A Sonic Boom . ... When an airplane travels at a speed faster than sound, density waves of sound emitted by the plane cannot precede the plane, and so accumulate in a cone behind the plane. ... As a plane accelerates to just break the sound barrier , however, an unusual cloud might form. ... A leading theory is that a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets. ... NASA Official: Jay Norris. ...