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The '''humidex''' (short for '''"humi'''dity in'''dex"''') is an index number used by [[Canada|Canadian]] [[meteorology|meteorologists]] to describe how hot the weather feels to the average person, by combining the effect of heat and [[humidity]]. The humidex is a unit-less number based on the [[dew point]], but it is equivalent to dry temperature in degrees Celsius. For example, if the temperature is {{convert|30|°C|F}}, and the calculated humidex is 40, then it indicates the humid heat feels approximately like a dry temperature of {{convert|40|°C|F}}.

According to the [[Meteorological Service of Canada]], a humidex of at least 30 causes "some discomfort", at least 40 causes "great discomfort" and above 45 is "dangerous". When the humidex hits 54, [[heat stroke]] is imminent.

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]].  

The record humidex in Canada occurred on July 14, 1961, when [[Castlegar, British Columbia]] recorded a humidex of 53.4.<ref>{{cite web |url = http://climate.weatheroffice.gc.ca/climate_normals/results_e.html?stnID=1105&lang=e&dCode=1&StationName=CASTLEGAR&SearchType=Contains&province=ALL&provBut=&month1=0&month2=12 |title = National Climate Data and Information Archive, Castlegar A |date = 2013-02-04 |accessdate = 2013-07-07}}</ref> This value was almost beaten on July 25, 2007 when [[Carman, Manitoba]] hit 53.0.<ref>http://www.cbc.ca/canada/manitoba/story/2007/07/26/storm-sewage.html</ref> 

== The humidex computation formula ==

When the [[temperature]] is 30 °C (86 °F) and the [[dew point]] is 15 °C (59 °F), the humidex is 34 (note that humidex is a [[dimensionless number]], but that the number indicates an approximate temperature in °C). If the temperature remains 30 °C and the dew point rises to 25 °C (77 °F), 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>http://climate.weatheroffice.gc.ca/prods_servs/normals_documentation_e.html</ref>
:<math>\text{Humidex} = \text{Air temperature}\ +\ 0.5555 \times (6.11 \times e^{5417.7530 \times \left(\frac{1}{273.16} - \frac{1}{\text{dewpoint in kelvins}}\right)} - 10)</math>
The humidity adjustment effectively amounts to one Fahrenheit degree for every millibar by which the partial pressure of water in the atmosphere exceeds 10 millibars.

== See also ==

*[[Heat index]]
*[[Wind chill]]

== References ==

{{Reflist}}

== External links ==
* [http://ec.gc.ca/meteo-weather/default.asp?lang=En&n=6C5D4990-1#humidex Humidex on Environment Canada web site]
*[http://ptaff.ca/humidex/?lang=en_CA Wind Chill and Humidex] Criticism about the use of Wind chill and humidex
*[http://everything2.com/?node_id=87523 More Humidex info]

{{Meteorological variables}}

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

[[ca:Temperatura de xafogor]]
[[de:Hitzeindex]]
[[es:Temperatura de bochorno]]
[[eo:Humidekso]]
[[fr:Indice humidex]]
[[pt:Índice de calor]]
[[ru:Humidex]]
[[simple:Humidex]]
[[fi:Kosteusindeksi]]
[[zh:酷热指数]]