Compression. HOWTO: Schlieren Photography. ( Back to my Schlieren Photography Page ) So how does Schlieren photography make pictures like the above?
The phenomena you are seeing is the difference in air density. When a gas is heated it expands, and when it cools it contracts. As you might remember from high school physics, when light passes through materials with different densities, it refracts. PicFog - real-time image search.