Humidex


The humidex 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 term humidex was coined in 1965. The humidex is a nominally dimensionless quantity based on the dew point.
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 used in the United States in being derived from the dew point rather than the relative humidity, though both dew point and relative humidity 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, Manitoba, hit 52.6.

Computation formula

When the temperature is and the dew point 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:
where is the vapour pressure in millibars given by:
also note:
  • is the air temperature in degrees Celsius
  • is the dewpoint in kelvins, which can be calculated from dewpoint in degrees Celsius using
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, and thus the humidex was originally based on that. The 1979 reformulation added the 0.5555 factor (from the relation to address metrication in Canada as the country switched to the Celsius scale.

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