Humidex: a Canadian invention

The tutor enjoys discussing scientific concepts, particularly ones with a Canadian dimension.

The humidex provides a meaningful measure of a day’s heat. By considering dew point and temperature together, it provides an “effective temperature” that suggests the comfort level of the conditions. The “effective temperature” is referred to as “the humidex”.

The humidex is calculated as follows:


where T is in Celsius, while dewpoint is, importantly, in Kelvin. To get Kelvin, you add 273 to the Celsius temperature. Therefore, a dew point of 20C is 293 K.

I have verified the formula using the following data set:

temp (°C) dewpoint (°C) humidex source
30 25 42 Wikipedia
28 20 36 Planetcalc: Humidex

When the dewpoint is 10C (283K) or less, the humidex is virtually the same as the Celsius temperature. For day-to-day life in Canada, it’s only on hot summer days, when the dew point can rise into the high teens or above, that the humidex becomes important.

Today’s humidex formula comes from J.M. Masterton and F.A. Richardson, Canada’s Atmospheric Environment Service, 1979. However, the humidex was first used in 1965, possibly with a different formula.


Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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