~ Images by Del Zane (Photography), Washington State.
Distinguish from iridescent clouds by the colour purity and saturation. And the reds are always uppermost.
Hexagonal plate crystals drifting with their large faces nearly horizontal generate the halo.
Sun rays enter a vertical side face, pass through the ice and leave through the lower horizontal facet.
The passage through what is effectively a 90° prism disperses the colours widely and with purity.
Summer is here, or close. The sun climbs higher and higher. We get more and more reports of rainbow coloured clouds.
Rainbows they are not. Despite the deceptive warming of the season, ice crystals in high cold cirrus haze and cloud form them. They are fragments of circumhorizon arcs, huge halos that - when allowed by extensive enough cloud -
light a bright spectral band parallel to the horizon and part of the way around the sky. When the necessary cirrus is in grudgingly small patches we see just fragments of the arc.
The sun must be high, higher than 58°. Relatively low latitudes are needed. Look beneath the sun around noon. Less fortunate parts of the world only glimpse them near the summer solstice. Higher latitudes still never see the solar CHA but the Moon with its tilted orbit might generate one.