The moisture content (also called
mixing ratio or
humidity ratio) is defined as the ratio of water vapor mass,
mv, to dry air mass,
ma:
where pv is the water vapor partial pressure,
pa is the dry air partial pressure, and
Ma and
Mv are the molar mass of dry air and water vapor, respectively. Without condensation, the moisture content is not affected by temperature and pressure. The moisture content represents a ratio of mass, and it is thus a dimensionless number.
The relative humidity of an air mixture is expressed as follows:
where pv is the water vapor partial pressure and
psat is the saturation pressure of water vapor.
The specific humidity is defined as the ratio of water vapor,
mv, to the total mass,
mtot = mv + ma:
where nv is the amount of water vapor (SI unit: mol) and
V is the total volume (SI unit: m
3). According to the ideal gas hypothesis, the saturation concentration is defined as follows: