Split Rainbows

rainbow over a house against a dark sky. The arc starts normal at the horizon, but loses its green and blue further up, instead fading into white, a dark band, and then white again. Jan Curtis

Notice how the rainbow starts to split further up the arc, creating white bands where the two rainbows overlap but not perfectly. Splitting in a primary rainbow is refered to as a twinned rainbow if it is split two ways, or a multisplit rainbow if it is split more.

Split rainbows are very rare, so rare that researchers don't yet have enough evidence to conclude how they are formed. A commonly accepted theory is that they are formed by different shaped raindrops. In a twinned rainbow the lower is formed by large raindrops slightly flattened by air resistance, the upper formed by raindrops small enough to remain spherical. These drops could be from the same shower, or from different locations (as long as they are within the viewers rainbow cone).

The rainbow is influenced by the shape of the drop, thus the outer rainbow formed by the small spherical drops is a perfect circle, where as the inner rainbow formed by the distorted drops is a slightly flattened circle (wider than it is tall).

Twinned Rainbows - AtOpt
A multi-split rainbow over a body of water and mountains. At the center is a bright primary rainbow. Just below it is a fainter slightly wider rainbow. Right bove it is a fainter rainbow, its outer red edge just visible, converging into the red of the primary near the horizon. A multi-split rainbow from south-east China, August 12th, 2014

Here is a multisplit rainbow. The bottom rainbow can be explained by flattened drops, but the cause of the faint rainbow above the primary is debatable. One hypothesis is that it is caused be elongated raindrops, but elongated drops are not as stable as flattened or normal drops, making this explanation debatable. A multi-split rainbow from south-east China, August 12th, 2014

Note that the secondary rainbow is not affected by splitting, which is typical of split rainbows.Twinned Rainbows - AtOpt

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