High Quality Content by WIKIPEDIA articles! Internal waves are gravity waves that oscillate within, rather than on the surface of, a fluid medium. They are one of many types of wave motion in stratified fluids (another example being Lee waves). A simple example is a wave propagating on the interface between two fluids of different densities, such as oil and water. Internal wave motions are ubiquitous in both the ocean and atmosphere, where they create wave clouds. Nonlinear solitary internal waves are called solitons.