Atmospheric Pressure & Density

Atmospheric Pressure

Atmospheric pressure is the weight of the atmosphere above a given area.

An instrument used to measure atmospheric pressure is called a barometer.

Standard sea-level atmospheric pressure is:

• 1013 hPa

As altitude increases, there is less air above and therefore atmospheric pressure decreases.

For practical purposes near sea level, pressure decreases by approximately:

• 1 hPa for every 30 ft increase in altitude.

Air Density

Air density is a measure of how much air is contained within a given volume.

Air density decreases as altitude increases.

This is because atmospheric pressure decreases with altitude and the air becomes less dense.

Air density also changes with temperature and pressure.

Air density decreases when:

• altitude increases;
• temperature increases; or
• atmospheric pressure decreases.

Air density increases when:

• altitude decreases;
• temperature decreases; or
• atmospheric pressure increases.

Lower air density reduces both aircraft and engine performance.

Therefore, aircraft performance suffers under conditions of:

• low atmospheric pressure; and
• high temperature.

International Standard Atmosphere – ISA

The International Standard Atmosphere (ISA) is a theoretical standard atmosphere used as a reference for aviation.