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I recently received a lovely email from a reader who shared an intriguing photo and wondered about it. John is an oceanographer – who apparently looks up, as well as down – and enjoys learning about optical illusions. By now you’ve had a good look at the photo and I’m sure you are as impressed as I am.
The photo was not taken locally, but it is as rare as it is beautiful. The lovely sight is known as a supernumerary rainbow.
Supernumerary means multiple and, in this case, multiple arcs of colour. These rainbows are predominantly green, purple and pink and their numbers and spacing can change from minute to minute.
How does this happen?
It has everything to do with the size of the raindrops that are reflecting and refracting the light. In the case of a typical rainbow, there is some variation in size of raindrops; supernumerary rainbows only form when falling water droplets are all nearly the same size.
Slightly different ray paths through a raindrop yield slightly different path lengths and slightly larger exit angle. As a result, a set of bows become visible inside the primary rainbow.
When raindrops are less than 0.4 mm in diameter, the spacing of supernumeraries widens as the drops get smaller.
A rare sight and another lovely example of the awesome connection between science and nature’s beauty.
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Cindy Day is the chief meteorologist for SaltWire Network.