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{{Short description|Index number to describe the feeling of heat in Canada}}
{{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]]
[[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]].
The '''humidex''' (short for '''humidity index''') is an index number used by 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>
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 |work=The Star|date=17 August 2015 |access-date=2016-09-23}}</ref>


* 20 to 29: Little to no discomfort
* 20 to 29: Little to no discomfort
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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>
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>\text{Humidex} = {T}_\text{air} + 0.5555(e - 10.0)</math>


<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 <math>e</math> is the vapour pressure in millibars given by:


where
<math display=block>e = 6.11 \exp \left [5417.7530 \left (\frac{1}{273.16} - \frac{1}{T_\text{dewpoint}} \right ) \right ]</math>
* ''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]].


also note:
* <math>{T}_\text{air}</math> is the air temperature in degrees Celsius
* <math>{T}_\text{dewpoint}</math> is the dewpoint in kelvins, which can be calculated from dewpoint in degrees Celsius using <math>\{T\}_\text{K} = \{T\}_{^\circ\text{C}} + 273.15</math>
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}}.
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.
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 added the 0.5555 factor (from the relation {{nowrap|1=1 °F = {{sfrac|5|9}} °C),}} to address [[metrication in Canada]] as the country switched to the Celsius scale.


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

Latest revision as of 14:12, 9 December 2025

Template:Short description

File:Humidex plot.svg
Plot of humidex depending on temperature and relative humidity

The humidex (short for humidity index) is an index number used by Canadian meteorologists 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.[1] The humidex is a nominally dimensionless quantity (though generally recognized by the public as equivalent to the degree Celsius) based on the dew point.

Range of humidex: Scale of comfort[2][3]

  • 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, the record humidex in Canada was set by Windsor, Ontario, which hit 52.1 on 20 June 1953, as reported by Environment Canada.[4]

This value was beaten on 25 July 2007 when Carman, Manitoba, hit 52.6.[5][6]

Computation formula

When the temperature is Script error: No such module "convert". and the dew point is Script error: No such module "convert"., the humidex is 34. If the temperature remains Script error: No such module "convert". and the dew point rises to Script error: No such module "convert"., 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:[7][8] Humidex=Tair+0.5555(e−10.0)

where e is the vapour pressure in millibars given by:

e=6.11exp⁡[5417.7530(1273.16−1Tdewpoint)]

also note:

  • Tair is the air temperature in degrees Celsius
  • Tdewpoint is the dewpoint in kelvins, which can be calculated from dewpoint in degrees Celsius using {T}K={T}∘C+273.15

5417.7530 is a rounded constant based on the molecular weight of water, latent heat of evaporation, and the universal gas constant. The humidity adjustment approximately amounts to one Fahrenheit degree for every millibar by which the partial pressure of water in the atmosphere exceeds Script error: No such module "convert"..

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 added the 0.5555 factor (from the relation 1 °F = Template:Sfrac °C), to address metrication in Canada as the country switched to the Celsius scale.

Table

Template:HumidexTable

See also

References

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  5. ↑ Cbc.ca Template:Webarchive
  6. ↑ Script error: No such module "citation/CS1".
  7. ↑ Script error: No such module "citation/CS1".
  8. ↑ Script error: No such module "citation/CS1".

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