CPL » Principles – Aeroplane » The Atmosphere

22.4 The Atmosphere

22.4.2 Explain the air density, and describe the effect of temperature, pressure and humidity on atmospheric 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).

22.4.4 Describe the normal changes in pressure, temperature and density with increased altitude in 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.

22.4.6 State the ISA sea-level pressure and temperature conditions, and the approximate lapse rates in the lower atmosphere.