Thermal and non-thermal internal gravity waves

This note deals with internal gravity waves in a single-component fluid and those in a two-component fluid in a physically reasonable manner. As a result, it becomes apparent that internal gravity waves can be classified, on the basis of their source of kinetic energy, into two categories: thermal and non-thermal internal gravity waves. The source of the kinetic energy of a thermal internal gravity wave is, in spite of its being called an internal "gravity" wave, the internal energy of the fluid. In contrast, that of a non-thermal internal gravity wave is the gravitational potential energy of the fluid: waves in this category can exist only in multi-component fluids.