What causes the "cloud" when a jet breaks the sound barrier?

Question

I used to have a photo on my pc of a jet breaking the sound barrier, and it showed a jet looking like it just emerged from a disc-shaped cloud stood on end. I just watched the following clip at MySpace:

http://vids.myspace.com/index.cfm?fuseaction=vids.individual&videoid=42042739

...and it showed the "cloud" when the jet broke the sound barrier. What causes the cloud?

Answer 1

The white halo is formed by condensed water droplets which are thought to result from a drop in air pressure around aircraft.

Answer 2

The Speed of Sound is the speed compression waves travel in air.  You probably know that sound is caused by waves of pressure on your ear.  As things move faster, air piles up in front of them.  Close to and above the speed of sound, the air can't move quickly enuf to flow out of the way and it must be "pushed" away.  This builds up a "wall" of pressurized air, often in a cone around the plane or rocket. Behind the "wall" is an area of lower pressure and lower temperature.  If the temperature is low enough and the humidity is high enough, water will condense to form a cloud.  The scientific name for this is the Prandtl-Glauert singularity.  This effect is not limited to just breaking the Sound Barrier, all speeds above sound also will cause it.

The reason why this cloud forms is still somewhat controversial.

Wikipedia says you can see the same thing at the tip of a cracking whip on a humid day, and in the shock wave from a thermonuclear detonation.