This is not a "rainbow". It instead is a circumhorizon arc. Rainbows are caused by internal reflections in water droplets. Halos such as the circumhorizon arc portrayed in the question are caused by internal reflections in ice crystals.

This is not a "rainbow". Quoting from the linked site, "not all colored patches in the sky are rainbows". It instead is a circumhorizon arc. Rainbows are caused by internal by refraction and reflection in water droplets. Halos such as the circumhorizon arc portrayed in the question are caused by internal refraction and reflection in ice crystals.