The humidex (short for humidity index) is an index number used by

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s 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 (meteorology), precipitation, dew, or fog t ...

. The term ''humidex'' was coined in 1965. The humidex is a nominally

dimensionless quantity Dimensionless quantities, or quantities of dimension one, are quantities implicitly defined in a manner that prevents their aggregation into unit of measurement, units of measurement. ISBN 978-92-822-2272-0. Typically expressed as ratios that a ...

(though generally recognized by the public as equivalent to the

degree Celsius The degree Celsius is the unit of temperature on the Celsius temperature scale "Celsius temperature scale, also called centigrade temperature scale, scale based on 0 ° for the melting point of water and 100 ° for the boiling point ...

) based on the

dew point The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air. When the air at a temperature above the ...

. ''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 shade (shadow), 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 (s ...

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 (meteorology), precipitation, dew, or fog t ...

, 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

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, Manitoba, hit 52.6.

Computation formula

When the

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

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\,

where * ''H'' denotes the Humidex * ''T''_air is the air temperature in °C * ''T''_dew is the dewpoint temperature in °C * exp is the exponential function. 5417.7530 is a rounded constant based on the molecular weight of water, latent heat of evaporation, and the

universal gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature, temperature ...

. 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 German-Polish physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he original ...

, and thus the humidex was originally based on that. The 1979 reformulation, which added the 0.5555 factor (from the relation 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. ...

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