What's the white cloud that appears when a airplane breaks the sound barrier?

Question

When a jet airplane breaks the sound barrier, there's always this kind of conical white cloud. I wonder also id the pilot can ear himself or the plane?

Answer 1

It has something to do with the air suddenly being accellerated and it's pressure suddently lowered.  In a given quantity of air it can contain only so much water vapor. When the volume of air is suddenly reduced the water tries to redistibute itself equally in all parts of that volume.  It's hard to put into words.

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When an airplane is traveling high up in the sky it is pressurized.  If something occurs like a window breaking and the air in the plane suddenly drops in pressure within a split second all the air that's left in the plane turns to a fog that is so thick you cannnot see your hand in front of your face.  I'm no scientist but I know it has something to do with the water vapor or humidity in the air.  In very humid conditions a cloud of vapor is sometimes seen developing on top of the wing of airliners as they land.  The air above the wing is always at a lower pressure and under certain high humidity conditions that cloud appears.

Answer 2

These vapor cones or "shock collars" (also called the Prandtl-Glauert singularity) actually don't always happen. It is just very common at low altitudes where people happen to be filming or photographing the plane. High humidity is also a must. Also, it doesn't have to be above the sound barrier, but only near it. In transsonic flight ,which is rougly .8 to 1 mach, shockwaves are beginning to form but aren't fully compressed into the sharp wave that makes a boom. At these speeds and above, the wave of compressed air causes the moisture in the air to condense and form a cloud for a moment. Depending on humidity and air pressure, this cloud can be fairly constant or seem to flicker as the plane flies through pockets of moisture. Often, if conditions are dry, there will be no cone at all. Basically, when you rapidly change air pressure you can create condensation. For the same reason, in wet weather airliners can create very large clouds above their wings when flaring to land. Here's a decent little blurb on wikipedia discussing vapor cones:

http://en.wikipedia.org/wiki/Prandtl-Gla...

Sources at the bottom of the article are pretty good.

EDIT: The fluid mechanics links at the bottome of the aforementioned article don't seem to be working anymore. If anyone reading this knows where to find more information on these subjects, please send me a message. Thanks.