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
humidity ?

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 = m**_{w} / m_{a}
= weight of water / weight of dry air

With :

ω = Absolute humidity of air (-)

m

_{w} = weight of water (kg water)

m

_{a} = weight of dry air (kg dry air)

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

With :

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 :

**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
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)