A rainbow, blurred
When a rainbow is caused not by a shower of rain, like it usually is, but by the fine droplets of a cloud it is known as a cloud bow. Unlike its colourful cousin, who gets all the glory, the arc of a cloud bow shows little more than very pale, insipid hues. Many cloud bows are completely white or show just the faintest red tinge to their outer edges, like this one spotted by Iestyn Marc Lewis from Mount Wellington, Tasmania, Australia.
A cloud bow owes its ghostly appearance to the tiny size of the cloud’s droplets. At less than 1/100th of a millimetre across, these are far smaller than the raindrops that cause the vivid colours of rainbows. While they do, just like raindrops, separate the sunlight into a spectrum as it shines through them and reflects back off their inner surfaces, the tiny cloud droplets scatter the sunlight much more. This effect, known as diffraction, tends to blur the bow’s colours back together again. As light passes around the droplets of a cloud it bends slightly. This bending, or diffraction, effect is more pronounced when the droplets are small enough to be comparable to the wavelengths of the light. The bow lacks colours because a spectrum of sunlight blurred back together appears to our eyes as white.
This cloud bow also has a dark band at its inner edge. This is the gap between the primary bow and the cloud-bow equivalent of those fine fringes of colour that sometimes appear at the edges of rainbows. Known as supernumerary bows, these subtle fringes at the edges of rainbow are also caused by the diffraction of sunlight, and they appear broader and more pronounced in cloud bows.
Since a cloud bow only appears when uninterrupted sunlight can shine onto cloud from directly behind the viewer, this optical effect is seen most often when looking down onto cloud. A peak like Mount Wellington is a great vantage point from which to see one. A cloud bow, like a rainbow, exists only in the eye of whoever sees it. The scattered sunlight haunts the icy winds of the mountain summit, waiting for someone like Iestyn Marc Lewis to climb up and spot it.
A cloud bow spotted in orographic Stratus cloud by Iestyn Marc Lewis over Mount Wellington, Tasmania, Australia.