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 humidex is a unit-less number based on the dew point, but it is equivalent to dry temperature in degrees Celsius. For example, if the temperature is 30 °C (86 °F), and the calculated humidex is 40, then it indicates the humid heat feels approximately like a dry temperature of 40 °C (104 °F).
According to the Meteorological Service of Canada, a humidex of at least 30 causes "some discomfort", at least 40 causes "great discomfort" and above 45 is "dangerous". When the humidex hits 54, heat stroke is imminent.
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.
The humidex computation formula 
When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34 (note that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C). 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:
See also 
- "Frequently Asked Questions", MSC, 2004.
- Wind Chill and Humidex Criticism about the use of Wind chill and humidex
- More Humidex info