Although the spectra of drop radii in rain showers are broad, the supernumerary bows are caused by only those drops with radii of about 0.25 mm. The angle of minimum deviation, the rainbow angle, is a function of drop size, being large for big drops, owing to drop distortion, and large for small drops, owing to interference. Between these extremes, there is a minimum rainbow angle. The drops that cause it give rise to the supernumerary bows.
© 1983 Optical Society of America
Alistair B. Fraser, "Why can the supernumerary bows be seen in a rain shower?," J. Opt. Soc. Am. 73, 1626-1628 (1983)