[aruanan]

If, as some have replied that the Prandtl-Glauart singularity can occur at well below trans-sonic speed, how can you say by looking at a photograph that the sound barrier is being broken?

Because those photos were from sites where the details of the photo were published and they were taken as the plane was approaching the speed of sound. Also because of the placement of the phenomenon. There are types of condensation that are observed behind wings or trailing behind wing tips, especially in very humid air. These are not examples of that phenomenon. You can see it in some turbofan engines at take-off. The plane is moving well below the speed of sound, but the blades in the fan are operating at trans-sonic speed.

For instance, if you see anything like this, it is caused by the aircraft approaching the speed of sound:



This is not:

[1raider1]

Oh.

[famousdayandyear]

Re the bottom photo — those are condensation trails. Also often observed flowing from rear wings of Indy-type cars. Re the top photo, I would guess the aircraft has broken M1.0. Below 1.0, conditions are unstable.