Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Density of Moist Humid Air

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

Sponsored Links

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)


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)


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)


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

moist air density temperature relative humidity

Sponsored Links

Related Topics

Related Documents

Tag Search

  • en: density air
  • es: aire densidad
  • de: Luftdichte
Sponsored Links

Search the Engineering ToolBox

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro .Add the Engineering ToolBox extension to your SketchUp from the SketchUp Pro Sketchup Extension Warehouse!

Translate this page to
About the Engineering ToolBox!


We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.

Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.

Google use cookies for serving our ads and handling visitor statistics. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected.

AddThis use cookies for handling links to social media. Please read AddThis Privacy for more information.


This page can be cited as

  • Engineering ToolBox, (2004). Density of Moist Humid Air. [online] Available at: [Accessed Day Mo. Year].

Modify access date.

. .


3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Scientific Online Calculator

Scientific Calculator

9 25

Sponsored Links