= Critical level of a shear-flow wave
{title2=$U(z_c)=c$}
= Critical level
{synonym}
For a wave with zonal <phase velocity> $c$ in a basic shear $U(z)$, a critical level is a height $z_c$ at which $U(z_c)=c$. The Doppler-shifted material frequency then vanishes. This means equality of the laboratory velocities; the $U$ and $-c$ terms cancel in the wave frame. It does not mean $U=-c$. A <semi-infinite Eady edge wave> with zero interior <potential vorticity> anomaly can remain regular through its steering height.
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