### Question

What is the Humidex formula used to find subjective temperature as a function of both measured temperature and relative humidity?

Humidex value is basically an equivalent temperature (one that human body would feel) given the actual temperature and the relative humidity of air. You have probably noticed that it feels hotter on a 30 deg C day when it is humid than on a 30 deg C day when it is dry!

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 (%)

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