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EFFECTS OF TEMPERATURE AND PRESSURE


The formula for the density of air is given by:

: ho = rac{p}{R \cdot T}

where ''ρ'' is the air density, ''p'' is pressure, ''R'' is the gas constant, and ''T'' is temperature.

The individual Gas Constant ''R'' for dry air is:

:R_\mathrm{dry\,air} = 287.05 rac{\mbox{J}}{\mbox{kg} \cdot \mbox{K}}

Therefore:
  • At Standard Temperature And Pressure (0 °C and 101.325 kPa), dry air has a density of ρSTP = 1.293 kg/m3.

  • At standard ambient temperature and pressure (25 °C and 100 kPa) dry air has a density of ρSATP = 1.168 kg/m3.



EFFECT OF WATER VAPOR



EFFECTS OF ALTITUDE


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 the Universal Gas Constant instead of the specific one:
  • sea level atmospheric pressure ''p''0 = 101325 Pa = 1013.25 mbar or hPa = 101.325 kPa (= 101325 kg/(m---s2))

  • sea level standard temperature ''T0'' = 288.15 K

  • Earth-surface gravitational acceleration ''g'' = 9.80665 m/s2.

  • Dry Adiabatic Lapse Rate ''L'' = −0.0065 K/ M

  • universal gas constant ''R'' = 8.31447 J/( Mol ·K)

  • molecular weight of dry air ''M'' = 0.0289644 kg/mol


Temperature at altitude ''h'' metres above sea level is given by the following formula (only valid below the Tropopause ):

:T = T_0 + L \cdot h

The pressure at altitude ''h'' is given by:

:p = p_0 \cdot \left(1 + rac{L \cdot h}{T_0} ight)^ rac{g \cdot M}{-R \cdot L}

Density can then be calculated according to a molar form of the original formula:

: ho = rac{p \cdot M}{R \cdot T}


IMPORTANCE OF TEMPERATURE


The below table demonstrates that the properties of air change significantly with temperature.

Table — Speed Of Sound in air ''c'', density of air ''ρ'',
Acoustic Impedance ''Z'' vs. Temperature °C


SEE ALSO




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