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The density of air ρ (Greek: rho) (air density) 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 temperature and humidity. At sea level and at 15°C air has a density of approximately 1.225 kg/m3 (1.225 x10-3 g/cm3, 0.0023769 slug/(cu ft), 0.0765 lb/(cu ft)) according to ISA (International Standard Atmosphere).

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]

Contents

Density of air calculationsEdit

Depending on the measuring instruments, use, and necessary rigor of the result, different sets of equations for the calculation of the density of air are used. Air is a mixture of gases and the calculations always simplify, to a greater or lesser extent, the properties of the mixture.

Density of air variablesEdit

Temperature and pressureEdit

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

 

where:

  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]

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:

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

Humidity (water vapor)Edit

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 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 (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:

   [13]

where:

  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, 0.028964 kg/mol
  Molar mass of water vapor, 0.018016 kg/mol
  Universal gas constant, 8.314 J/(K·mol)
 
The movement of the helicopter rotor leads to a difference in pressure between the upper and lower blade surfaces, allowing the helicopter to fly. A consequence of the pressure change is local variation in air density, strongest in the boundary layer or at transonic speeds.

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

 

where:

  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 [14] used to find the saturation vapor pressure is:

 

where   is in degrees C.

note:
  • This equation will give the result of pressure in hPa (100 Pa, equivalent to the older unit millibar, 1 mbar = 0.001 bar = 0.1 kPa)

The partial pressure of dry air   is found considering partial pressure, resulting in:

 

Where   simply denotes the observed absolute pressure.

AltitudeEdit

 
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 for calculation the universal gas constant instead of the air specific constant:

  sea level standard atmospheric pressure, 101.325 kPa
  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, 0.0289644 kg/mol

Temperature at altitude   meters above sea level is approximated by the following formula (only valid inside the troposphere):

 

The pressure at altitude   is given by:

 

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

 

where:

  molar mass
  ideal gas constant
  absolute temperature
  absolute pressure

CompositionEdit

Composition of dry atmosphere, by volume[▽note 1]
Gas (and others) Volume by various[15][▽note 2] Volume by CIPM-2007[16] Volume by ASHRAE[17] Volume by Schlatter[18] Volume by ICAO[19] Volume by US StdAtm76[20]

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 ppmv[▽note 3] percentile  ppmv percentile  ppmv percentile  ppmv percentile  ppmv percentile  ppmv percentile
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]
  1. ^ ▽Concentration pertains to the troposphere
  2. ^ ▽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.
  3. ^ ▽ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, see volume (thermodynamics))
  4. ^ a b c d e f ▽values disregarded for the calculation of total dry air
  5. ^ 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
  6. ^ ▽volumetric composition value adjustment factor (sum of all trace gases, below the (CO2), and adjusts for 30 ppmv)


See alsoEdit

NotesEdit

  1. ^ a b c d In the SI unit system. However, other units can be used.
  2. ^ as dry air is a mixture of gases, its molar mass is the weighted average of the molar masses of its components

ReferencesEdit

  1. ^ Olson, Wayne M. (2000) AFFTC-TIH-99-01, Aircraft Performance Flight
  2. ^ ICAO, Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), Doc 7488-CD, Third Edition, 1993, ISBN 92-9194-004-6.
  3. ^ Grigorie, T.L., Dinca, L., Corcau J-I. and Grigorie, O. (2010) Aircrafts’ [sic] Altitude Measurement Using Pressure Information:Barometric Altitude and Density Altitude
  4. ^ A., Picard, R.S., Davis, M., Gläser and K., Fujii (CIPM-2007) Revised formula for the density of moist air
  5. ^ S. Herrmann, H.-J. Kretzschmar, and D.P. Gatley (2009), ASHRAE RP-1485 Final Report
  6. ^ F.R. Martins, R.A. Guarnieri e E.B. Pereira, (2007) O aproveitamento da energia eólica (The wind energy resource).
  7. ^ Andrade, R.G., Sediyama, G.C., Batistella, M., Victoria, D.C., da Paz, A.R., Lima, E.P., Nogueira, S.F. (2009) Mapeamento de parâmetros biofísicos e da evapotranspiração no Pantanal usando técnicas de sensoriamento remoto
  8. ^ Marshall,John and Plumb,R. Alan (2008), Atmosphere, ocean, and climate dynamics: an introductory text ISBN 978-0-12-558691-7.
  9. ^ Pollacco, J. A., and B. P. Mohanty (2012), Uncertainties of Water Fluxes in Soil-Vegetation-Atmosphere Transfer Models: Inverting Surface Soil Moisture and Evapotranspiration Retrieved from Remote Sensing, Vadose Zone Journal, 11(3), doi:10.2136/vzj2011.0167.
  10. ^ Shin, Y., B. P. Mohanty, and A.V.M. Ines (2013), Estimating Effective Soil Hydraulic Properties Using Spatially Distributed Soil Moisture and Evapotranspiration, Vadose Zone Journal, 12(3), doi:10.2136/vzj2012.0094.
  11. ^ Saito, H., J. Simunek, and B. P. Mohanty (2006), Numerical Analysis of Coupled Water, Vapor, and Heat Transport in the Vadose Zone, Vadose Zone J. 5: 784-800.
  12. ^ Perry, R.H. and Chilton, C.H., eds., Chemical Engineers’ Handbook, 5th ed., McGraw-Hill, 1973.
  13. ^ Shelquist,R (2009) Equations - Air Density and Density Altitude
  14. ^ Shelquist,R (2009) Algorithms - Schlatter and Baker
  15. ^ Partial sources for figures: Base constituents, Nasa earth factsheet, (updated 2014-03). Carbon dioxide, NOAA Earth System Research Laboratory, (updated 2014-03). Methane and Nitrous Oxide, The NOAA Annual greenhouse gas index(AGGI) Greenhouse gas-Figure 2, (updated 2014-03).
  16. ^ A., Picard, R.S., Davis, M., Gläser and K., Fujii (2008), Revised formula for the density of moist air (CIPM-2007), Metrologia 45 (2008) 149–155 doi:10.1088/0026-1394/45/2/004, pg 151 Table 1
  17. ^ S. Herrmann, H.-J. Kretzschmar, and D.P. Gatley (2009), ASHRAE RP-1485 Final Report Thermodynamic Properties of Real Moist Air,Dry Air, Steam, Water, and Ice pg 16 Table 2.1 and 2.2
  18. ^ Thomas W. Schlatter (2009), Atmospheric Composition and Vertical Structure pg 15 Table 2
  19. ^ ICAO, Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), Doc 7488-CD, Third Edition, (1993), ISBN 92-9194-004-6. pg E-x Table B
  20. ^ U.S. Committee on Extension to the Standard Atmosphere (COESA) (1976) U.S. Standard Atmosphere, 1976 pg 03 Table 3
  21. ^ a b Wallace, John M. and Peter V. Hobbs. Atmospheric Science; An Introductory Survey.Elsevier. Second Edition, 2006. ISBN 978-0-12-732951-2. Chapter 1

External linksEdit