Humidex (Canadian Humidity Index)

The humidex is Canada's answer to the question of how hot it feels when the air is wet. J. M. Masterton and F. A. Richardson devised it in 1979 at the Atmospheric Environment Service by adding to the air temperature a term proportional to the excess of the actual vapour pressure over 10 hPa, a value chosen to represent comfortably dry air. When the air is that dry the correction is zero and the humidex is simply the temperature.

Environment Canada is careful to call the result a unitless index rather than a temperature, and this page follows that convention. It sits on the Celsius scale by construction, so a humidex of 40 feels roughly like 40 °C in dry air, but it is a comfort scale and not a reading you could take with a thermometer. The published guidance runs: below 29 no discomfort, 30 to 39 some discomfort, 40 to 45 great discomfort with outdoor exertion to be curtailed, and above 45 dangerous.

The vapour pressure comes from the dew point through a Clausius-Clapeyron style exponential, which is why the humidex climbs so steeply on muggy days. A rise of a few degrees in dew point moves the vapour pressure far more than the same rise in air temperature moves anything, and the index reflects a real physical fact: sweat is your cooling system, and humid air will not accept it.