Waves form when a fast layer moves over a slower layer, dragging the top along to create a curled shape.
If the differences between the air speed and the temperatures of the layers of the atmosphere are small they will glide over each other. If the difference between the layers is too great then where the layers meet turns into turbulence. When the atmospheric conditions between the two layers are just right, waves form. The example in the photo is one where the air between the two atmospheric layers is close to the threshold for turbulence.