= Effective-potential stratification coefficient
{title2=$\mathcal N^2=-\rho/(\gamma p)-(d\rho/d\Psi)/\rho$}
For barotropic equilibrium $dp/d\Psi=-\rho$, this coefficient equals $-\rho^{-1}(dp/d\Psi)[\gamma^{-1}d\ln p/d\Psi-d\ln\rho/d\Psi]$. It weights displacement across effective-potential surfaces in the <Cowling energy principle for a rotating barotropic star>. Its sign distinguishes stabilizing from destabilizing buoyancy there. It is not itself a squared <frequency>: the physical local buoyancy-<frequency> square contains an additional factor $|\nabla\Psi|^2$.
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