Density of air

The density of air or atmospheric density, denoted ρ (Greek: rho), is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature and humidity. At 101.325 kPa (abs) and 15 °C, air has a density of approximately 1.225 kg/m³ (or 0.00237 slug/ft³), about 1/1000 that of water according to ISA (International Standard Atmosphere).^{[citation needed]}

Air density is a property used in many branches of science, engineering, and industry, including aeronautics;^[1][2][3] gravimetric analysis;^[4] the air-conditioning^[5] industry; atmospheric research and meteorology;^[6][7][8] agricultural engineering (modeling and tracking of Soil-Vegetation-Atmosphere-Transfer (SVAT) models);^[9][10][11] and the engineering community that deals with compressed air.^[12]

Depending on the measuring instruments used, different sets of equations for the calculation of the density of air can be applied. Air is a mixture of gases and the calculations always simplify, to a greater or lesser extent, the properties of the mixture.

Temperature[]

Other things being equal, hotter air is less dense than cooler air and will thus rise through cooler air. This can be seen by using the ideal gas law as an approximation.

Dry air[]

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

${\displaystyle \rho ={\frac {p}{R_{\text{specific}}T}}}$

where:

${\displaystyle \rho }$, air density (kg/m³)^{[note 1]}

${\displaystyle p}$, absolute pressure (Pa)^{[note 1]}

${\displaystyle T}$, absolute temperature (K)^{[note 1]}

${\displaystyle R_{\text{specific}}}$, specific gas constant for dry air (J/kg·K)^{[note 1]}.

${\textstyle R_{\text{specific}}={\frac {R}{M_{\text{d}}}}}$, where ${\displaystyle R}$ is the universal gas constant and ${\displaystyle M_{\rm {d}}}$ is the molar mass of dry air. The specific gas constant for dry air is 287.058 J/(kg·K) in SI units, and 53.35 ft·lbf/(lb·°R) in United States customary and Imperial units. This quantity may vary slightly depending on the molecular composition of air at a particular location.

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 0.074887 lb/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 (°C)	Speed of sound, c (m/s)	Density of air, ρ (kg/m3)	Characteristic specific acoustic impedance, z0 (Pa·s/m)
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

Humid air[]

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 when the molar mass of water vapor (18 g/mol) is less than the molar mass of dry air^{[note 2]} (around 29 g/mol). For any ideal gas, at a given temperature and pressure, the number of molecules is constant for a particular volume (see Avogadro's Law). So when water molecules (water 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 by treating it 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:

${\displaystyle \rho _{\text{humid air}}={\frac {p_{\text{d}}}{R_{\text{d}}T}}+{\frac {p_{\text{v}}}{R_{\text{v}}T}}={\frac {p_{\text{d}}M_{\text{d}}+p_{\text{v}}M_{\text{v}}}{RT}}}$  ^[13]

where:

${\displaystyle \rho _{\text{humid air}}}$, density of the humid air (kg/m³)

${\displaystyle p_{\text{d}}}$, partial pressure of dry air (Pa)

${\displaystyle R_{\text{d}}}$, specific gas constant for dry air, 287.058 J/(kg·K)

${\displaystyle T}$, temperature (K)

${\displaystyle p_{\text{v}}}$, pressure of water vapor (Pa)

${\displaystyle R_{\text{v}}}$, specific gas constant for water vapor, 461.495 J/(kg·K)

${\displaystyle M_{\text{d}}}$, molar mass of dry air, 0.0289652 kg/mol

${\displaystyle M_{\text{v}}}$, molar mass of water vapor, 0.018016 kg/mol

${\displaystyle R}$, universal gas constant, 8.31446 J/(K·mol)

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

${\displaystyle p_{\text{v}}=\phi p_{\text{sat}}}$

where:

${\displaystyle p_{\text{v}}}$, vapor pressure of water

${\displaystyle \phi }$, relative humidity (0.0–1.0)

${\displaystyle p_{\text{sat}}}$, saturation vapor pressure

The saturation vapor pressure of water at any given temperature is the vapor pressure when relative humidity is 100%. One formula is Tetens' equation from^[14] used to find the saturation vapor pressure is:

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

where:

${\displaystyle p_{\text{sat}}}$, saturation vapor pressure (hPa)

${\displaystyle T}$, temperature (°C)

See vapor pressure of water for other equations.

