Home > Density of air

1 Effects of temperature and pressure

The formula for the density of air is given by:

where ρ is the air density, p is pressure, R is the gas constant, and T is temperature.

The individual gas constant R for dry air is:

Therefore:

• At Standard temperature and pressure (0 °C and 101.325 kPa), dry air has a density of ρ_STP = 1.293 kg/m³).
• At standard ambient temperature and pressure (25 °C and 100 kPa) dry air has a density of ρ_SATP = 1.168 kg/m³).

2 Effect of water vapor

For moist air, the partial pressure of the water vapor must be considered as well. In this case, the density of the air is the sum of the density of the dry air and the density of the water vapor:

The gas constant for water vapor is:

3 Effects of altitude

To calculate the density of air as a function of altitude, one requires aditional parameters. They are listed below, along with their values according to the International Standard Atmosphere:

• sea level atmospheric pressure p₀ = 101325 Pa = 1013.25 mbar = 1013.25 hPa
• sea level standard temperature T₀ = 288.15 K
• Earth-surface gravitational acceleration g = 9.80665 m/s²).
• dry adiabatic lapse rate L = -6.5 K/km
• gas constant R = 8.31432 J/ mol·K
• molecular weight of dry air M = 28.9644 gm/mol

Temperature at altitude h above sea level is given by the following formula (only valid below the tropopause):

The pressure at altitude h is given by:

Density can then be calculated according to the original formula:

4 Importance of temperature

The below table demonstrates that the properties of air change significantly with temperature.

Table - speed of sound in air c, density of air ρ,
acoustic impedance Z vs. temperature °C

5 See also

• NRLMSISE-00

6 External links

• Conversions of density units ρ
• Air density and density altitude calculations