What causes the cone shaped cloud to appear around an aircraft as it breaks the sounds barrier?
The cone shape is the shock wave created by the aircraft as it flies above the speed of sound. When the aircraft first accelerates above the speed of sound, a drop in pressure behind the shock wave causes moisture to condense out of the air, producing the white fog inside the cone. Sound waves are indeed invisible to the eye. What you are seeing in this case is water droplets (mist). Shock waves can sometimes be seen as an optical distortion of objects behind them, although that’s difficult in this case because the mist behind the shock wave is much more visible.
