Thehumidex (short forhumidity index) is an index number used by Canadianmeteorologists to describe how hot the weather feels to the average person, by combining the effect of heat andhumidity. The termhumidex was coined in 1965.^[1] The humidex is a nominallydimensionless quantity (though generally recognized by the public as equivalent to thedegree Celsius) based on thedew 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

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 theheat index used in the United States in being derived from thedew point rather than therelative 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 whenCarman, Manitoba, hit 52.6.^[5][6]

When thetemperature is 30 °C (86 °F) and thedew 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]$${\displaystyle \text{Humidex}=T_{\text{air}}+0.5555(e-10.0)}$$

where${\displaystyle e}$ is the vapour pressure in millibars given by:

$${\displaystyle e=6.11\exp \left[5417.7530\left(\frac{1}{273.16}-\frac{1}{T_{\text{dewpoint}}}\right)\right]}$$

also note:

• ${\displaystyle T_{\text{air}}}$ is the air temperature in degrees Celsius
• ${\displaystyle T_{\text{dewpoint}}}$ is the dewpoint in kelvins, which can be calculated from dewpoint in degrees Celsius using${\displaystyle \{T{\}}_{\text{K}}=\{T{\}}_{{}^{\circ }\text{C}}+273.15}$

5417.7530 is a rounded constant based on the molecular weight of water,latent heat of evaporation, and theuniversal gas constant. The humidity adjustment approximately amounts to one Fahrenheit degree for every millibar by which thepartial 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 theFahrenheit scale, and thus the humidex was originally based on that. The 1979 reformulation added the 0.5555 factor (from the relation1 °F =⁠5/9⁠ °C), to addressmetrication in Canada as the country switched to the Celsius scale.

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

• Atmospheric thermodynamics
• 1979 introductions
• Climate of Canada
• Meteorological indices

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