F-18 Super Hornet Vapor Cone (World Fighter Aircraft Collection)

An F-18F Super Hornet pulls a vapor cone during the 2004 airshow at Naval Air Weapons Station Point Mugu, on the coast north of Los Angeles.

In spite of common belief, a vapor cone like this doesn't necessarily mean that the aircraft is flying at supersonic speed, though it is usually associated with very high subsonic flight.   The water condensation is formed by what is called a "Prandtl-Glauert singularity", caused when a fast-moving object creates an area of low pressure in its wake.   The low air pressure causes a drop in temperature, which can force the invisible water vapor in the air to fall below its dew point and condense, resulting in the vapor cone.   This effect can also be seen during atomic bomb tests and, when the conditions are right, even when a whip is cracked.