View source for Humidex

{{Short description|Index number to describe the feeling of heat in Canada}}
[[File:Humidex plot.svg|thumb|upright=2|Plot of humidex depending on temperature and relative humidity]]
The '''humidex''' (short for '''humidity index''') is an index number used by [[Canada|Canadian]] [[meteorologist]]s to describe how hot the weather feels to the average person, by combining the effect of heat and [[humidity]]. The term ''humidex'' was coined in 1965.<ref>{{Cite web|url=https://ec.gc.ca/meteo-weather/default.asp?lang=En&n=6C5D4990-1|title=Spring and Summer Hazards|website=Environment and Climate Changes|publisher=Government of Canada|access-date=2016-09-22}}</ref> The humidex is a nominally [[dimensionless quantity]] (though generally recognized by the public as equivalent to the [[Celsius scale|degree Celsius]]) based on the [[dew point]].

'''''Range of humidex: Scale of comfort''''':<ref name=SMC>{{cite web| url= https://www.canada.ca/en/environment-climate-change/services/seasonal-weather-hazards/warm-season-weather-hazards.html#toc7 |title=Humidex | work=Spring and Summer Weather Hazards| author = Meteorological Service of Canada | publisher= Environment Canada | access-date= 20 June 2016}}</ref><ref>{{cite web|last1=Hong|first1=Jackie|title=7 things you probably didn't know about the Humidex|url=https://www.thestar.com/news/starweather/2015/08/7-things-you-probably-didn-t-know-about-the-humidex.html |publisher=The Star|access-date=2016-09-23}}</ref>

* 20 to 29: Little to no discomfort
* 30 to 39: Some discomfort
* 40 to 45: Great discomfort; avoid exertion
* Above 45: Dangerous; heat stroke quite possible

== History ==
The current formula for determining the humidex was developed by J. M. Masterton and F. A. Richardson of Canada's Atmospheric Environment Service in 1979. Humidex differs from the [[heat index]] used in the United States in being derived from the [[dew point]] rather than the [[relative humidity]], though both dew point and relative humidity (when used in conjunction with air temperature) are directly related to atmospheric moisture.

For a long time,<!--Specify how long if possible--> the record humidex in Canada was set by Windsor, Ontario, which hit 52.1 on 20 June 1953, as reported by Environment Canada.<ref>{{Cite web |url=https://canada.ca/en/environment-climate-change/services/seasonal-weather-hazards/spring-summer.html#heat_and_humidity |title=Spring and Summer Weather Hazards: Heat and Humidity |publisher=Environment and Climate Change Canada |access-date=5 July 2018 |archive-url=https://web.archive.org/web/20180714221713/https://www.canada.ca/en/environment-climate-change/services/seasonal-weather-hazards/spring-summer.html#heat_and_humidity |archive-date=14 July 2018 |url-status=dead }}</ref>

This value was beaten on 25 July 2007 when [[Carman, Manitoba|Carman]], Manitoba, hit 52.6.<ref>[https://www.cbc.ca/news/canada/manitoba/city-error-sends-sewage-into-winnipeg-basements-1.664422 Cbc.ca] {{webarchive |url=https://web.archive.org/web/20090116104229/https://www.cbc.ca/news/canada/manitoba/city-error-sends-sewage-into-winnipeg-basements-1.664422 |date=16 January 2009 }}</ref><ref>{{Cite web|url=https://ec.gc.ca/meteo-weather/default.asp?lang=En&n=5EBCEA59-1 |title=Canada's Top Ten Weather Stories for 2007 |publisher=Environment and Climate Change Canada|access-date=15 January 2017}}</ref>

== Computation formula ==
When the [[temperature]] is {{convert|30|°C|°F|abbr=on}} and the [[dew point]] is {{convert|15|°C|°F|abbr=on}}, the humidex is 34. If the temperature remains {{convert|30|°C|°F|abbr=on}} and the dew point rises to {{convert|25|°C|°F|abbr=on}}, the humidex rises to 42. The humidex is higher than the U.S. heat index at equal temperature and relative humidity.

The humidex formula is as follows:<ref>{{cite web|url=http://climate.weatheroffice.gc.ca/prods_servs/normals_documentation_e.html|title=Calculation of the 1981 to 2010 Climate Normals for Canada|access-date=4 October 2014|url-status=dead|archive-url=https://web.archive.org/web/20130627223738/http://climate.weatheroffice.gc.ca/prods_servs/normals_documentation_e.html|archive-date=27 June 2013|df=dmy-all}}</ref><ref>{{Cite web|url=https://climate.weather.gc.ca/glossary_e.html|title=Glossary - Climate - Environment and Climate Change Canada|last=Canada|first=Environment and Climate Change|date=2011-10-31|website=climate.weather.gc.ca|language=en|access-date=2019-12-27}}</ref>

<math display=block>H = T_\text{air} + 0.5555 \left\{6.11\times \exp\left[5417.7530 \left(\frac{1}{273.15} - \frac{1}{273.15+T_\text{dew}}\right)\right] - 10\right\},</math>

where
* ''H'' denotes the Humidex
* ''T''<sub>air</sub> is the air temperature in °C
* ''T''<sub>dew</sub> is the dewpoint temperature in °C
* exp is the [[exponential function]].

5417.7530 is a rounded constant based on the molecular weight of water, [[Enthalpy of vaporization|latent heat of evaporation]], and the [[universal gas constant]]. The humidity adjustment approximately amounts to one Fahrenheit degree for every millibar by which the [[Vapour pressure of water|partial pressure of water]] in the atmosphere exceeds {{convert|10|mb}}.

At the time the humidex was originally developed in 1965, Canada was still on the [[Fahrenheit scale]], and thus the humidex was originally based on that. The 1979 reformulation, which added the 0.5555 factor (from the relation {{nowrap|1=1 °F = {{sfrac|5|9}} °C),}} was largely to address [[metrication in Canada]] as the country switched to the Celsius scale.

===Table===
{{HumidexTable}}

== See also ==
* [[Heat index]] (with temperature chart in both °F and °C)
* [[AccuWeather#RealFeel temperature|AccuWeather RealFeel temperature]]
* [[Thermal_comfort#Thermal_stress|Thermal stress on humans]]
* [[Wind chill]]

== References ==
{{Reflist}}

{{Meteorological variables}}
{{Use dmy dates|date=March 2017}}


[[Category:Atmospheric thermodynamics]]
[[Category:1979 introductions]]
[[Category:Climate of Canada]]
[[Category:Meteorological indices]]