Density of Moist Humid Air

Density of dry air, water vapor or the mixture of air and water vapor - moist or humid air

The density of humid air varies with the temperature and the moisture content in the air. When temperature increase a higher molecular motion results in expansion of volume and a decrease in 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 as:

ρa = 0.0035 pa / T         (1)


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 expressed as:

ρw = 0.0022 pw / T         (2)


pw = partial pressure 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 will influence the density. Water vapor is a relatively light gas compared to diatomic Oxygen and diatomic Nitrogen - the dominant components in air.

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

Based on specific volume of moist air density can be expressed as:

ρ = 1 / v

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


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)


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 the moist air - dry air is more dense than moist air.

moist air density temperature relative humidity

Related Topics

  • Air Psychrometrics - The study of moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content
  • Gases and Compressed Air - Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves

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