Optical phenomenon — ice crystals
Circumzenithal Arc
The 'upside-down rainbow' overhead
The circumzenithal arc is a brilliant, smile-shaped band of colour that curves around a point near the zenith, high overhead, its bow opening away from the sun. It forms when sunlight enters the flat top face of horizontally-floating hexagonal plate ice crystals and leaves through a side face, refracting through an effective 90° prism. This disperses the light into unusually pure colours, with red on the outer edge and violet on the inner.
Worth knowing
- The circumzenithal arc switches off when the sun climbs above about 32°: at higher angles the refracted light exits the bottom face of the crystals instead, so the arc cannot form. Its colours are notably purer than a rainbow's because the colour bands overlap far less.
Note
Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.