Wake turbulence is caused by wing tip vortices and is a by-product of lift. The higher air pressure under the wings tries to move to the lower air pressure on top of the wings by flowing towards the wing tips where it rotates and flows into the lower pressure on top of the wings. This results in a twisting rotary motion which is very pronounced at the wing tips and continues to spill over the top in a downward spiral. Therefore, the wake consists of two counter-rotating cylindrical vortices.