This US Navy photo of an F-18A transonic flight shows what it looks like the moment a pilot crosses the sound barrier. Or does it? An article in the Cambridge University Press called "Sonic Boom, Sound Barrier, and Condensation Clouds: Prandtl-Glauert Condensation Clouds" questions what exactly these cloud rings really are.
At near-sonic speeds, these clouds are sometimes said to visualize shock waves. But others maintain it’s the aircraft bursting through the sound barrier. According to this article, the clouds are the result of a "high-lift maneuver resulting in very low pressures on the upper surface of the wings." The truth is, there is no simple explanation for this phenomena.