The buoyancy frequency, $N$ is defined as $N^2=\frac{gd\theta}{\theta dz}$, where $\theta$ is the potential temperature (I see it as a measure of the deviation from the adiabatic nature), $z$ is the vertical co-ordinate and $g$ is the acceleration due to gravity and sometimes $\theta$ is replaced with potential density. My question is why not use $T(z)$ (usual temperature) instead of $\theta(z)$ to define $N$?
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2$\begingroup$ If you start from the derivation of the frequency (en.wikipedia.org/wiki/…), you see that in reality it is based on density. In the atmosphere, potential temperature has the same properties with regards to buoyancy as density and they can be substituted. $\endgroup$ – arkaia Jun 22 '16 at 17:23
