So how does Schlieren photography make pictures like the above?
The phenomena you are seeing is the difference in air density. When 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. As light rays pass through a patch of air that is hotter than the surrounding air they get refracted and change their direction. Just like light does when it passes into or out of water. You can see this effect on hot days as the shimmering over roads in the distance.
We tend to think of the air as uniform because we can’t see the tiny changes all around us. But in reality the air is constantly moving and being disturbed by small heat sources. Even your body is causing the air to circulate upward around you.The Schlieren technique greatly magnifies this effect, making even tiny changes visible, such as the heat rising from human skin.