The partial pressure of dry air ${\displaystyle p_{\text{d}}}$ is found considering partial pressure, resulting in:

${\displaystyle p_{\text{d}}=p-p_{\text{v}}}$

Where ${\displaystyle p}$ simply denotes the observed absolute pressure.

Variation with altitude[]

Troposphere[]

To calculate the density of air as a function of altitude, one requires additional parameters. For the troposphere, the lowest part (~10 km) of the atmosphere, they are listed below, along with their values according to the International Standard Atmosphere, using for calculation the universal gas constant instead of the air specific constant:

${\displaystyle p_{0}}$, sea level standard atmospheric pressure, 101325 Pa

${\displaystyle T_{0}}$, sea level standard temperature, 288.15 K

${\displaystyle g}$, earth-surface gravitational acceleration, 9.80665 m/s²

${\displaystyle L}$, temperature lapse rate, 0.0065 K/m

${\displaystyle R}$, ideal (universal) gas constant, 8.31446 J/(mol·K)

${\displaystyle M}$, molar mass of dry air, 0.0289652 kg/mol

Temperature at altitude ${\displaystyle h}$ meters above sea level is approximated by the following formula (only valid inside the troposphere, no more than ~18 km above Earth's surface (and lower away from Equator)):

${\displaystyle T=T_{0}-Lh}$

The pressure at altitude ${\displaystyle h}$ is given by:

${\displaystyle p=p_{0}\left(1-{\frac {Lh}{T_{0}}}\right)^{\frac {gM}{RL}}}$

Density can then be calculated according to a molar form of the ideal gas law:

${\displaystyle \rho ={\frac {pM}{RT}}={\frac {pM}{RT_{0}\left(1-{\frac {Lh}{T_{0}}}\right)}}={\frac {p_{0}M}{RT_{0}}}\left(1-{\frac {Lh}{T_{0}}}\right)^{{\frac {gM}{RL}}-1}}$

where:

${\displaystyle M}$, molar mass

${\displaystyle R}$, ideal gas constant

${\displaystyle T}$, absolute temperature

${\displaystyle p}$, absolute pressure

Note that the density close to the ground is ${\textstyle \rho _{0}={\frac {p_{0}M}{RT_{0}}}}$

It can be easily verified that the hydrostatic equation holds:

${\displaystyle {\frac {dp}{dh}}=-g\rho .}$

Exponential approximation[]

As the temperature varies with height inside the troposphere by less than 25%, ${\textstyle {\frac {Lh}{T_{0}}}<0.25}$ and one may approximate:

${\displaystyle \rho =\rho _{0}e^{\left({\frac {gM}{RL}}-1\right)\ln \left(1-{\frac {Lh}{T_{0}}}\right)}\approx \rho _{0}e^{-\left({\frac {gM}{RL}}-1\right){\frac {Lh}{T_{0}}}}=\rho _{0}e^{-\left({\frac {gMh}{RT_{0}}}-{\frac {Lh}{T_{0}}}\right)}}$

Thus:

${\displaystyle \rho \approx \rho _{0}e^{-h/H_{n}}}$

Which is identical to the isothermal solution, except that H_n, the height scale of the exponential fall for density (as well as for number density n), is not equal to RT₀/gM as one would expect for an isothermal atmosphere, but rather:

${\displaystyle {\frac {1}{H_{n}}}={\frac {gM}{RT_{0}}}-{\frac {L}{T_{0}}}}$

Which gives H_n = 10.4 km.

