An interesting tidbit from the Seforno on Ber’ 9:13 “vehaysa le’os beris”:
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And it will be as a covenantal sign: When the rainbow is double. The scientific experts grew tired of trying to give a ta’am for the order of the color of the secondary rainbow, which is the reverse of the order of the colors in the primary, usual, rainbow. It will be a sign to the righteous of the generation that their generation is guilty. (As when it says [Kesuvos 77b; about truly righteous Leviim] never seeing a rainbow in their entire lifetimes.) So that [the righteous] will pray, rebuke others, and teach the nation wisdom.
Seforno is saying that the rainbow that the sign of the covenant with Noach, which Chazal talk about being a symbol of Divine “Displeasure” with us, is not the usual rainbow, but the second of a doubled rainbow. The Seforno emphasizes the fact that the colors are reversed. A primary rainbow has red on the top, outer, curve, and violet on the bottom, inner, one.
A secondary rainbow is about it some distance — red on the inside curve (nearest the red of the primary) and violet on the outside.
(I took the picture from HowStuffWorks: Science, “How Rare Are Double Rainbows”. That page also has the scientific explanation that the Natural Philosophers of the Seforno’s day apparently despaired of finding.)
I don’t know why the Seforno mentions the reversed color sequence. Maybe he considers it a significant part of the symbol of Hashem’s promise not to destroy the world. But in any case, it solves a problem: On the one hand, we make the berakhah of Oseh Maaseh Bereishis when seeing a rainbow, which implies that rainbows are awe-inspiring examples of Creation and positive thing to see. On the other hand that are talmudic quotes like the one the Seforno mentions that says that seeing the rainbow is a sign that the world deserves destruction, that it’s only Hashem’s beris with Noach stopping Him from doing so.
According to Seforno, each is about a different kind of rainbow:
We make a berakhah whenever we see a rainbow. Which means that the berakhah must be on the usual, primary, rainbow.
A secondary rainbow is rare and therefore more exciting. (Ask Hungrybear9562, Paul Vasquez, whose excitement about seeing a “double rainbow” in Yosemite National Park become a viral video.) But based on what we just learned, this reaction is ironic. Seeing a the secondary of a rare double rainbow is a bad thing. Righteous generations that earned their right to exist wouldn’t see one.