# Humid Air : Absolute air humidity

## Definition of absolute humid air and associate air psychometric calculations

1. Definition of air absolute humidity
2. Calculation of air absolute humidity
3. Absolute humidity at saturation

## 1. Definition of air absolute humidity

### What is absolute air humidity ?

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 = 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)

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

With :
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 :

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