Air - Altitude Density and Volume

The density of air varies with the elevation above sea level

The density of air varies with the air pressure (the Ideal Gas Law) and the altitude above sea level. Mean absolute pressure at sea level is approximately 760 mmHg (101.325 kPa, kN/m²). The variation is about +/- 5%.

Altitude and Air Pressure

The air pressure varies with altitude as shown in the table below:

Altitude and Air Density

The air density is proportional with the absolute pressure. If the pressure at an altitude is named H, the ratio H/H_o decreases from 1 at sea level, to 0.74 at altitude 2,400 m. At 2,400 m the pressure is decreased with approximately 26%.

The ratio H/H_o can be expressed as in the chart below:

TP - Standard Temperature and Pressure

• STP - Standard Temperature and Pressure is 0^oC and 101.325 kN/m².

NTP - Normal Temperature and Pressure

• NTP - Normal Temperature and Pressure, common used for testing and documentation of fan capacities, is 20^oC and 101.6 kN/m² (60^oF , 30 in Hg)

Altitude and Air Volume

The volume of standard air at a certain level can be calculated with the volume correction factor below:

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