>c_{15} = 8.43296 \times 10^{-10},\,\!</math> <math>c_{16} = -4.81975 \times 10^{-11}.\,\!</math>
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| For example, using this last formula, with temperature {{convert|90|F|C}} and relative humidity (RH) of 85%, the result would be: {{heat index|90|85}}.
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| == See also ==
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| * [[Apparent temperature]]
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| * [[Humidex]]
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| * [[Wind chill]]
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| ==References==
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| {{Reflist}}
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| == External links ==
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| *[http://www.bom.gov.au/info/thermal_stress/ Description of wind chill & apparent temperature] Formulae in metric units
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| *[http://www.wpc.ncep.noaa.gov/html/heatindex.shtml Heat Index Calculator] Calculates both °F and °C
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| {{Meteorological variables}}
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| {{DEFAULTSORT:Heat Index}}
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| [[Category:Atmospheric thermodynamics]]
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| [[Category:Weather]]
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