As a child, on bright sunny mornings, I would dreamily gaze through the air above the toaster, at the dancing sunflowers in the Van Gogh picture on the wall behind. This is called convection.
Light rays passing through such a mixture are slightly bent or distorted.
The cooler, heavier, blobs slow up the light more than the warmer, lighter, ones. So the beam is bent as it moves through regions of varying density. A similar effect is observed when a straight stick appears bent when held partly in water. However, the positions of the warm and cold blobs are ever changing, and so are their sizes. So we see the picture dance.