How are these images captured? They look like Schlieren photography, but as I understand it that requires a mirror on the other side of the target being photographed.
It is a Schlieren technique, but in this case a series of images taken over a textured background are then processed with software in a way that amplifies the differences in air density. This page goes into a little more detail:
Someone posted this [0] on an article from another website. It's NASA's method for taking these sorts of pictures from the ground. The process for taking airborne images is probably similar, but must require so much more coordination.