Nice photo of a jet with condensation disk. Popular thinking is that the white ring is a shock wave phenomenon. More likely, according to the author, it demonstrates Radial Momentum. The air, set in radial motion by the nose of the aircraft, fans out into a cone. Eventually, the density and pressure of the air falls to a point where it cools the water vapor enough to form droplets and fog.
Another demonstration of Radial Momentum inducing fog.