Why do clouds sometimes form a ring around the Sun?
This optical phenomenon is called a 22° halo which is a subset of other halos.  This arises from sunlight refracting through hexagonal ice crystals, which can be found in high level cirrus clouds.  Light that would otherwise not make it to your eye enters an ice crystal and then exits at an angle of approximately 22 degrees.  This produces the arc of light you see in the video.  You see (inverted) rainbow coloring of the halo because the light is not uniformly refracted but varies from 21.7 degrees for red photons to 22.5 degrees for violet photons.  


Image by donalbein, Wikemedia Commons, CC-By-SA-2.5 https://commons.wikimedia.org/wiki/File:Path_of_rays_in_a_hexagonal_prism.png

There are many more optical phenomenon you can see, but all are based around sunlight and the optical properties of ice crystals and water droplets.  Here is a phenomenal picture taken in winter showing many examples: