Internal Gravity Waves

Vertical velocity field of an internal gravity wave in a density stratified fluid, illustrating the 90-degree angle between the velocity of propagation of the wave crests (the phase velocity) and the velocity of propagation of the envelope (the group velocity) (blue is positive, white zero and black negative). The whiter patches indicate regions of vertically stationary fluid, forming a clear structural component of the envelope visible for all times. The envelope propgates towards the bottom right, whereas the wave crests move towards the top right, at right-angles to the movement of the envelope, illustrating a remarkable feature of these waves.