I usually say “check out this awesome video and then stick around for the science,” but the video explains the science pretty completely. So enjoy.
But for the sake of completeness, here’s a little background on the photographing technique via Wikipedia:
Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects. Its role is changing due to the increasing use of computational fluid dynamics, which reduces the need for all such experimental fluid flow measurement techniques.
More information here.