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.
In the case of aircraft, the cloud is caused by supersonic 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.
A vapor cone is caused by the formation of so-called ‘Prandtl–Meyer’ expansion fans, which temporarily decrease the air pressure, density and temperature below the air’s dew point. It is not the same thing as the Mach Cone (which is an invisible pressure front), but the two often occur in tandem, allowing us to pretend that we have just seen the sound barrier broken. In this incredible clip of a Boeing F/A-18 Hornet flying at a height of 25 feet, you can see both the Vapor Cone and evidence of the Mach Cone on the surface of the water…