I was just ‘tweeted’ by Anne who had spotted what initially appeared to be an unusual light phenomenon appearing in some cirrus clouds. I think it is just a small arc of a standard ‘primary rainbow’, but part of me desperately wanted it to be a ‘fire rainbow’ which I have never knowingly seen.
Fire rainbows (see photo) are very rare and form in cirrus ice crystals at high altitudes; their coloured arcs are near horizontal and parallel to the horizon. Fire rainbows can only come into being if the sun and atmospheric conditions meet very stringent criteria, another of nature’s beautiful balancing acts. There is more background to these extraordinary light shows on the National Geographic website.
As this website explains, fire rainbows can only occur when the sun is high enough in the sky (58 degrees or more) and sadly that rules them out from highish latitudes like the UK for most of the year.