I've been looking around for a way to calculate the humidity ratio by the partial vapor pressure and the atmospheric pressure of the environment.

I found a procedure for such calculation on engineering toolbox (https://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html). There, it is explained that:

Based on the Ideal Gas Law, the humidity ratio can be expressed as:

x = 0.62198 pw / (pa - pw)


pw = partial pressure of water vapor in moist air (Pa, psi)

pa = atmospheric pressure of moist air (Pa, psi)

However, it is very unclear to me where they got the value 0.62198 from. I couldn't find any explanation as to why this value is used and how is it obtained.


1 Answer 1


It is the ratio between the molar mass of water (about 18.01528g/mol) and the one of dry air (about 28.97g/mol).


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