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 humidex is a dimensionless quantity based on the dew point, but it is equivalent to dry temperature in degrees Celsius (omitting degree symbol '°'). For example, if the temperature is 30 °C (86 °F), and the calculated humidex is 40, then it indicates the humid heat feels approximately like a dry temperature of 40 °C (104 °F).

According to the Meteorological Service of Canada, a humidex of at least 30 causes "some discomfort", at least 40 causes "great discomfort" and above 45 is "dangerous". When the humidex hits 54, heat stroke is imminent.

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 occurred on 14 July 1961, when Castlegar, British Columbia, recorded a humidex of 53.4. This value was almost beaten on 25 July 2007 when Carman, Manitoba, hit 53.0.

The humidex computation formula

When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34 (note that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C). 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:

$\text{Humidex} = T_\text{air} + 0.5555 \left[6.11 e^{5417.7530 \left(\frac{1}{273.16} - \frac{1}{T_\text{dew}}\right)} - 10\right]$

where

• $\scriptstyle T_\text{air}$ is the air temperature in °C
• $\scriptstyle T_\text{dew}$ is the dewpoint in K

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.

See also

• Heat index
• Wind chill

External links

• Humidex on Environment Canada web site
• Wind Chill and Humidex Criticism about the use of Wind chill and humidex
• More Humidex info