Atmospheric pressure can be thought of as the weight of the atmosphere above the point where a measurement is made. As a consequence, air pressure decreases as altitude increases. An aircraft flying at 20,000 ft will experience a lower ambient pressure compared to an aircraft on the ground.

The actual pressure drop experienced by an aircraft depends on geographic location, temperature, and other local conditions. The International Standard Atmosphere (ISA) models pressure at different altitudes based on the following assumptions:

• The pressure at mean sea level is 1013.25 hPa (29.92 inHg).
• The air temperature changes with altitude as described by the ISA Temperature Model.
• Air is a perfect gas.

The graph below shows the relationship between altitude and pressure according to the ISA model:

As can be seen, this relationship is non-linear. Air pressure changes more rapidly close to the surface than at higher altitudes.

The altitude that corresponds to a given pressure in the graph above is called the pressure altitude. E.g. in ISA conditions, a pressure reading of 376 hPa gives a pressure altitude of 25,000 ft.

Because the atmosphere expands with higher temperatures, the pressure layers will move upwards. As an example, an aircraft flying at an ambient pressure of 697 hPa will be at 10,000 ft in ISA conditions. In ISA+10 conditions the same pressure corresponds to 10,340 ft.

Conversely, if the same aircraft is flying in ISA-10 conditions, 697 hPa corresponds to an altitude of only 9,646 ft. At low altitudes and very low temperatures this must be considered, especially when flying non-precision approaches or while flying over high terrain.

Sources:

• getting to grips with aircraft performance, Airbus Customer Services, January 2002
• Interplanetary Air Pressure at Altitude Calculator, read 2024-12-31