The bow shock-wave - very high subsonic flight, just below the speed of sound. Some surfaces of the aircraft are already supersonic - there is local supersonic flow. The mass condensation occurs in the low pressure area directly behind the bow shock-wave.
Super Hornet from in distinctive "Digital" camouflage and bearing the words Years Of Naval Aviation" bursts through its own vapor over EAA AirVenture 2011 in Oshkosh. I've flown in in the backseat of one of the trainers for these.
A cloud forms as this Hornet aircraft speeds up to supersonic speed. Aircraft flying this fast push air up to the very limits of its speed, forming what's called a bow shock in front of them. Credit: Ensign John Gay, USS Constellation, U.