photo #5409311

What’s up with these rainbows? The peculiar triplet of bows appeared one afternoon over hills in Rogaland, Norway as Dave Roberts (Cloud Appreciation Society Member 54,479) was driving near the village of Jelsa. The thin, faint inner-most one is known as a supernumerary bow. It is an occasional, though not rare, effect formed by light diffracting around very small raindrops, and it appears as fine fringes of colour on the inner edge of a primary rainbow – the one that is here the inner of the two main bows. More unusual by far here is the outer bow. This is not the common secondary rainbow, whose arrangement of colours is the other way around and whose arc is concentric to the primary bow. This bow appeared to diverge from the primary rainbow at its upper end, and to identify it, you need to know what was directly behind Dave as he spotted it.Off to the west, where the Sun was lowering in the sky, was a large body of water called Boknafjord. The rare outer rainbow was a reflection bow, produced by sunlight illuminating the rain shower after first reflecting up from the surface of the calm fjord behind Dave. Since these reflected rays hit the raindrops from a very different angle to those coming directly from the Sun, the reflection bow was centred on a different point from the primary rainbow. For this reason, a reflection bow appears at a different angle to its related primary rainbow. The two always, however, meet at the horizon line (the level of the reflecting water). ‘What treasure,’ wondered Dave, ‘might one find at the foot of such an abundance of rainbows?’