The phenomenon is not well studied. Scientists refer to it as a vapor cone, shock collar, or shock egg, and it's thought to be created by what's called a Prandtl-Glauert singularity.
Here's what scientists think happens:
A layer of water droplets gets trapped between two high-pressure surfaces of air. In humid conditions, condensation can gather in the trough between two crests of the sound waves produced by the jet. This effect does not necessarily coincide with the breaking of the sound barrier, although it can.
From the Upcoming ueue, submitted by .