A vapor cone, also known as shock collar or shock egg, is a visible cloud of condensed water which can sometimes form around an object. A vapor cone is typically observed as an aircraft, or object, approaches transonic velocity. Atmospheric water then condenses, and thus becomes visible, as air pressure varies brutally across shock waves associated with supersonic flow speed.
Atmospheric condensation around aircraft is also associated with low-pressure regions in subsonic flight, and can typically be observed in damp conditions; but this does not result in conical condensation clouds.
Condensation cones appear in what was described as a Prandtl-Glauert singularity. In addition to making the shock waves themselves visible, water condensation can also occur in the trough between two crests of the shock waves produced by the passing of the object. However, this effect does not necessarily coincide with the breaking of the sound barrier, or the acceleration of an aircraft through Mach 1.