<P> A vapor cone, also known as shock collar or shock egg, is a visible cloud of condensed water which can sometimes form around an object moving at high speed through moist air, for example an aircraft flying at transonic speeds . When the localized air pressure around the object drops, so does the air temperature . If the temperature drops below the saturation temperature a cloud forms . </P> <P> In the case of aircraft, the cloud is caused by expansion fans decreasing the air pressure, density and temperature below the dew point . Then pressure, density and temperature suddenly increase across the stern shock wave associated with a return to subsonic flow behind the aircraft . Since the local Mach number is not uniform over the aircraft, parts of the aircraft may be supersonic while others remain subsonic--a flight regime called transonic flight . </P> <P> In addition to making the shock waves themselves visible, water condensation can also occur in the trough between two crests of the shock waves produced by the passing of the object . However, this effect does not necessarily coincide with the acceleration of an aircraft through the speed of sound or Mach 1 . </P> <P> These condensation clouds can often be seen appearing around space - bound rockets as they accelerate through the atmosphere . For example, they were frequently seen during Space Shuttle launches, about 25 to 33 seconds after launch, when the vehicle was traveling at transonic speeds . Similar effects were also visible in archival footage of some nuclear tests . Scientists observing the Operation Crossroads nuclear tests in 1946 named the transitory cloud a "Wilson cloud" for its superficial similarity to the Wilson cloud chamber effect . </P>

Why is there a cloud when you break the sound barrier