Note that for different gasses, the value of H_n differs, according to the molar mass M: It is 10.9 for nitrogen, 9.2 for oxygen and 6.3 for carbon dioxide. The theoretical value for water vapor is 19.6, but due to vapor condensation the water vapor density dependence is highly variable and is not well approximated by this formula.

The pressure can be approximated by another exponent:

${\displaystyle p=p_{0}e^{\left({\frac {gM}{RL}}\right)\ln \left(1-{\frac {Lh}{T_{0}}}\right)}\approx p_{0}e^{-{\frac {gM}{RL}}{\frac {Lh}{T_{0}}}}=p_{0}e^{-{\frac {gMh}{RT_{0}}}}}$

Which is identical to the isothermal solution, with the same height scale H_p = RT₀/gM. Note that the hydrostatic equation no longer holds for the exponential approximation (unless L is neglected).

H_p is 8.4 km, but for different gasses (measuring their partial pressure), it is again different and depends upon molar mass, giving 8.7 for nitrogen, 7.6 for oxygen and 5.6 for carbon dioxide.

Total content[]

Further note that since g, Earth's gravitational acceleration, is approximately constant with altitude in the atmosphere, the pressure at height h is proportional to the integral of the density in the column above h, and therefore to the mass in the atmosphere above height h. Therefore the mass fraction of the troposphere out of all the atmosphere is given using the approximated formula for p:

${\displaystyle 1-{\frac {p(h=11{\text{ km}})}{p_{0}}}=1-\left({\frac {T(11{\text{ km}})}{T_{0}}}\right)^{\frac {gM}{RL}}\approx 76\%}$

For nitrogen, it is 75%, while for oxygen this is 79%, and for carbon dioxide, 88%.

Tropopause[]

Higher than the troposphere, at the tropopause, the temperature is approximately constant with altitude (up to ~20 km) and is 220 K. This means that at this layer L = 0 and T = 220 K, so that the exponential drop is faster, with H_TP = 6.3 km for air (6.5 for nitrogen, 5.7 for oxygen and 4.2 for carbon dioxide). Both the pressure and density obey this law, so, denoting the height of the border between the troposphere and the tropopause as U:

${\displaystyle {\begin{aligned}p&=p(U)e^{-{\frac {h-U}{H_{\text{TP}}}}}=p_{0}\left(1-{\frac {LU}{T_{0}}}\right)^{\frac {gM}{RL}}e^{-{\frac {h-U}{H_{\text{TP}}}}}\\\rho &=\rho (U)e^{-{\frac {h-U}{H_{\text{TP}}}}}=\rho _{0}\left(1-{\frac {LU}{T_{0}}}\right)^{{\frac {gM}{RL}}-1}e^{-{\frac {h-U}{H_{\text{TP}}}}}\end{aligned}}}$

Composition[]

