The ISA
- defines a standard atmosphere model.
- is referenced from the equator.
- is taken from 45 degrees latitude.
Explanation from PART66Online
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.
P
Prodromos asked · 3 Jun 2026
Why
Community Comments (1)
T
Tomash
3 Jun 2026
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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