Twinned rainbow, Norway - OPOD

Last updated on September 16, 2023

Twinned Rainbow: A Captivating Atmospheric Phenomenon

Have you ever witnessed a rainbow splitting into two, only to disappear within seconds, leaving you wondering if it was real? Well, let me assure you, twinned rainbows are indeed real and have been captured in numerous photographs. One such mesmerizing occurrence was photographed by András Uhrin during an afternoon thunderstorm in Stavanger, Norway.

In the captivating image, we can clearly observe a twinned primary rainbow, where the colors of the rainbow appear to split into two distinct arcs. However, intriguingly, there is only a single secondary rainbow visible. This phenomenon has puzzled scientists and enthusiasts alike, as the traditional explanation for rainbows does not fully account for this unique occurrence.

One popular theory suggests that twinning may be caused by a combination of ice spheres and water droplets in the atmosphere. The hypothesis proposes that ice-generated bows would be slightly larger than those formed by water droplets. However, this theory raises questions about the transparency and spherical perfection of the ice spheres, as well as the absence of a split secondary rainbow.

An alternative explanation that holds greater plausibility is the presence of raindrops of different sizes. In stormy conditions, it is likely that two separate rain sheets or regions contain raindrops of varying sizes. Smaller raindrops tend to be spherical in shape, while larger raindrops become increasingly flattened due to air resistance as they descend from the clouds.

The surface tension forces acting on a raindrop, which tend to maintain its spherical shape, are relatively weaker compared to the distortion caused by air resistance in larger drops. As a result, a primary rainbow formed by slightly flattened drops appears flattened at its top. Intriguingly, ray tracing calculations reveal that the secondary rainbow remains unaffected by this flattening effect.

To create a twinned primary rainbow and a single secondary rainbow, we require two populations of raindrops, each nearly monosized. This, of course, seems improbable. However, even ordinary rainbows necessitate the presence of perfect drops and specific sunlight conditions to manifest their awe-inspiring beauty. The formation of twinned rainbows pushes the boundaries of atmospheric optics, presenting us with a delightful conundrum.

While twinned secondary rainbows have yet to be observed, the existence of twinned primary rainbows has been well-documented through photographs. Sometimes the twinning effect is subtly indistinct, while other instances, such as the one captured in Stavanger, Norway, offer a clear and vivid display.

As we continue to unravel the mysteries of atmospheric optics, the phenomenon of twinned rainbows serves as a reminder of the intricate and fascinating interactions between light, water droplets, and our atmosphere. Each occurrence presents an opportunity to deepen our understanding of the natural world and appreciate the awe-inspiring beauty that surrounds us.

So, the next time you find yourself gazing at a rainbow after a storm, keep an eye out for any signs of twinning. You never know when nature might surprise you with its captivating displays of optical wonder.

Twinned Rainbow ~ Photographed during an afternoon thunderstorm at Stavanger, Norway by András Uhrin. OPOD has slightly enhanced the images. ©András Uhrin, shown with permission

Colour subtraction enhancement shows a clear twinned primary but only one secondary bow

Large raindrops are increasingly flattened. Very large drops eventually break up into smaller ones.

A media and children's book raindrop

Usually during stormy conditions a rainbow will appear and then, perhaps just for a few seconds, split into two. Then it is all over and you wonder whether it was real. Twinned bows are real and by now there are many photographs. Sometimes the twinning is tantalizingly indistinct. At other times, like here, it is clear.

So far though we have never seen a twinned secondary rainbow. The enhancement at right shows obvious separation into two primary bows but only a single secondary. Single, but perhaps just a trifle widened?

A popular explanation of twinning was that a mixture of ice spheres and water droplets made the the bows. An ice bow would be slightly larger than a water generated bow. But the ice spheres would have to be improbably transparent and near perfect spheres, and why is the secondary not split?

A mixture of raindrops of two different sizes is much more likely to be responsible, either in the same rain sheet or in two separate ones. Small raindrops are spherical. Larger raindrops are increasingly flattened by air resistance as they fall. The surface tension forces pulling a raindrop into a sphere are proportionality weaker compared to air resistance distortion in large drops.

A primary rainbow from slightly flattened drops is itself flattened at its top – but – ray tracing calculations show that the secondary is not. Two populations of drops of different flattening will thus create a double primary but a single secondary.

We need two populations of drops and each almost monosized. That too is improbable. Yet, even ordinary rainbows need such perfect drops and sunlight conditions that they are a wonder.

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Reference Atmospheric Optics

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