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PPL » Aircraft Tech Knowledge (A) » The Atmosphere

12.4 The Atmosphere

12.4.2 Name the principal gases which constitute the atmosphere.

The atmosphere is a mixture of gases that surrounds the Earth.

The 3 main gases are:

Nitrogen                    - 78%

Oxygen                      - 21%

Other                         - 1%      

(argon (0.9%), carbon dioxide (0.04%), neon, helium, ozone, methane, krypton, and hydrogen)


12.4.4 Define air density.

Air Density ρ (rho), is a fundamental property of the atmosphere.


It refers to the amount of mass per unit volume of air.

In other words, it is a measure of how much air molecules are present in a given amount of space.


Air Density is affected by temperature, pressure, and humidity.

  • temperature increases = density decreases
  • pressure increases = density increases
  • humidity increases = density decreases


At sea level at standard temperature and pressure, air density is 1.225 kilograms per cubic metre (kg/m³).


12.4.6 Explain how air density varies with altitude within the atmosphere.

Air is compressible so the atmosphere is more dense at lower levels because the weight above compresses the air molecules.

As you go up in altitude, the amount of molecules in the air decreases, so less pressure.

Less pressure = Lower density

With increasing altitude, density decreases because the decrease in pressure overrides the decrease in temperature.

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