Dalton’s Law of Partial Pressures:
It states, "The total pressure exerted by the mixture of air and water vapour is equal to the sum of the pressures, which each constituent would exert, if it occupied the same space by itself".
In other words, the total pressure exerted by air and water vapour mixture is equal to the barometric pressure.
Mathematically, barometric pressure of the mixture,
Pb = Pa+Pv
Where Pa = Partial pressure of dry air, and
Pv = Partial pressure of water vapour.