<a href="https://en.wikipedia.org/wiki/Shock_waves" class="extiw" title="en:Shock waves">Shock waves</a> 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.<br>
Shock waves are a normal occurrence above airliner wings, particularly during flights made at speeds in high <a href="https://en.wikipedia.org/wiki/transonic" class="extiw" title="en:transonic">transonic</a> regime. whose cruise speeds are in the transonic range. They may not always be observable, depending on flight cruise, atmospheric and light conditions.
<a href="//commons.wikimedia.org/wiki/User:Ariadacapo" title="User:Ariadacapo">Olivier Cleynen</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>