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Principles of flight
When manuals and performance data are developed for aircraft, they are based
on a so-called "standard atmosphere."
The following has been adopted for an international standard atmosphere (ISA):
Air Pressure: MSL 1013.25 hPa, decreasing by one hPa per 30 ft
Temperature: MSL 15°C, decreasing by 2°C per 1000 feet
The standard atmosphere is completely dry
If these values do not match the conditions on a given day, conversion methods
(see the aircraft's performance section) are available to convert to standard
atmospheric conditions.
There is always some moisture in the atmosphere. The moisture we refer to is
the invisible amount of water vapor that the air contains.
Humid air is not as dense as dry air, so the aircraft performs worse in humid air.
The invisible humidity in the air not only negatively affects aerodynamic
performance but also impacts the engine's performance.
The higher the pressure (the weight of the air above), the denser the air (more
compressed).
The denser the air, the better the aircraft engine performs. The aircraft's wings
generate more lift.
Air resistance is greater in dense air than in the thinner air found higher up in the
atmosphere.
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Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 442
