What is the white cloud formed when a plane breaks through the sound barrier?

A sharp rise in aerodynamic drag, occurs when a plane reaches the speed of sound. For an aircraft flying at subsonic speed, the pressure waves generated by it extend in all directions, and, outspeeding the craft, transmit the disturbance uniformly. However, for supersonic speeds, the pressure field is confined to an area extending in a Mach cone from the rear of the craft. The waves are unable to get out of the way of the aircraft, and strong local shockwaves act on the wing and body of the plane. These shockwaves squeeze moisture in the air to give rise to a cloud-like effect.