Jet Aircraft Breaking the Sound Barrier

Some scientists describe it as the visible manifestation of jet aircraft "breaking the sound barrier" (or creating a "sonic boom"). Others contend that these vaporous cones only form at "transonic" speeds (just under or over the speed of sound), may or may not be accompanied by a sonic boom, and strictly speaking have nothing to do with breaking the sound barrier. They are caused by a sudden drop in air pressure around the aircraft (see Prandtl-Glauert singularity). (about.com)

Thanks Julie