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. 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. See Saturation State for the definition of saturation pressure psat as a function of temperature.

According to Dalton’s law, the total pressure of a mixture of gases is the sum of all the partial pressures of each individual gas; that is, p = pv + pa, where pa is the dry air partial pressure.

The relative humidity formulation is often used to quantify humidity. However, for the same quantity of moisture content, the relative humidity changes with temperature and pressure, so in order to compare different values of ϕw it has to be at the same temperature and pressure conditions. Then the thermodynamical properties of moist air can be deduced through the mixture formula described below.

The relative humidity is useful to study the condensation as it defines the boundary between the liquid phase and the vapor phase. In fact, when the relative humidity ϕw reaches unity, it means that the vapor is saturated and that water vapor may condense.

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: m3). According to the ideal gas hypothesis, the saturation concentration is defined as follows: