# Moist Air - Daltons Law of Partial Pressure

## The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.

Daltons Law states that

*the total pressure of a mixture of gases is equal to the sum of the partial pressures of the constituent gases*

The partial pressure is the pressure each gas would exert if it alone occupied the volume of the mixture.

Daltons Law of partial pressure for moist air can be expressed as

p = p_{a}+ p_{w}(1)

where

p= total pressure of air (Pa, N/m^{2})

p_{a}= partial pressure dry air (Pa, N/m^{2})

p_{w}= partial pressure water vapor (Pa, N/m^{2})

With the partial pressure for dry air and water vapor *(1)* can be modified to:

p = ρ_{a}286.9T + ρ_{w}455T(2)

where

ρ_{a}= density of air (kg/m^{3})

ρ_{w}= density of water vapor (kg/m^{3})