Sforno on Genesis 9:13/ספורנו על בראשית ט׳:י״ג

1את קשתי נתתי בענן, I have arranged that the rainbow become a natural phenomenon.

2והיתה לאות ברית, there is a dual function to the rainbow; it is true that the scientists have already tired of trying to explain the phenomenon of the various colours of the second rainbow, something which appears in the opposite order of the colours in the original rainbow, [according to the encyclopedia Britannica, there are primary and secondary rainbows, the secondary rainbow having a colour pattern in a reverse order of that known as the primary rainbow. Our author may understand that which has baffled scientists in his time, as a hint of the dual nature of the rainbow, i.e. as a warning, or a promise respectively. Ed.] According to the view of the sages in Ketuvot 77 the very appearance of the rainbow is a reminder that the generation in which it appears is a guilty generation. It is reported there that two generations were fortunate enough that in their time no rainbow was ever observed. This was interpreted as a sign that the people of that generation did not need the phenomenon of the rainbow to alert them to become penitents. Their religious leaders did an outstanding job in prevailing on the people to be Torah-observant. [according to the של'ה, it is not reasonable to assume that there were periods when the laws of nature were suspended and there was no rainbow; rather if the Jewish people during certain periods were very deserving, had lots of merits, so that they did not need to be reminded of the deluge, the rain descended at night when no rainbow would become visible even if they had not been a deserving generation. Ed.]