The instability of Kelvin-Helmholtz

The instability of Kelvin-Helmholtz is an undulating movement that is formed when two fluids are superimexposured and move at different speeds at their contact surface.

Canon EOS 350D Astrodon modded, Canon EF 17mm F4 L @ F5.6, ISO 800. Exposure time: 1/25 of a second.