A peculiar quadruple rainbow imaged by Yuan-Shu, Chang and Tien-Chu,Chang in Ilan, Taiwan. Just three minutes later (below) the fourth bow had disappeared but the third and mysterious bow remained.
©Yuan-Shu, Chang and Tien-Chu,Chang
Could they be reflection rainbows formed by upwards mirrored sunlight from smooth water?
When that happens, falling raindrops see two suns, one above the horizon and one below it, and thus form two separate sets of bows. Yet these Taiwan bows cannot come from reflections. They are always outside the primary - diagram below. They meet at the horizon. When the sun is away from the horizon as here, the reflection bow is angled sharply away from the ordinary primary. Only two immense mirrors tilted so that sunlight almost grazed their surfaces could have manufactured the Taiwan bows.
A clue to their origin is that there are no extra colour reversed secondary bows visible even when the images are severely enhanced. That suggests twinned bows. Sometimes, during a very violent and stormy shower, the primary splits into two but the secondary stays single. Two separate populations of drops with different amounts of flattening make the twins. The different light paths generating the secondary are much less sensitive to the drops’ oblateness and that bow is unsplit.
|A twinned bow is usually inside the primary but needs very special conditions to form. Ray tracing by a HaloSim modification.|
|A reflection rainbow would not be inside the primary|