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 dimensionless quantity based on the dew point.
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 record humidex in Canada occurred on 14 July 1961, when Castlegar, British Columbia, recorded a humidex of 53.4. This value was almost beaten on 25 July 2007 when Carman, Manitoba, hit 53.0.
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. 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:
- is the air temperature in °C
- is the dewpoint in K
The humidity adjustment effectively amounts to one Fahrenheit degree for every millibar by which the partial pressure of water in the atmosphere exceeds 10 millibars (10 hPa).
|Temperature (°C, for range 59–109 °F)|
- Meteorological Service of Canada. "Humidex". Spring and Summer Weather Hazards. Environment Canada. Retrieved June 20, 2016.
- "National Climate Data and Information Archive, Castlegar A". 2013-02-04. Retrieved 2013-07-07.
- Cbc.ca Archived January 16, 2009, at the Wayback Machine.
- "Calculation of the 1981 to 2010 Climate Normals for Canada". Retrieved 4 October 2014.