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 1/1000th that of water according to ISA (International Standard Atmosphere).
Air density is a property used in many branches of science, engineering, and industry, including aeronautics; gravimetric analysis; the air-conditioning industry; atmospheric research and meteorology; agricultural engineering (modeling and tracking of Soil-Vegetation-Atmosphere-Transfer (SVAT) models); and the engineering community that deals with compressed air.
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.
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.
- air density (kg/m³)[note 1]
- absolute pressure (Pa)[note 1]
- absolute temperature (K)[note 1]
- specific gas constant for dry air (J/(kg·K))[note 1].
, where is the universal gas constant and 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.
- 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:
|Speed of sound
|Density of air
|Characteristic specific acoustic impedance|
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 molar mass of water (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:
- Density of the humid air (kg/m³)
- Partial pressure of dry air (Pa)
- Specific gas constant for dry air, 287.058 J/(kg·K)
- Temperature (K)
- Pressure of water vapor (Pa)
- Specific gas constant for water vapor, 461.495 J/(kg·K)
- Molar mass of dry air, 0.0289654 kg/mol
- Molar mass of water vapor, 0.018016 kg/mol
- Universal gas constant, 8.314 J/(K·mol)
- Vapor pressure of water
- Relative humidity
- 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  used to find the saturation vapor pressure [Pa] is:
where is in degrees C. See vapor pressure of water for other equations.
The partial pressure of dry air is found considering partial pressure, resulting in:
Where simply denotes the observed absolute pressure.
Variation with altitudeEdit
To calculate the density of air as a function of altitude, one requires additional parameters. For the troposphere, the lowest part 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:
- sea level standard atmospheric pressure, 101325 Pa
- sea level standard temperature, 288.15 K
- earth-surface gravitational acceleration, 9.80665 m/s²
- temperature lapse rate, 0.0065 K/m
- ideal (universal) gas constant, 8.31447 J/(mol·K)
- molar mass of dry air, 0.0289654 kg/mol
Temperature at altitude 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)):
The pressure at altitude is given by:
Density can then be calculated according to a molar form of the ideal gas law:
Note that the density close to the ground is
It can be easily verified that the hydrostatic equation holds:
As the temperature varies with height inside the troposphere by less than 25%, and one may approximate:
Which is identical to the isothermal solution, except that Hn, the height scale of the exponential fall for density (as well as for number density n), is not equal to RT0/g M as one would expect for an isothermal atmosphere, but rather:
Which gives Hn = 10.4 km.
Note that for different gasses, the value of Hn 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:
Which is identical to the isothermal solution, with the same height scale Hp = RT0/gM. Note that the hydrostatic equation no longer holds for the exponential approximation (unless L is neglected).
Hp 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.
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:
For nitrogen, it is 75% , while for oxygen this is 79%, and for carbon dioxide - 88%.
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= 220K, so that the exponential drop is faster, with HTP = 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:
|Gas (and others)||Volume by various[▽note 2]||Volume by CIPM-2007||Volume by ASHRAE||Volume by Schlatter||Volume by ICAO||Volume by US StdAtm76|
|Carbon monoxide||(CO)||0.1||(0.00001% )||0.2||(0.00002%)||0.2||(0.00002%)||-||-||-||-||-||-|
|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%)||-||-|
|Sulphur dioxide||(SO2)||trace||trace||-||-||-||-||-||-||up to 1.00||up to (0.0001%)||-||-|
|Ozone||(O3)||0.02 to 0.07||(2 to 7×10−6%)||-||-||-||-||0.01 to 0.10||(1 to 10×10−6%)||up to 0.02 to 0.07||up to (2 to 7×10−6%)||-||-|
|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.||~0.25% by mass over full atmosphere, locally 0.001%–5% by volume.|
- In the SI unit system. However, other units can be used.
- as dry air is a mixture of gases, its molar mass is the weighted average of the molar masses of its components
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