I know how to calculate the density under normal ISA conditions at a given altitude, but I'm unsure about how to go about calculating it at ISA+10ºC, or at any ISA+XºC conditions.

  • $\begingroup$ Do you know how to calculate (or get from the standard table) the pressure? Density is calculated from it and the temperature. Check Density of air. $\endgroup$
    – Zeus
    Aug 19, 2021 at 0:40
  • 1
    $\begingroup$ Right now that's a physics question without relation to aviation, which would be off topic here. I recommend asking about the underlying issue; I'm guessing calculating density altitude at different ISA deviations? If yes, see: How do I calculate density altitude by hand? and the related topics there on the right-hand side. Tip: it helps to search the site before asking, see: How do I ask a good question? $\endgroup$
    – user14897
    Aug 19, 2021 at 1:07
  • $\begingroup$ at a pressure altitude of 11705 m or a geometric altitude of 11705? $\endgroup$
    – DeltaLima
    Aug 19, 2021 at 9:58

1 Answer 1


Just going to answer my own question here, I found a very simple solution.

ISA + X ºC changes only the temperature. Pressure is ISA, because the model is static. So calculate ISA pressure with standard ISA formulae, calculate ISA temperature, add 10ºC, and obtain density through ideal gas equation. The ISA model presupposes that air is an ideal gas anyway.

  • $\begingroup$ Yep, they may use Kelvin for that application. Seeing similarities to Clausius-Clapeyron equation. $\endgroup$ Aug 19, 2021 at 15:03

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