Asked by: Sean Haz

The Humidex formula is based on the work of J.M. Masterton and F.A. Richardson at AES, 1979. It's a standard for Canada, but variations are used around the world. The dewpoint temperature should be given in Kelvin for the formula to work. The magic number 5417.7530 is a rounded constant; it's based on the molecular weight of water, latent heat of vaporization, and the universal gas constant.

humidex = (air temperature) + h

where, h = (0.5555)*(e - 10.0); and,

e = 6.11 * exp(5417.7530 * ((1/273.16) - (1/dewpoint)))

Also a related and simplified formula that takes temperature and the relative humidity inputs is:

Humidex = T + 5/9 * (e-10)

where:

e = vapour pressure(6.112*10^(7.5*T/(237.7+T))*H/100)

T= air temperature (degrees Celsius)

H= humidity (%)

Click here to visit our online Humidex and Windchill calculator.

Also, for more information on various weather frequently asked questions visit: Environment Canada Web Site

Answered by: Anton Skorucak, M.S. Physics, PhysLink.com Creator

'Physicists like to think that all you have to do is say, these are the conditions, now what happens next?'**Richard Phillips Feynman**

(*1918-1988*)