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.
What is the difference between absolute humidity and relative
Air absolute humidity should not be confused with air relative humidity, which is
representing a ratio of the partial pressure of water at the given
condition vs the partial pressure of water at saturation.
2. Calculation of air absolute humidity
How to calculate air absolute humidity ?
ω = mw / ma
= weight of water / weight of dry air
ω = Absolute humidity of air (-)
= weight of water (kg water)
= weight of dry air (kg dry air)
The absolute air humidity can also be calculated from the molar
fraction of water in the gas.
= molar weight of water = 18 g/mol
= molar weight of air = 29 g/mol
y = molar ratio of water in air = Pwater / Ptotal
= partial pressure of water
If y is small,
which is quite often the case, then the absolute
humidity can be approximated easily :
3. Absolute humidity at saturation
How to calculate absolute air 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 accommodate more
ωsat = Absolute humidity of air at saturation
at temperature T (-)
= saturation pressure of water at temperature
P = total pressure (Pa)