## 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 = 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 relations

The absolute air humidity can 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 :

## 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 :

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)