Slopes close to zero show as yellow. The movie is in time lapse, so that the waves appear to have higher than real frequency. The movie starts from rest, and after the paddle motion begins, the wave field starts to fill the tank outwards from the paddle. In the movie clip, wave energy appears to travel at an angle from the horizontal, and the wave crests sweep at right angles to this direction. In a density-stratified fluid at rest, each fluid parcel sits at the level of others of the same density.
Lighter parcels sit at a higher level, and denser ones lie below.
The parcel now floats upwards towards its rest level. The fluid parcel therefore bobs up and down, pushed by the buoyancy force generated by gravity acting on slight density differences, and balanced by the inertial forces due to the fluid's mass. The buoyancy oscillation described above is a special case of an internal gravity wave. Waves of a specific frequency can only propagate at a specific angle. Note also that since , the maximum allowed frequency is N, the frequency of the buoyancy oscillation.