Shock waves are visible above the wing of an A320 family airliner during a regular flight. The shock waves appear as faint vertical "creaks" or "wrinkles" in the photo, and move back and forth a few centimeters along the top surface of the wing. They are in fact changes in air density over a very narrow amount of space, causing optical refraction. Shock waves are a normal occurrence above airliner wings, particularly during flights made at speeds in high transonic regime. whose cruise speeds are in the transonic range. They may not always be observable, depending on flight cruise, atmospheric and light conditions.