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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 52.6.⁵⁶

Computation formula

When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34. If the temperature remains 30 °C (86 °F) 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:⁷⁸

H = T air + 0.5555 { 6.11 × exp ⁡ [ 5417.7530 ( 1 273.15 − 1 273.15 + T dew ) ] − 10 } , {\displaystyle 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\},}

where

• H denotes the Humidex
• Tair is the air temperature in °C
• Tdew is the dewpoint temperature in °C
• exp is the exponential function.

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 10 millibars (10 hPa).

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 1 °F = ⁠5/9⁠ °C), was largely to address metrication in Canada as the country switched to the Celsius scale.

Table

See also

• Heat index (with temperature chart in both °F and °C)
• AccuWeather RealFeel temperature
• Thermal stress on humans
• Wind chill

References

1. "Spring and Summer Hazards". Environment and Climate Changes. Government of Canada. Retrieved 22 September 2016. https://ec.gc.ca/meteo-weather/default.asp?lang=En&n=6C5D4990-1 ↩

2. Meteorological Service of Canada. "Humidex". Spring and Summer Weather Hazards. Environment Canada. Retrieved 20 June 2016. https://www.canada.ca/en/environment-climate-change/services/seasonal-weather-hazards/warm-season-weather-hazards.html#toc7 ↩

3. Hong, Jackie. "7 things you probably didn't know about the Humidex". The Star. Retrieved 23 September 2016. https://www.thestar.com/news/starweather/2015/08/7-things-you-probably-didn-t-know-about-the-humidex.html ↩

4. "Spring and Summer Weather Hazards: Heat and Humidity". Environment and Climate Change Canada. Archived from the original on 14 July 2018. Retrieved 5 July 2018. https://web.archive.org/web/20180714221713/https://www.canada.ca/en/environment-climate-change/services/seasonal-weather-hazards/spring-summer.html#heat_and_humidity ↩

5. Cbc.ca Archived 16 January 2009 at the Wayback Machine https://www.cbc.ca/news/canada/manitoba/city-error-sends-sewage-into-winnipeg-basements-1.664422 ↩

6. "Canada's Top Ten Weather Stories for 2007". Environment and Climate Change Canada. Retrieved 15 January 2017. https://ec.gc.ca/meteo-weather/default.asp?lang=En&n=5EBCEA59-1 ↩

7. "Calculation of the 1981 to 2010 Climate Normals for Canada". Archived from the original on 27 June 2013. Retrieved 4 October 2014. https://web.archive.org/web/20130627223738/http://climate.weatheroffice.gc.ca/prods_servs/normals_documentation_e.html ↩

8. Canada, Environment and Climate Change (31 October 2011). "Glossary - Climate - Environment and Climate Change Canada". climate.weather.gc.ca. Retrieved 27 December 2019. https://climate.weather.gc.ca/glossary_e.html ↩