Density of air

The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity. At sea level and at 15 °C according to ISA (International Standard Atmosphere), air has a density of approximately 1.225 kg/m³ (0.0023769 slugs/ft³).

Temperature and pressure

The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure:

$\rho = \frac{p}{R_{\rm specific} T}$

where ρ is the air density, p is absolute pressure, R_specific is the specific gas constant for dry air, and T is absolute temperature.

The specific gas constant for dry air is 287.058 J/(kg·K) in SI units, and 53.35 (ft·lb_f)/(lb_m·R) in United States customary and Imperial units.

Therefore:

At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of 1.2754 kg/m³.
At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m³.
At 70 °F and 14.696 psi, dry air has a density of 0.074887lb_m/ft³.

The following table illustrates the air density–temperature relationship at 1 atm or 101.325 kPa:

Effect of temperature on properties of air
Temperature T in °C	Speed of sound c in m·s⁻¹	Density of air ρ in kg·m⁻³	Acoustic impedance Z in N·s·m⁻³
+35	351.88	1.1455	403.2
+30	349.02	1.1644	406.5
+25	346.13	1.1839	409.4
+20	343.21	1.2041	413.3
+15	340.27	1.2250	416.9
+10	337.31	1.2466	420.5
+5	334.32	1.2690	424.3
0	331.30	1.2922	428.0
−5	328.25	1.3163	432.1
−10	325.18	1.3413	436.1
−15	322.07	1.3673	440.3
−20	318.94	1.3943	444.6
−25	315.77	1.4224	449.1

Water vapor

The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear counter-intuitive.

This occurs because the molecular mass of water (18 g/mol) is less than the molecular mass of dry air (around 29 g/mol). For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume (see Avogadro's Law). So when water molecules (vapor) are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the mass per unit volume of the gas (its density) decreases.

The density of humid air may be calculated as a mixture of ideal gases. In this case, the partial pressure of water vapor is known as the vapor pressure. Using this method, error in the density calculation is less than 0.2% in the range of −10 °C to 50 °C. The density of humid air is found by:

$\rho_{\,\mathrm{humid~air}} = \frac{p_{d}}{R_{d} T} + \frac{p_{v}}{R_{v} T} = \frac{p_{d}M_{d}+p_{v}M_{v}}{R T} \,$ ^[1]

where:

$\rho_{\,\mathrm{humid~air}} =$ Density of the humid air (kg/m³)

$p_{d} =$ Partial pressure of dry air (Pa)

$R_{d} =$ Specific gas constant for dry air, 287.058 J/(kg·K)

$T =$ Temperature (K)

$p_{v} =$ Pressure of water vapor (Pa)

$R_{v} =$ Specific gas constant for water vapor, 461.495 J/(kg·K)

$M_{d} =$ Molar mass of dry air, 0.028964 (kg/mol)

$M_{v} =$ Molar mass of water vapor, 0.018016 (kg/mol)

$R =$ Universal gas constant, 8.314 J/(K·mol)

The vapor pressure of water may be calculated from the saturation vapor pressure and relative humidity. It is found by:

$p_{v} = \phi p_{\mathrm{sat}} \,$

Where:

$p_{v} =$ Vapor pressure of water

$\phi =$ Relative humidity

$p_{\mathrm{sat}} =$ Saturation vapor pressure

The saturation vapor pressure of water at any given temperature is the vapor pressure when relative humidity is 100%. A simplification of the regression ^[1] used to find this, can be formulated as:

$p_{\mathrm{sat}} = 6.1078 \times 10^{\frac{7.5 T}{T+237.3}}$

where T is in degrees C. Note:

This will give a result in hPa (100 Pa, equivalent to the older unit millibar, 1 mbar = 0.001 bar = 0.1 kPa)
$p_{d}$ is found considering partial pressure, resulting in:

$p_{d} = p-p_{v} \,$

Where p simply denotes the absolute pressure in the observed system.

Altitude

Standard Atmosphere: p₀=101.325 kPa, T₀=288.15 K, ρ₀=1.225 kg/m³

To calculate the density of air as a function of altitude, one requires additional parameters. They are listed below, along with their values according to the International Standard Atmosphere, using the universal gas constant instead of the specific one: