# Canadian Humidity Index (HUMIDEX)

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Equation / Last modified by KurtHeckman on 2018/07/10 16:22
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The Canadian HUMIDEX calculator computes the Canadian Humidity Index (HUMIDEX) based on the dry-bulb ambient temperature and the dew-point temperature.

INSTRUCTIONS: Choose units and enter the following:

• (Tair) This is the dry bulb temperature
• (Tdew) This is the dew-point temperature.

HUMIDEX:  The calculator returns the HUMIDEX.

#### The Math / Science

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. It is not equivalent to a dry temperature in degrees Celsius, it is just calculated to look like it.(1) Don't know the dew point? No problem, you can calculate it using the conversion from Relative Humidity.

#### Range of humidex: Degree of comfort(2)

• Less than 29: No discomfort
• 30 to 39: Some discomfort
• 40 to 45: Great discomfort; avoid exertion
• Above 45: Dangerous; Heat stroke possible
##### HUMIDEX Equation

Humidex = Tair + h;

h = (5/9)(e-10);

e = 6.11exp[(MwL/R)*(1/273.16 - 1/Tdew)].

Where

• Humidex is the Canadian Humidity Index
• Tair is the dry ambient temperature in oC
• Tdew is the dew point temperature in K
• h is the humidity adjustment factor
• Mw is the molecular weight of water
• L is the Latent Heat of Vaporization for water
• R is the ideal gas constant(3).

The Humidex is designed to represent human heat stress conditions and is not meaningful at cool temperatures.(4)

# References

1. Humidex. (n.d.). Retrieved May 19, 2016, from https://en.wikipedia.org/wiki/Humidex
2. Weather and Meteorology. (n.d.). Retrieved May 19, 2016, from http://ec.gc.ca/meteo-weather/default.asp?lang=En
3. Masterton JM, Richardson FA (1979) A method of quantifying human discomfort due to excessive heat and humidity. Downsview, Ontario, Canada:AES, Env. Canada, CLI 1-79.
4. Smoyer-Tomic, K. E., & Rainham, D. G. (2001). Beating the heat: development and evaluation of a Canadian hot weather health-response plan. Environmental health perspectives, 109(12), 1241.