Hi Mae,

That's a really interesting question. The effect is known as a
Prandtl-Glauert condensation cloud or more simply, a condensation cloud.
There are some excellent descriptions of the effect at these web sites:

http://www.fluidmech.net/tutorials/sonic/prandtl-glauert-clouds.htm

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

You can read about the details at the web sites above, but in simple terms,
a cloud is created when air is cooled to a point where moisture in the air
condenses. The air cools due pressure changes, which in turn cause
temperature changes. Contrary to popular belief, it is not necessarily and
indication of breaking the sounds barrier. It can happen at lower,
transonic speeds.

Contrails are created by a different effect, as they are similar to
naturally occurring cirrus clouds -- tiny ice crystals. Prandtl-Glauert
condensation clouds are more like naturally occurring fair weather cumulous
clouds. You can read more about contrails and types of clouds here:

http://en.wikipedia.org/wiki/Contrail

http://en.wikipedia.org/wiki/Cloud

Many of the pictures and videos of Prandtl-Glauert condensation clouds are
made by high speed aircraft, but usually not
travelling at supersonic speeds. I am not aware of any countries that allow
supersonic travel over populated areas. This is primarily due to high noise
levels of sonic booms. The supersonic transport (SST) or Concorde failed in
the market partly due to restrictions on supersonic flights over populated
areas. Eventually, the Concorde flew at supersonic speeds over oceans only,
namely, the Atlantic. You can read more about the Concorde here:

http://en.wikipedia.org/wiki/Concorde

Aircraft at air shows often fly at very high speeds, but I doubt that any
actually exceed the speed of sound, at least not at close range.

I hope that answers your question!

Madhu