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Density of Moist Humid Air

Density of dry air and water vapor - moist or humid air

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The density of humid air varies with water content and temperature. When the temperature increases a higher molecular motion results in expansion of volume and a decrease of density.

The density of a gas, dry air, water vapor - or a mixture of dry air and water vapor like moist or humid air - can be calculated with the Ideal Gas Law.

Density of Dry Air

The density of dry air can be calculated

ρa = 0.0035 pa / T                                   (1)

where

ρa = density dry air (kg/m3)

pa = partial pressure of air (Pa, N/m2)

T = absolute dry bulb temperature (K)

Density of Water Vapor

The density of water vapor can be calculated

ρw = 0.0022 pw / T                                 (2)

where

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

ρw = density of water vapor (kg/m3)

T = absolute dry bulb temperature (K)

Density of Moist Air - an Air Vapor Mixture

The amount of water vapor in air influence density. Water vapor is relatively light compared to diatomic Oxygen and diatomic Nitrogen - the dominant components in air.

When vapor content increases in moist air the amount of Oxygen and Nitrogen are decreased per unit volume and the density of the mix decreases since the mass is decreasing.

Based on specific volume of moist air the moist air density can be calculated

ρ = 1 / v

    = (p / Ra T) (1 + x) / (1 + x Rw / Ra)                                          (3)

where

v = specific volume of moist air per mass unit of dry air and water vapor (m3/kg)

Ra = 286.9 - individual gas constant air (J/kg K)

Rw = 461.5 - individual gas constant water vapor (J/kg K)

x = humidity ratio (kg/kg)

p = pressure in the humid air (Pa)

Density of dry air can be expressed as:

ρda = p / Ra T         (4)

where

ρda = density dry air (kg/m3)

Combining (4) and (3):

ρ = ρda (1 + x) / (1 + x Rw / Ra)                                       (5)

The gas constant ratio between water vapor and air is

Rw / Ra = (461.5 J/kg K) / (286.9 J/kg K)

    = 1.609

Inserting the ratio in (5):

ρ = ρda (1 + x) / (1 + 1.609 x )                                         (6)

Note! As we can see from (6) - increased moisture content reduces the density of moist air - dry air is more dense than moist air.

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