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Internal gravity waves (IGWs, internal waves) are analogous to surface waves (e.g., on water) but are internal to the fluid: instead of being ripples on the surface of a fluid, they are, for example, ripples on a surface between two regions of the fluid of different densities. Such waves can also occur where 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 wave-patterns in clouds (wave clouds) or can be illustrated by clouds, such as cirrus clouds, which can make visible such a wave's breaking (i.e., a 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.