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

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

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: :

H = T_\text + 0.5555 \left .11\times e^ - 10\right /math>

where
* ''H'' denotes the Humidex
* ''T''

_air is the air temperature in °C * ''T''_dew is the dewpoint temperature in °C * ''e'' = 2.71828 is an approximation of Euler's number 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 . At the time the humidex was originally developed in 1965, Canada was still on the

Fahrenheit scale The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined h ...

, and thus the humidex was originally based on that. The 1979 reformulation, which added the 0.5555 factor, was largely to address

metrication in Canada Metrication in Canada began in 1970 and ceased in 1985. While Canada has converted to the metric system for many purposes, there is still significant use of non-metric units and standards in many sectors of the Canadian economy and everyday life t ...

as the country switched to the Celsius scale.

Table

References

{{Use dmy dates, date=March 2017 Atmospheric thermodynamics 1979 introductions Climate of Canada Meteorological indices

History

Computation formula

Table

See also

References