Conditions were exactly right this weekend (high humidity, fast aircraft) to produce a few shock collars (sometimes known as Prandtl–Glauert singularities (only a theoretical result), shock eggs or vapor cones). at the Fleet Week airshow in San Francisco.
This is not the aircraft breaking the sound barrier, but traveling close to it at transonic speed. Around the plane air is compressed into higher- and lower-pressure regions which cause instability in the flow. This builds up into a shockwave, and as the plane passes through it causes moisture to precipitate out of the air producing the beautiful but transient shock collar, as seen above. See some shots of the shock collar developing and dissipating here.
Explored 08/10/11 #8!