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The Kelvin-Helmholtz instability (KH instability or KHI) can occur in fluids when there is a velocity-shear, e.g., region where some of the fluid is flowing faster than fluid adjacent to it. It can occur between two adjacent fluids or within a single fluid. For example, such an instability is the instigator of waves on the surface of water, the water and the atmosphere being the two fluids. A sign of the instability's occurrence is a repeating, wave-like pattern. Some kinds of (Earth-weather) cloud-patterns show such repeats, caused by the instability. An example is clouds in parallel strips, with spaces between. Characteristic patterns have been detected in other solar system planets' atmospheres, e.g., Jupiter and Mars. This type of instability is also of interest in astrophysics, as a potential mechanism in star formation and planet formation/protoplanetary disks.