Kelvin–Helmholtz Instability Two streams slide past each other — wind over water, warm air over cold. A hundred thousand tracer particles ride the shear layer as it buckles and winds into a row of curling “cat’s-eye” vortices — the shape of billow clouds and of surf about to break. The journey: Helmholtz (1868) and Kelvin (1871) proved a shear layer is unstable to every ripple. Stuart (1967) wrote down the exact rolled-up state; the slider dials continuously from a straight shear layer to the fully wound street.