The circumzenithal arc, oft mistaken for a rainbow, can be ephemeral. Always close to the zenith, the ice crystal cirrus clouds forming it scud across quickly, brightening and then dimming the arc in seconds.
It has the purest and brightest colours (with its matching circumhorizon arc) of all the halos.
Plate crystals generating it are aerodynamic drag aligned with their large hexagonal faces usually within a degree or better of horizontal. Nearly parallel sun rays stream into the upper face and leave through a vertical side face. Nature creates an almost ideal laboratory spectrum demonstration where collimated light shines through a clamped 90° prism. Have dark Fraunhofer lines ever been observed in a CZA?