The panorama above features a wonderful example of supernumerary bows just beneath the upper reaches of this high arching rainbow. It was taken from Richmond, Virginia, late in the day on October 16, 2014. Tiny water drops are responsible for forming these supernumeraries, unlike the larger drops that give the primary and secondary rainbows their color.
Sunlight is diffracted by the minute droplets in such a way to produce waves of constructive (colors) and destructive (darkened regions) interference. Because the supernumeraries shown here are so clearly visible, the droplets forming them must be of nearly uniform size.
(via the Earth Science Picture of the Day http://epod.usra.edu)