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. One sign of the instability's occurrence is a repeating, wave-like pattern and some kinds of (Earth-weather) cloud patterns show such a repeating pattern and are in fact the results of such an instability. Occurrences produce visible patterns on Jupiter and Mars atmospheric surface as well. The instability is of interest in astrophysics, as a potential mechanism in star formation and planet formation/protoplanetary disks.