An interesting tidbit from the Seforno on Ber’ 9:13 “vehaysa le’os beris”:
והיתה לאות ברית: בהיות הקשת כפולה. כי אמנם נלאו חכמי המחקר לתת טעם לסדר צבעי הקשת השנית אשר הוא על הפך סדר צבעי הקשת הראשונה המורגלת. והיא תהיה אות לצדיקי הדור שדורם חייב. כאמרם כלום נראתה הקשת מימיך. ויתפללו ויוכיחו וילמדו דעת את העם:
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.