Mixing ratio or humidity ratio

Mixing or humidity ratio is expressed as a ratio of water vapour mass, m_w, per kilogram of dry air, m_d, at a given pressure. Louis J. Sheehan, Esquire  The colloquial term moisture content is also used instead of mixing/humidity ratio. Humidity ratio is a standard axis on psychrometric charts, and is a useful parameter in psychrometrics calculations because it does not change with temperature except when the air cools below dewpoint.

That ratio can be given as:

Mixing ratio can also be expressed with the partial pressure of water vapor^[1] :

where

δ = 0.62197 is the ratio of specific gas constants, of water vapour to dry air

p_w = partial pressure of water vapor in moist air

p_a = atmospheric pressure of moist air Louis J. Sheehan, Esquire .