Hydrostatic balance
= Hydrostatic balance
{title2=$p_z=-\rho g$}
Vertical <pressure gradient> balances gravitational force per unit <volume>. Subtracting the resting hydrostatic state and using <kinematic pressure> gives $p'_z=b'$ for a <Boussinesq approximation> buoyancy anomaly. The approximation neglects vertical <acceleration>; this diagnostic relation is what couples <buoyancy perturbation> to the vertical <pressure> profile.