Humidex

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.

Plot of humidex depending on temperature and relative humidity

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 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:[7][8]

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
Humidex for range 15–43 °C
  Temperature (°C)
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
Dew
point
(°C)
10 1617181920212223242526272829303132333435363738394041424344
11 1718192021222324252627282930313233343536373839404142434445
12 1718192021222324252627282930313233343536373839404142434445
13 1819202122232425262728293031323334353637383940414243444546
14 1819202122232425262728293031323334353637383940414243444546
15 1920212223242526272829303132333435363738394041424344454647
16 21222324252627282930313233343536373839404142434445464748
17 222324252627282930313233343536373839404142434445464748
18 2425262728293031323334353637383940414243444546474849
19 26272829303132333435363738394041424344454647484950
20 282930313233343536373839404142434445464748495051
21 2930313233343536373839404142434445464748495051
22 31323334353637383940414243444546474849505152
23 333435363738394041424344454647484950515253
24 3536373839404142434445464748495051525354
25 37383940414243444546474849505152535455
26 394041424344454647484950515253545556
27 4243444546474849505152535455565758
28 44454647484950515253545556575859

See also

References
  1. "Spring and Summer Hazards". Environment and Climate Changes. Government of Canada. Retrieved 22 September 2016.
  2. Meteorological Service of Canada. "Humidex". Spring and Summer Weather Hazards. Environment Canada. Retrieved 20 June 2016.
  3. Hong, Jackie. "7 things you probably didn't know about the Humidex". TheStar.com. The Star. Retrieved 23 September 2016.
  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.
  5. Cbc.ca Archived 16 January 2009 at the Wayback Machine
  6. "Canada's Top Ten Weather Stories for 2007". Environment and Climate Change Canada. Retrieved 15 January 2017.
  7. "Calculation of the 1981 to 2010 Climate Normals for Canada". Archived from the original on 27 June 2013. Retrieved 4 October 2014.
  8. Canada, Environment and Climate Change (31 October 2011). "Glossary - Climate - Environment and Climate Change Canada". climate.weather.gc.ca. Retrieved 27 December 2019.

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