It looks like a rainbow, but the colourful spectacle is actually a rare weather phenomenon called a sundog. It is caused when the sun refracts through hexagonal-shaped ice crystals that have formed in clouds. Photographer Robert Arn, spotted the scene in Heyworth, Illinois.
that is not a Sun Dog… Sun dogs (or sundogs), mock suns or phantom suns, scientific name parhelia (singular parhelion), are an atmospheric phenomenon that consists of a pair of bright spots on either side on the Sun, often co-occurring with a luminous ring known as a 22° halo.
I believe this is what is called an ‘Iridescent Cloud’. Or maaaaybe the bottom half of a ‘Halo’. I’d guess it more likely the former though. But for sure NOT a Sundog.