On internal gravity waves associated with the stable boundary layer

The stable planetary boundary layer at the base of the residual layer supports internal waves that are unambiguously `boundary layer' in character. Some of these waves are instabilities and some are neutrally stable modes, but they all have critical levels in the residual layer. These waves exist for a broad range of conditions and should be a major component of any ducted disturbance that propagates within ninety degrees of the wind direction. The wave properties can be computed without the numerical difficulties usually associated with critical-level systems.