According to Dalton's law of partial pressures, (where pb = Barometric pressure, pa = Partial pressure of dry air and pv = Partial pressure of water vapour)

According to Dalton's law of partial pressures, the total pressure by a mixture of gases is equal to the sum of the partial pressures of each of the constituent gases. The partial pressure is defined as the pressure each gas would exert if it alone occupied the volume of the mixture at the same temperature -According to Dalton's law of partial pressures, Pb = pa + pv.
(Where pb = Barometric pressure, pa = Partial pressure of dry air and pv = Partial pressure of water vapour)