The coloured rings around your own shadow on a cloud, which follow you and nobody else — backscattered light from droplets by a surface wave that cannot be explained by geometric optics at all; seen from a mountain it is the Brocken spectre, and the ring size gives the droplet size.
Drawn with: a WebGL2 shader.
Reacts to and looks like: gpu, webgl2 and audio-reactive.
Family: Visualiser.
Runs in your browser. The preview above is the same program the renderer uses.