Puzzling triple rainbow - Atoptics Highlights OPOD

Sighted by Terry Wooten at Winston-Salem, North Carolina with the sun 23.6° high.

There are two equally wide and coloured primary bows. Above them and almost as bright is a secondary bow with its colours reversed as is usual. All three arcs appear concentric in this iPhone image. Image ©Terry Wooten, shown with permission

Supernumerary bow

Light wave interference often produces coloured fringes inside a primary rainbow and rather more rarely outside the secondary.

The image�s inner arc does not have a supernumerary structure nor its colours. At left, Philip Laven has computed supernumeraries and compared them with the image. Attempts for many different drop sizes just do not fit.

Reflection from water

Water can reflect sunlight upwards to generate extra rainbows. The raindrops see two 'suns', one above and one below the horizon. The parking lot is wet but this double primary could not be generated from horizontal water - the geometry is all wrong. We would need many smooth wet roofs tilted within 2� of the sun elevation and all with the same precise azimuth angles. A search of Google Earth shows none. In any event, the reflections would also double the secondary bow.

"Twinned" bows

Sometimes, but rarely, we see a primary bow split into two. A violent shower seems to help.

The twinning most likely occurs when a rain shower has drops with two distinctly different sizes. Large drops are more flattened as they fall and these produce the lower �twin�. Small drops are nearly spherical and they produce the 'normal' upper arc of the twins. By a quirk of ray paths through the raindrops the secondary bow is not split.

Isn�t that what�s here?

Maybe, it is perhaps the least unlikely explanation. The double bow was transient and during "a short and heavy downpour". But the twins are not usually so distinct nor so well separated. Furthermore, the expected separation is greatest at the bow top whereas these bows appear equally spaced throughout. At right is a more typical example of twinning.

A primary rainbow forms when sunlight reflects once inside near spherical raindrops. Two internal reflections give an outer secondary rainbow. The geometry is fixed, only one primary and one secondary. Both are centred on a point directly opposite the sun. A doubled primary is an oddity. Here are some ways that two primaries could form - but not necessarily like those of the image.

"Two suns"

Two suns about 4� apart would give a doubled primary - But a doubled secondary too! We cannot invoke a bright cloud reflection as a second sun. It would not be intense nor point-like enough compared to the bright direct sunlight casting strong shadows on the grass.