A Normal Shock is created by a blunt body in supersonic flow. The same body in a subsonic flow produces waves of sound that propagate ahead of the body, basically “warning” the approaching air stream of the approaching body. These sound waves cause the molecules in the air stream to begin to diverge around the body well in advance of the actual body. When the object is traveling supersonically, however, these sound waves cannot outrun the object, and they pile up a short distance in front of the object. This stacking of sound waves is a Normal shock wave, and it serves to instantaneously force the air to change direction around the body. This effect is also referred to as a Bow Shock, and is shown in the figure below, depicting a supersonic bullet.