Pictured: Rare upside-down rainbow spotted in the UK
Rainbows are formed when sunlight is refracted in a raindrop.
But in a circumzenithal arc, the colours are in reverse order from a rainbow, with violet on the top and red at the bottom.
The arc usually vanishes quickly because the cirrus clouds containing the ice crystals shift their position.
Ice particles in high cirrus clouds occur all year round, but circumzenithal arcs are usually obscured by lower level clouds.
Circumzenithal arcs are so named as they go around the zenith – the point in the sky directly above the observer- rather than the sun.
(Pedantic man notes that rainbows actually refract the light twice)
More on circumzenithal arcs
h/t to Caroline