Pressure in Moist Air - Daltons Law

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

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 for moist air can be expressed as

p = p_a + p_w (1)

where

p = total pressure of air (Pa, N/m²)

p_a = partial pressure dry air (Pa, N/m²)

p_w = partial pressure water vapor (Pa, N/m²)

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

p = ρ_a 286.9 T + ρ_w 455 T (2)

where

ρ_a = density of air (kg/m³)

ρ_w = density of water vapor (kg/m³)

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