The K-H wave is formed due to the vertical wind shear and gravitational shear instability. The horizontal wind speeds of the upper and lower fluids are different, so there is a vertical wind shear. The vortex generated by the wind shear brings the fluid in the lower layer to the upper layer, and the fluid in the upper layer is brought to the lower layer causing the upper fluid to be heavier than the lower fluid, thus becoming unstable. (This picture is taken by Mr. Wang Jiantang on the roof of Dayi Hall of Wenhua University on July 29, 2013.)