Critical Mach (MCrit) over a Boeing 757-200 airfoil

This is the fourth time I've witnessed this phenomenon. Once was similar to the somewhat faint vertical lines you see here. Another was a razor-sharp standing shockwave (concave in front, vertical line at rear), and finally I once saw the shadow of the wave running the length of the wing, although the shockwave itself was not visible.

Look carefully, and you will see some diffraction forward of the leading edge near the wing tip, and a little more clearly where the trailing edge of the darker grey paint meets the shiny light grey paint.

MCrit occurs when the aircraft is flying at a sub-sonic speed, but the wind traveling over the airfoil accelerates over Mach 1. This flight was reported by ADS-B to be at FL 390 and approximately 520 knots.

The 757 is the only passenger aircraft I've ever personally seen this on.