Billow clouds are created when different layers of wind flow at different velocities resulting in unstable turbulence between the layers. The common name for this instability is Kelvin-Helmholtz instability. Billow clouds are created in the upward branch of each of the "eddies" if the air within this branch has a high enough relative humidity that, upon lifting, the air parcel reaches saturation. Individual billow clouds generally last a few minutes. Billow clouds provide a visible signal to pilots of potentially dangerous turbulence.