One of my favorite photos from last weeks Ares 1-X launch showed a very defined shock collar developing around the vehicle. Shock collars, shock eggs, or vapor cones are also known as the Prandtl-Glauert singularity occur when a drop of air pressure cause condensation around a craft traveling at transonic speeds.
Wikipedia article on Prandtl-Glauert singularity
Here are a few of my favorite rocket shock collars:


Ares 1-X shock collar



Apollo 11 shock collar


Shuttle shock collar