Humid Air : Absolute air humidity

Air psychrometrics

1. Introduction

The 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. Calculation

ω = mw / ma = weight of water / weight of dry air
With :
ω = Absolute humidity of air (-)
mw = weight of water (kg water)
ma = weight of dry air (kg dry air)

3. Other relations

The absolute air humidity can be calculated from the molar fraction of water in the gas.

Calculation of air absolute humidity from molar ratio of water in air
Mwater = molar weight of water = 18 g/mol
Mair = molar weight of air = 29 g/mol
y = molar ratio of water in air = Pwater / Ptotal
Pwater = 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 saturation

When 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 :
ωsat = Absolute humidity of air at saturation at temperature T (-)
Pwatersat = saturation pressure of water at temperature T (Pa)
P = total pressure (Pa)