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Concept
Density of air
Summary
The density of air or atmospheric density, denoted ρ, is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmospheric pressure, temperature and humidity. At 101.325 kPa (abs) and 20 °C (68 °F), air has a density of approximately , according to the International Standard Atmosphere (ISA). At 101.325 kPa (abs) and , air has a density of approximately , which is about that of water, according to the International Standard Atmosphere (ISA). Pure liquid water is .
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.
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)
absolute pressure (Pa)
absolute temperature (K)
is the gas constant, 8.31446261815324 in J⋅K−1⋅mol−1
is the molar mass of dry air, approximately 0.0289652 in kg⋅mol−1.
is the Boltzmann constant, 1.380649e-23 in J⋅K−1
is the molecular mass of dry air, approximately 4.81e-26 in kg.
the specific gas constant for dry air, which using the values presented above would be approximately 287.0500676 in J⋅kg−1⋅K−1.
Therefore:
At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of approximately 1.2754 kg/m3.
Official source
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