So I'm A Geek. Big Surprise.
Yesterday two of my co-workers were looking at picture on the internet of a jet (apparently) breaking the sound barrier. It’s an amazing picture and one worth admiring. My contribution to the conversation was “oh, that’s a great example of a Prandtl-Glauert singularity”. Both of my co-workers did a slow head turn in my direction and gave me an accusing look. And so I went to the white board and drew the classic compression-rarefaction-recovery curve, showing how the negative pressure can result in condensation. I pointed out that this phenomenon is not limited to transonic jets, citing the classic bullwhip example, and that this can also occur at speeds below the transonic threshold. They both called me a pathetic paste-eating geek for having that term in my head and for being able to explain the physics behind it. Yum. Paste.
During my three years aboard an aircraft carrier, I saw this many times. However, as John mentions, you can see it at much lower speeds. For example, on a very humid day, when your flight is leaving SeaTac, watch the area just behind the highest point in the chord of the wing – you may see pockets of “fog” appearing above the wing, particularly when the wing is in a high lift (flaps down) configuration.
The urban legend is that the photograph illustrates a “sonic boom.” Not quite…
Yep. This isn’t (necessarily) a sonic boom. It’s caused by the “N” wave (so named for the approximate shape of the compression/rarefaction/recovery profile) and is visible because the moisture in the air is suddenly exposed to a big pressure drop. You get an instant mini-cloud that dissipates as soon as the pressure normalizes. A sonic boom initiates a shock profile that can be visible if there is sufficient water vapor.
Cautiously, our intrepid explorer observes the odd beasts. Perterbed by their strange behavior, she carefully backs away to avoid drawing their attention.