The rain coming only with the Flood would explain how a large rainbow would not have been a key component of everyday life prior to the flood since it would only exist as small mist refractions. That a postdiluvian world which was dryer, with clear skies and water collected in rivers or other bodies would generate large rainbows for all to see.

For the longest time the idea of waters in the deep of the earth that could do this was a laughable topic but recently massive water bodies have recently been discovered deep beneath the Earth’s crust, specifically within the mantle. These “reservoirs,” sometimes called “hidden oceans,” are not liquid water as typically imagined, but are instead vast quantities of water trapped within the crystal structure of mantle minerals such as ringwoodite and wadsleyite. The transition zone, located roughly 250–400 miles (410–660 kilometers) below the surface, is estimated to hold an amount of water comparable to several times the volume of all Earth’s surface oceans.

The Midrashic writings in many places also allude to the world being cracked open and remade. Even [2nd Peter 3:5-7] makes that connection clear.

The point being that the flood destroyed the layout and surface of the earth, moving lands, reforming spaces and that it will do it again at the second Cataclysm of Fire where the earth will be melted to its elements with a great noise [2 Peter 3:10–13] and a new Earth and new Heavens will be formed after the 1,000 year reign of Messiah.

The two Cataclysms of Water and Fire were written about in the Testaments of the Patriarchs and Testament of Adam that are alluded to by Josephus:

And that their inventions might not be lost before they were sufficiently known, upon Adam’s prediction that the world was to be destroyed at one time by the force of fire, and at another time by the violence and quantity of water, they made two pillars, the one of brick, the other of stone: they inscribed their discoveries on them both, 71that in case the pillar of brick should be destroyed by the flood, the pillar of stone might remain, and exhibit those discoveries to mankind; and also inform them that there was another pillar of brick erected by them. Now this remains in the land of Siriad to this day.

 

-The Antiquities of the Jews, 1.70-71

 This is another example of a prerequisite understanding the writers of the Tanakh and New Covenant presumed the readers had since no where in the Tanakh does it explicitly state there are two destructions, one by water and one by fire. 

In Summary

In Summary the point of all this is that not everything in the Tanakh and the New Covenant writings explicitly spell out the events they cover but allude to aspects of it throughout the writings to the finer details and presume that the reader knows things that were covered in testaments, books and other materials that were written earlier that we may have today, might have pseudepigraphical retellings of or might have lost all together.

The small string of the Antediluvian Mist that is one and a half verses in total can lead to deeper and broader understandings when explored in context of the biblical literature and the parabiblical writings from a second temple Semitic understanding.