= Absolute-frequency conservation in steady shear
{title2=$\dot\omega=0,\quad\widehat\omega=\omega-kU$}
A time-independent ray Hamiltonian conserves the stationary-observer <absolute frequency>. A horizontally uniform background also conserves horizontal wavenumber. The <intrinsic frequency> $\widehat\omega=\omega-kU(z)$ nevertheless changes when the ray moves through shear. These follow from the <Hamiltonian ray-tracing equations> and are essential to identifying a <critical level of an internal gravity wave>.
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