The Physics Factbook™
Edited by Glenn Elert -- Written by his students
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|Bibliographic Entry||Result |
|Cutnell, John D. & Kenneth W. Johnson. Physics. 3rd Edition, New York: Wiley. 1995: 315.||"Substance, Air, Mass Density (kg/m3), 1.29, *Unless otherwise noted, densities are given at 0 °C and 1 atm pressure."||1.29 kg/m3|
|"Atmosphere." Encarta. CD-ROM. Redmond, WA: Mircosoft, 1994.||"The density of dry air at sea level is about 1/800th the density of water."||1.25 kg/m3|
|CRC Handbook of Chemistry & Physics. 61st ed. Florida: CRC Press, 1980-1981: F-10.||"The density of moist air may be determined by a similar relation: D = 1.2929 (273.13/T) [(B – 0.3783e)/760] where T is the absolute temperature; B, the barometric pressure in mm, and e the vapor pressure of the moisture in the air in mm."||1.2929 kg/m3|
|CRC Handbook of Chemistry & Physics. 48th ed. Ohio: Chemical Rubber Co. 1967-68: A-10.||"Density of Dry air at 0 °C & 760 mm = 1.2929 g/liter"||1.2929 kg/m3|
|Horowitz, Irving L. Contemporary Earth Science. New York: Amsco: 1976, 13-15.||"The density of air at sea level is approximately 1/800th the density of water."||1.25 kg/m3|
Density (D) is the mass of a given volume of a substance. Density can be obtained by dividing the mass (m) of an object by the volume of that object …
D = m / V
A mass that is concentrated in a small volume has a greater density than a substance of equal mass that occupies a larger volume. Thus, gases have the smallest densities as compared to solids and liquids because gas molecules contain mostly empty space while molecules in liquids are more tightly packed together.
The density of a substance (mainly gases) depends on temperature and pressure. Gases are usually compared at a standard temperature and standard pressure. These are the freezing point (0 °C) and normal air pressure at sea level (760 torr), respectively.
The density of dry air at sea level is 1.2929 kg/m3 or about 1/800th the density of water. But as altitude increases, the density drops dramatically. This is because the density of air is proportional to the pressure and inversely proportional to temperature. And the higher you go into the atmosphere, the lower the pressure gets. Pressure is approximately halved for each additional increase of 56 km in altitude. To determine the density of dry air at a given altitude we could use the relation
D = D0 × (T0 / T) × (P / P0)
Where D0 is the known density at absolute temperature T0 and pressure P0 and D, the unknown density at absolute temperature T and pressure P.
Just as there is a density of dry air, there is also the density of moist air, or air that contains moisture (humidity). To obtain this density you can use the relation
D × (273.15 / T) × [(B–0.3783 e)/760]
D is the density of dry air at sea level,
T is the absolute temperature in kelvin,
B is the barometric pressure in torr, and
e is the vapor pressure of the moisture in the air in torr.
Rachel Chu -- 2000
|Bibliographic Entry||Result |
|Eubanks, Steven W. Standard Atmosphere Computations, NASA Lewis Research Center.||"1.22500 kg/m^3"||1.225 kg/m3|
|Carmichael, Ralph. A Sample Atmosphere Table (SI units), Public Domain Aeronautical Software.||"1.225E+0 kg/cu.m"||1.225 kg/m3|
|Beaufils, Jean-Louis. Email to the editor. 5 July 2006.||"Another way to estimate the density of air is through its composition; i.e., roughly 78% nitrogen, 21% oxygen and 1% argon. With respective molar masses for those gases being 28, 32 and 40 g/mol, the average molar mass of air is therefore: |
0.78 × 28 + 0.21 × 32 + 0.01 × 40 = 28.96
At 0 °C and normal sea level pressure a mole of gas occupies 22.4 liters, so a cubic meter of air has a mass of:
1000 / 22.4 × 0.02896 = 1.293 kg."
Editor's Supplement -- 2001, 2006
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