A Vapor Cone on an F-22 Raptor.
Vapor cones on a plane is believed to be the result of a plane reaching supersonic speed or breaking the sound barrier, but it has nothing to do with it. The cone only forms when a plane flies past a particularly humid area, it's pressure envelope increases the air pressure around and causes the water vapor to instantly condense. Once the plane passes, the pressure is released and the condensed water would just as quickly re-vaporize. Of course the faster a plane goes, the greater the difference of pressure and so the effect will be more evident.