The ISA
- defines a standard atmosphere model.
- is referenced from the equator.
- is taken from 45 degrees latitude.
Explanation
The International Standard Atmosphere is referenced to conditions at 45 degrees latitude, giving the agreed baseline of 15 degrees C, 1013.25 hPa and a 1.98 degrees C per 1000 ft lapse rate at mean sea level. It is not referenced from the equator, where surface conditions are warmer, and it is a fixed standard rather than the live atmosphere.
Prodromos asking:
Why
Community Comments (1)
T
Tomash Posts: 3 03.06.2026 / 19:55
It's C. We lean on those ISA numbers all the time when we set altimeters and read engine performance against deviation from standard, 15 degrees C and 1013 millibars at sea level. The whole model is built on average mid latitude air, so 45 degrees is the datum, not the equator.
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