Air is a mixture of different gases. Under normal environmental conditions the gases have an ideal behaviour, i.e. each gas molecule can act independently from all others. Dalton’s law is valid :
The partial pressure p is defined as the pressure of a gas, if it would occupy alone the whole volume of the gas mixture.
Water in its gaseous phase (vapour) is also a component of air mixture. Under normal conditions it behaves like an ideal gas. With Dalton ́s law p becomes: