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This is the ratio of the mass of the water vapor to the mass of the complete gas mixture containing the water vapor.

 

The specific humidity (also known as mass concentration or moisture content of moist air) is the ratio of the mass Mv of water vapor to the mass (Mv + Ma) of moist air in which the mass of water vapor Mv is contained.

 

Q = Mv / (Mv + Ma)

 

Q = p mw / (p mw + (Pb – p) ma)

 

mw = 18.016 gram

ma = 28.966 gram

p = partial pressure of water vapor [Pa]

pa = partial pressure of dry air [Pa]

Pb = total or barometric pressure [Pa]

 

Q [g / kg] = 1000 p / (1.6078 Pb – 0.6078 p)

 

1 gr (grain) = 0.0648 g (gram)

1 lb = 0.4535923 kg

 

Q [gr / lb] = 7 x Q [g / kg]