Supernumerary rainbows are faint bow lines that appear just inside the main rainbow.
Look at the rainbow above and you may just be able to make out some of the normal colours. From top to bottom: red, orange, yellow, green, blue, indigo and violet. (Richard Of York Gave Battle In Vain – or however you remember it.)
At the bottom, just below the violet, there are some fainter lines with little colour. These are the supernumerary bows.
They are caused by interference between the different wavelengths as the light travels towards our eyes. They are so sensitive to changes in raindrop size that you will see them fluctuating from one moment to the next and even from one side of the rainbow to the other in the same moment.
Every colour in a rainbow is telling us something about the rain. A deep red colour means that the rain is especially heavy. “Lots of red, means a wet head.”
Supernumerary bows mean the opposite to a deep red at the top: the raindrops are very small. For this reason it is very unlikely you will see the two together. Look again at the bow above and you’ll spot that there is a good orange colour at the top, but not much true red.
You will have seen that there is a ‘secondary rainbow’ above the main one. Secondary bows are quite common and always:
- Appear fainter than the main bow
- Have colours reversed, with red on the inside of the bow and violet on the outside.
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The Secret World of Weather – The Book