Composition of dry atmosphere, by volume^{[▽ note 1]}

Gas (and others)		Various[15][▽ note 2]		CIPM-2007[16]		ASHRAE[17]		Schlatter[18]		ICAO[19]		US StdAtm76[20]		▼ Tap to expand or collapse table ▲
		ppmv [▽ note 3]	percentage	ppmv	percentage	ppmv	percentage	ppmv	percentage	ppmv	percentage	ppmv	percentage
Nitrogen	N2	780,800	78.080%	780,848	78.0848%	780,818	78.0818%	780,840	78.084%	780,840	78.084%	780,840	78.084%
Oxygen	O2	209,500	20.950%	209,390	20.9390%	209,435	20.9435%	209,460	20.946%	209,476	20.9476%	209,476	20.9476%
Argon	Ar	9,340	0.9340%	9,332	0.9332%	9,332	0.9332%	9,340	0.9340%	9,340	0.9340%	9,340	0.9340%
Carbon dioxide	CO2	397.8	0.03978%	400	0.0400%	385	0.0385%	384	0.0384%	314	0.0314%	314	0.0314%
Neon	Ne	18.18	0.001818%	18.2	0.00182%	18.2	0.00182%	18.18	0.001818%	18.18	0.001818%	18.18	0.001818%
Helium	He	5.24	0.000524%	5.2	0.00052%	5.2	0.00052%	5.24	0.000524%	5.24	0.000524%	5.24	0.000524%
Methane	CH4	1.81	0.000181%	1.5	0.00015%	1.5	0.00015%	1.774	0.0001774%	2	0.0002%	2	0.0002%
Krypton	Kr	1.14	0.000114%	1.1	0.00011%	1.1	0.00011%	1.14	0.000114%	1.14	0.000114%	1.14	0.000114%
Hydrogen	H2	0.55	0.000055%	0.5	0.00005%	0.5	0.00005%	0.56	0.000056%	0.5	0.00005%	0.5	0.00005%
Nitrous oxide	N2O	0.325	0.0000325%	0.3	0.00003%	0.3	0.00003%	0.320	0.0000320%	0.5	0.00005%	-	-
Carbon monoxide	CO	0.1	0.00001%	0.2	0.00002%	0.2	0.00002%	-	-	-	-	-	-
Xenon	Xe	0.09	0.000009%	0.1	0.00001%	0.1	0.00001%	0.09	0.000009%	0.087	0.0000087%	0.087	0.0000087%
Nitrogen dioxide	NO2	0.02	0.000002%	-	-	-	-	-	-	Up to 0.02	Up to 0.000002%	-	-
Iodine	I2	0.01	0.000001%	-	-	-	-	-	-	Up to 0.01	Up to 0.000001%	-	-
Ammonia	NH3	trace	trace	-	-	-	-	-	-			-	-
Sulphur dioxide	SO2	trace	trace	-	-	-	-	-	-	Up to 1.00	Up to 0.0001%	-	-
Ozone	O3	0.02 to 0.07 [▽ note 4]	2 to 7×10−6% [▽ note 4]	-	-	-	-	0.01 to 0.10 [▽ note 4]	1 to 10×10−6% [▽ note 4]	Up to 0.02 to 0.07 [▽ note 4] [▽ note 5]	Up to 2 to 7×10−6% [▽ note 4] [▽ note 5]	-	-
Trace to 30 ppm [▽ note 6]	—	-	-	-	-	2.9	0.00029%	-	-	-	-	-	-
Dry air total	air	1,000,065.265	100.0065265%	999,997.100	99.9997100%	1,000,000.000	100.0000000%	1,000,051.404	100.0051404%	999,998.677	99.9998677%	1,000,080.147	100.0080147%
Not included in above dry atmosphere
Water vapor	H2O	~0.25% by mass over full atmosphere, locally 0.001–5% by volume.[21]						~0.25% by mass over full atmosphere, locally 0.001–5% by volume.[21]
▽ notes ^ Concentration pertains to the troposphere ^ The NASA total value do not add up to exactly 100% due to roundoff and uncertainty. To normalize, N2 should be reduced by about 51.46 ppmv and O2 by about 13.805 ppmv. ^ ppmv: parts per million by volume. Volume fraction is equal to mole fraction for ideal gas only, see volume (thermodynamics). ^ Jump up to: a b c d e f Values disregarded for the calculation of total dry air ^ Jump up to: a b O3 concentration up to 0.07 ppmv (7×10−6%) in summer and up to 0.02 ppmv (2×10−6%) in winter. ^ Volumetric composition value adjustment factor (sum of all trace gases, below the CO2, and adjusts for 30 ppmv)

See also[]

• Air
• Atmospheric drag
• Lighter than air
• Density
• Atmosphere of Earth
• International Standard Atmosphere
• U.S. Standard Atmosphere
• NRLMSISE-00

Notes[]

References[]

External links[]

• Conversions of density units ρ by Sengpielaudio
• Air density and density altitude calculations and by Richard Shelquist
• Air density calculations by Sengpielaudio (section under Speed of sound in humid air)
• Air density calculator by Engineering design encyclopedia
• Atmospheric pressure calculator by wolfdynamics
• Air iTools - Air density calculator for mobile by JSyA