How it's calculated
ρ = Pd ÷ (Rd·T) + e ÷ (Rv·T)
Pd = dry-air partial pressure (Pa) = P − e, e = water-vapour pressure, T = temperature (K), Rd = 287.058, Rv = 461.495 J/(kg·K). e = RH·eₛ, eₛ = 6.1094·exp(17.625·Tc/(243.04+Tc)). Humid-air (CIPM/Magnus) density.
Common questions
Does humidity make air denser or lighter?
Humid air is actually lighter than dry air at the same pressure and temperature, because water vapour is less dense than the nitrogen and oxygen it displaces.
What is ISA altitude equivalent?
It is the altitude in the standard atmosphere model that would produce the same air density, handy for performance comparisons.