Aircraft performing at airshows in America do not, as a rule, break the speed of sound. I assume this is an FAA rule, and I see the value of it, but all the same I would love to attend a show where such a restriction were not in place.
Planes can still get close to Mach (for an explanation of that term, see here), and they do that all the time. If the air is humid enough, the rapid changes in air pressure lead to the creation of very small clouds. The location of these clouds correlates with areas of maximum cross sectional area, and ties in very closely with Frank Whittle’s are-rule theory that still shapes high-performance aircraft to this day. For more discussion, read this.
If all that theory bores you, then how’s this – I will stop yammering and move straight to Flickr user nelnov‘s picture of a Boeing F/A-18F Super Hornet. Taken at Scott AFB on September 20, 2008, this is a terrific capture of a spectacular phenomenon.
If you like this, I encourage you to check out this summary of all of the aviation photography I have featured here.
[Update: I try not to do this, but I am compelled to add a shot to this post. Wow!]