Twinned or Triple Rainbows

Unlike a double rainbow which consists of two separate and concentric rainbow arcs, the very rare twinned rainbow appears as two rainbow arcs that split from a single base. The colors in the second bow, rather than reversing as in a double rainbow, appear in the same order as the primary rainbow. It is sometimes even observed in combination with a double rainbow. The explanation for a twinned rainbow is the combination of different sizes of water drops falling from the sky. Due to air resistance raindrops flatten as they fall and flattening is more prominent in larger water drops. When two rain showers with different sized raindrops combine they each produce slightly different rainbows which may combine and form a twinned rainbow.

How Strange Twinned Rainbows Form   Live Science - August 9, 2012
A twinned primary rainbow produced through computer simulation. The rainbow is split because of the interaction of light with two types of water drops: some smaller, spherical ones, and some larger water drops that become nonspherical. The different shapes cause light to leave the water drops in two different directions, which causes the rainbow to split into two arcs, a study presented in August 2012 found. Double rainbows had their fifteen minutes of fame on the Internet. Now get ready for their even more mysterious cousins: twinned rainbows.

New research has suggested an explanation for these exotic shows of color. Rainbows are known to form when sunlight interacts with tiny water drops in the atmosphere. As sunlight gets both reflected and refracted within the drops, it gets separated into its basic color components. Still, all the secrets of the more complex behavior of rainbows have long remained a puzzle. The most common rainbow has a single arc. The less common double rainbow, which consists of two separate, concentric arcs, has inspired Internet memes. Triple and quadruple rainbows have even been spotted. Even rarer, however, is the twinned rainbow, where two arcs split from a single base rainbow.

Secret of 'twinned rainbows' simulated on a computer   MSNBC - December 11, 2011

Researchers in San Diego say they've discovered how a rare "twinned rainbow" works and can now simulate one on a computer. By studying virtual rainbows, they figured out that the twinned rainbow needs sunlight to reflect off both small and large size water droplets at just the right angle. "A double rainbow everyone has seen," said Iman Sadeghi, a Google software engineer and former doctoral student at the University of California San Diego's Jacobs School of Engineering. Sadeghi says the image of any rainbow is dependent on the shape of the water droplets in the air, as well as the angle between the observer, the water droplets and the sun behind the observer.

Quadruple rainbow caught on film for the first time   BBC - October 6, 2011

The third (l) and fourth (r) rainbows appear closer to the sun, rather than opposite it as with the first and second, which would have been behind the photographer.
Scientists have captured the first image of a "quaternary" rainbow - the fourth rainbow caused by the bending of light through water in the air. This refraction frequently creates a visible second rainbow, but until now, no one had caught sight of the fainter third and fourth arcs that the process creates in a different part of the sky. The first tertiary, or third, rainbow has only just been caught on film. Digitally enhanced pictures of the two effects appear in Applied Optics.

September 20, 2010 - Photo by Daryl Pederson of Anchorage, Alaska

August 30, 2010 - Beluga Point South of Anchorage Alaska