A vapor cone, also known as shock collar or shock egg, is a visible cloud of condensed water that 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.
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.
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.