The figure below shows an F-14 Tomcat off of the aircraft carrier Constellation as it breaks the sound barrier at 741 miles per hour.  The pressure wave that results from the aircraft outrunning the sound waves causes the surrounding atmosphere to expand rapidly and cool adiabatically condensing the moisture behind the adiabatic front and forming the egg shaped cloud of water droplets seen in the picture below.  This picture was taken by Ensign John Gay on July 7, 1999.