Internal gravity waves (internal waves) are like surface waves (e.g., on water) but are internal to the fluid and instead of being ripples on the surface of a fluid, are, for example, ripples on a surface between two regions of different density. They can also occur if the change in density is continuous (smooth) rather than discontinuous (abrupt, i.e., a surface). Like surface waves, the gravitational force is the "spring" that is being exercised by the wave.
Internal gravity waves occur in atmospheres, including Earth's, where they are sometimes made visible by clouds: the waves themselves can instigate cloud formation in patterns (wave cloud) or can be illustrated by clouds (e.g., cirrus clouds) which can make visible their breaking (breaking atmospheric wave). These wave phenomena are of interest in the study of other atmospheres, e.g., of solar system planets and moons and extra-solar planets.