The humidex (short for humidity index) is an index number used by
Canadian
Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, many (or all) of these connections exist and are collectively the source of ...
meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and
humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
. The term ''humidex'' was coined in 1965. The humidex is a nominally
dimensionless quantity (though generally recognized by the public as equivalent to the
degree Celsius) based on the
dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will ...
.
''Range of humidex: Scale of comfort'':
* 20 to 29: Little to no discomfort
* 30 to 39: Some discomfort
* 40 to 45: Great discomfort; avoid exertion
* Above 45: Dangerous; heat stroke quite possible
History
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
The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is al ...
used in the United States in being derived from the
dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will ...
rather than the
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
, though both dew point and relative humidity (when used in conjunction with air temperature) are directly related to atmospheric moisture.
For a long time, the record humidex in Canada was set by Windsor, Ontario, which hit 52.1 on 20 June 1953, as reported by Environment Canada.
This value was beaten on 25 July 2007 when
Carman
In Celtic mythology, Carman or Carmun was a warrior and sorceress from Athens who tried to invade Ireland in the days of the Tuatha Dé Danann, along with her three sons, Dub ("black"), Dother ("evil") and Dian ("violence"). She used her magica ...
, Manitoba, hit 52.6.
Computation formula
When the
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
is and the
dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will ...
is , the humidex is 34. If the temperature remains and the dew point rises to , 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:
: