About Humidex

What is Humidex?

Humidex is an index to indicate how hot or humid the weather feels to the average person. It is derived by combining temperature and humidity values into one number to reflect the perceived temperature. For example, a humidex of 40 means that the sensation of heat when the temperature is 30 degrees and the air is humid feels more or less the same as when the temperature is 40 degrees and the air is dry. Hourly Humidex values are only displayed when the air temperature is 20C or greater and the humidex value is at least 1 degree greater than the air temperature.

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.

What is the formula of Humidex?

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 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:


  1. H denotes the Humidex
  2. Tair is the air temperature in °C
  3. Tdew is the dewpoint temperature in °C
  4. e = 2.71828

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 5⁄9 factor, was largely to address metrication in Canada as the country switched to the Celsius scale.