Humidex

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 term humidex is a Canadian innovation and was first coined in 1965. The humidex is a dimensionless quantity based on the dew point.

Range of humidex: Degree 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

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 first occurred in Windsor, Ontario with measurement at 52.1 degrees Celsius on 20 June 1953 as reported by Environment Canada.

This value was beaten on 25 July 2007 when Carman, Manitoba, hit 53.0.

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:

where

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).

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