Humid Air : Absolute air humidity 
Air psychrometrics

1. IntroductionThe absolute humidity is defined as the weight of water in the air per weight of dry air. From a given state, if there is no water / air transfer the absolute humidity is not changing.2. Calculationω = m_{w} / m_{a}
= weight of water / weight of dry air
With :ω = Absolute humidity of air () mw = weight of water (kg water) m_{a} = weight of dry air (kg dry air) 3. Other relationsThe absolute air humidity can be calculated from the molar fraction of water in the gas.M_{water} = molar weight of water = 18 g/mol M_{air} = molar weight of air = 29 g/mol y = molar ratio of water in air = P_{water} / P_{total} P_{water} = partial pressure of water If y is small, which is quite often the case, then the absolute humidity can be approximated easily : 4. Absolute humidity at saturationWhen the water partial pressure
in the air reaches the saturation pressure at the
corresponding temperature, then the air is saturated  it
cannot accomodate more water.
With :
ω_{sat} = Absolute humidity of air at
saturation at temperature T ()
P_{water}^{sat} = saturation pressure of water
at temperature T (Pa)P = total pressure (Pa)
