This picture was mailed to me with the following description:

Every so often, just the right combination of conditions and events
occur to create an unbelievable event -- in this case an F-18 passing
through the sound barrier. There was just enough water vapor in the
air that the shock waves caused the water to precipitate, but the
density and temperature were just right, AND there just happened to
be a camera in the vicinity to capture the moment. The F-18 is
actually in transonic flight, with normal shock waves emanating from
behind the canopy and across the wings and fuselage. The condition
will last for only an instant, and once supersonic flow exists
completely around the aircraft, sharp-angled sonic cones replace the
normal shock waves. The odds of getting a shot like this are
staggering.

  (click on thumbnail to get larger picture)