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Moist Air - Density vs. Water Content and Temperature

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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

ρ a = p a / R a T

= 0.0035 p a / T (1)


ρ a = density dry air (kg/m3 )

p a = partial pressure of dry air (Pa, N/m2)

R a = 286.9 - individual gas constant of dry air (J/kg K)

T = absolute dry bulb temperature (K)

Density of Water Vapor

The density of the water vapor can be calculated

ρ w = p w / R w T

= 0.0022 p w / T (2)


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

ρ w = density of water vapor (kg/m3 )

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

T = absolute dry bulb temperature (K)


Density of Moist Air - an Air Vapor Mixture

The amount of water vapor in air influences the 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 / R a T) (1 + x) / (1 + x R w / R a ) (3)


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

x = m w / m a = specific humidity or humidity ratio (kg h2o /kg dry_air )

m w = mass of water vapor (kg)

m a = mass of dry air (kg)

p = pressure in the moist air (Pa)

The density of dry air with the same total pressure p as in the mixture (dry air + vapor partial pressure) - can be calculated as a reference as

ρ ref = p / R a T (4)


ρ ref = reference density of dry air with the same pressure as in the air vapor mixture (kg/m3 )

The density of moist air related to dry air can be expressed by combining (4) and (3) as

ρ = ρ ref (1 + x) / (1 + x R w / R a ) (5)

The gas constant ratio between water vapor and air is

R w / R a = (461.5 J/kg K) / (286.9 J/kg K)

= 1.609

and inserting the ratio in (5) the density of moist air related to dry air can be expressed as

ρ = ρ ref (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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Related Documents

Air - Composition and Molecular Weight

Dry air is a mechanical mixture of nitrogen, oxygen, argon and several other gases in minor amounts.

Air - Humidifying by Adding Steam or Water

Air can be humidified by adding water or steam.

Air - Humidifying with Steam - Imperial Units

Estimate the amount of steam required (lb/h in 100 cfm) in humid air.

Air - Humidifying with Steam, SI units

Using steam to humidify air.

Air - Humidity Measurement from Dry and Wet Bulb Temperature

Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.

Air - Humidity Ratio

The mass of water vapor present in moist air - to the mass of dry air.

Air - Maximum Moisture Carrying Capacity

Maximum water content in humid air vs. temperature.

Air - Moisture Holding Capacity vs. Temperature

The moisture holding capacity of air increases with temperature.

Air - Molecular Weight and Composition

Dry air is a mixture of gases where the average molecular weight (or molar mass) can be calculated by adding the weight of each component.

Air and Steam Mixtures

Air in the steam will lower the surface temperatures in heat exchangers - and less heat will be transferred.

Dry Air - Thermodynamic and Physical Properties

Thermodynamic properties of dry air - specific heat, ratio of specific heats, dynamic viscosity, thermal conductivity, Prandtl number, density and kinematic viscosity at temperatures ranging 175 - 1900 K.

Dry Air and Water Vapor - Density and Specific Volume vs. Temperature - Imperial Units

Density and specific volume of dry air and water vapor at temperatures ranging 225 to 900 degF (107 to 482 degC).

Humid Air and the Ideal Gas Law

Pressure, temperature and volume in a perfect ideal gas like moist air (air with water vapor).

Mixing of Humid Air

The change in state wwhen mixing moist air - enthalpy, heat, temperature and specific humidity.

Moist Air - Degree of Saturation

Humidity ratio of moist air to humidity ratio of saturated moist air.

Moist Air - Density vs. Pressure

Density of moist air vs. pressure ranging 75 - 1000 mmHg.

Moist Air - Partial Pressure and Daltons Law

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.

Moist Air - Psychrometric Table for Pressure 29.92 inHg

Dry and wet bulb temperatures, saturation pressure, water vapor weight, specific volume, heat and more.

Moist Air - Specific Volume

Specific volume of moist air is defined as the total volume of humid air per mass unit of dry air

Moist Air - Specific vs. Relative Humidity

Specific humidity of moist air vs. relative humidity, water vapor and air density.

Moist Air - Water Vapor and Saturation Pressure

Saturation pressure of water vapor in moist air vs. temperature.

Moist Air - Weight of Water Vapor

Weight of water vapor in air

Saturated Salt Solutions - Controlling Air Humidity

A salt solutions can be used maintain a particular value of relative humidity.

Wet Steam - Specific Volume

Wet steam and specific volume.

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