Did rainbows exist before the Mabul?

Thee Ramban writes in this weeks parsha (Noach) that the simple understanding of the pasuk is that after the mabul Hashem created rainbows. However, the Ramban writes that this can't be true because the Greeks stated that a rainbow is caused by refraction of the light in the moist air and therefore it existed from the time of creation (before the mabul). Here again, the Ramban writes explicitly that he accepts the opinions of the Greek scientists and bases pshat in a pasuk based on that.

Here is the translation of the Ramban
Ramban Bereishis 9:12
"This is the sign of the covenant that I give" - It would seem from this sign that the rainbow which appears in the clouds is not part of the acts of creation, and only now did God create something new, to make a rainbow appear in the sky on a cloudy day… But we are compelled to believe the words of the Greeks, that the rainbow is a result of the sun's rays passing through moist air, for in any container of water that is placed before the sun, there can be seen something that resembles a rainbow. And when we look again at the wording of the verse, we will understand it thus. For it says, "I have set my rainbow in the cloud," and it did not say "I am setting it in the cloud," (in the present tense) as it said, "this is the sign of the covenant that I am giving." And the word "My rainbow" indicates the rainbow previously existed.