Kelvin–Helmholtz instability

An instability that develops where fluids move at different speeds, producing growing waves and often vortices. It occurs in settings ranging from clouds and oceans to planetary atmospheres.

Velocity shear: A spatial change in fluid velocity that can arise between adjacent layers or across a boundary. The speed difference across the interface supplies energy for disturbances to grow.
Kelvin–Helmholtz clouds: Cloud formations shaped into repeated wave-like billows by atmospheric wind shear. Their cloud tops can trace shear-driven waves as they grow and overturn.
Fluid interface: A boundary separating two fluid regions with differing properties or motion. The instability develops along an interface or across a sharply changing flow layer.
Richtmyer–Meshkov instability: An instability caused when a shock impulsively accelerates a corrugated interface between fluids of different densities. Its waves arise from shock acceleration, not sustained velocity differences.
Richardson number: A dimensionless ratio comparing the stabilizing effects of buoyancy with the destabilizing effects of velocity shear. It estimates whether stratification can suppress shear-driven instability.
Clear-air turbulence: Atmospheric turbulence occurring outside visible clouds, often near strong wind shear. Kelvin–Helmholtz waves can break into turbulence near aircraft cruising altitudes.
Incompressible flow: Fluid motion in which density changes are negligible, so velocity has zero divergence. Many classic treatments assume incompressibility to isolate shear-driven wave growth.
Magnetorotational instability: An instability in a weakly magnetized, differentially rotating conducting fluid. It extracts energy from rotational shear through magnetic tension rather than interface billows.
Kelvin–Helmholtz billow: A rolling, coherent vortex formed as a shear-driven wave overturns and breaks. Billows are the characteristic large-scale structures produced by the instability.
Ocean mixing: The transport and blending of water, heat, salt, and other properties within the ocean. Shear-driven billows help mix water across stratified ocean layers.

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