Internal waves

When two fluids of different densities do not mix, gravity waves may occur at the interface. When the two densities are very similar the effective value of g (the acceleration due to gravity) is very low, and the wave velocity is correspondingly slow.

The satellite image shows apparently stationary internal waves of very long wavelength at a fresh-water/salt-water interface beneath the surface of the Mediterranean, just inside the Straights of Gibraltar. The editor has seen (2005) apparently stationary 200 meter internal waves off the coast of Java.