Causal envelope of stationary internal waves
= Causal envelope of stationary internal waves
{title2=$(x-Ut/2)^2+z^2=(Ut/2)^2$}
In uniform positive flow $U$, upward stationary <internal gravity waves> have laboratory <group velocity> $U(\cos^2\theta,\sin\theta\cos\theta)$. Packets emitted by a localized source at startup lie on $(x-Ut/2)^2+z^2=(Ut/2)^2$, with later emissions inside. This envelope belongs to the permanent stationary component, not every possible startup transient.