Density of air

The air density ρ (also: density of air or air density ) indicates how much mass of air contained in a certain volume. At sea level the air with approximately 1.2041 kg / m³ ( ​​0.0012 g / cm ³ ) at 20 ° C by about oppressive air mass is compressed more than at higher altitudes: The air is so dense in proportion.

Height dependence

The air has on the ground getting the highest density and the highest air pressure - and except for inversions also the highest temperature. At higher altitudes the air is thinner. If the temperature at all heights are equal, air pressure and air density would also together with increasing altitude after take off gas law (see barometric formula ). However, the temperature at different heights varies greatly.

The theoretical decrease of pressure and density of air per 5000 meters - which they would have to fall by half - not strictly true; but the differences are small.

If air is considered as an ideal gas, the air density ρ calculated in kg / m³ to:

With the air pressure p, the molar mass M (note: in SI units), the universal gas constant R and the temperature T in Kelvin.

Substituting the specific gas constant for dry air RS is obtained:

The specific gas constant for dry air is RS:

With energy in J (= N · m); {T } = { Kelvin in temperature in ° C 273.15 }.

Atmospheric air pressure p0 = 101325 Pa = 1013.25 mbar = 1013.25 hPa and R = 287.058 J / kg · K.

At T0 = 273.15 K (0 ° C) ( standard conditions) is the air density:

When T25 = 298.15 K (25 ° C) ( standard conditions) is the air density:

It is furthermore common T20 = 293.15 K ⇔ 20 ° C and this is the air density ρ = 1.204 kg / m³.

Also widespread is T15 = 288.15 K ⇔ 15 ° C, while the air density ρ = 1.225 kg / m³.

As can be seen, these values ​​are temperature dependent.

Precise determination of the air density

An exact determination of the air density requires a consideration of the relative humidity, as this changes the gas constant of air. After the gas was adjusted constant, the equation

Be used. The gas constant of moist air is calculated by:

In which

In order to minimize the measurement error, is recommended to determine the humidity a Aspirationspsychrometer and for determining the ambient pressure, a mercury barometer, the barometric pressure must still be corrected by capillarity, bulge height of the mercury level, temperature dependent density of mercury and local acceleration of gravity.

Temperature dependence

Use in meteorology

In the field of meteorology one often also uses the reciprocal of the density and referred to as the size of α specific volume:

• Meteorological size