TRIPLE RAINBOWS EXIST, PHOTO EVIDENCE SHOWS

Single rainbows are inspiring, double rainbows are rare, but tertiary rainbows have been elusive until a meteorologist provided guidelines that showed how to find them. Few people have ever claimed to see three rainbows arcing through the sky at once. In fact, scientific reports of these tertiary rainbows were so rare that until now many scientists believed sightings were as fanciful as Leprechaun's gold at a rainbow's end. These legendary optical rarities have finally been confirmed, thanks to photographic perseverance and a new meteorological model.

In addition to the confirmed photo of a tertiary rainbow, the optical treasure hunt went one step further, as revealed in another photo that shows the shimmering trace of a fourth (quaternary) rainbow.

Raymond Lee, a professor of meteorology at the U.S. Naval Academy, did not snap those pictures, but he did make them possible. One year ago, Lee predicted how tertiary rainbows might appear and challenged rainbow chasers to find them.

Although staggeringly rare, tertiary and quaternary rainbows are natural products of the combination of refraction, dispersion, and reflection inside raindrops. These are the same processes that create all rainbows, yet they are taken to their most extreme to produce these higher order variants. Refraction is when sunlight bends as it moves from air into water and vice versa. Water droplets bend each of the colors in sunlight by a slightly different angle. This is called dispersion, and it separates the colors to create a rainbow.

Most of that multicolored light passes through the other side of the raindrop, but some is reflected. The raindrop's spherical curves concentrate those reflections at 138 degrees from the Sun. This concentrated light is bright enough to create a visible primary rainbow.

A double rainbow occurs because not all that light exits the raindrop. Some is reflected back into the raindrop and goes through the whole process again. Although this light is dimmer, sometimes it is bright enough to produce a secondary rainbow just outside the first.

A third series of reflections creates a tertiary rainbow. It is even dimmer than the secondary rainbow, and much harder to find because instead of forming away from the Sun, a tertiary rainbow forms around the Sun. To see it, observers have to look into the Sun's glare.

This may be why only five scientifically knowledgeable observers had described tertiary rainbows during the past 250 years.

Lee reviewed each description. He eliminated one questionable account and found common elements in the others. All described tertiary rainbows that appeared for a few seconds against a dark background of clouds about 40 degrees from a brightly shining sun.

Along with colleague Philip Laven, Lee used a mathematical model to predict what conditions might produce visible tertiaries. First, they needed dark thunderclouds and either a heavy downpour or a rainstorm with nearly uniformly sized droplets. Under these conditions, if the Sun broke through the clouds, it could project a tertiary rainbow against the dark clouds nearby. The contrasting colors would make the dim tertiary visible.

Thanks to: sciencedaily.com