https://twitter.com/gunsnrosesgirl3/status/1752203259576488390?t=sK-znCYE__KHTrZ46wds0Q

Interesting

Indeed.  And we know that God sent the rainbow as a sign that He would not destroy the world by Flood anymore.

So, we can presume that something changed at the Flood to cause rainbows, which presumably didn't exist before the Flood?

As to what, who knows?  If one believes the scientific explanation that rainbows are caused by raindrops or moisture in the atmosphere, perhaps the overall humidity of the earth's atmosphere increased after the Flood, since there was definitely a net increase of water on earth.  Or perhaps it has something to do with how the sun shines from within the firmament onto the earth and rainbows are actually caused by some kind of prism effect through the firmament.

Yes, there is a mysterious statement in the beginning of Genesis that it didn't rain when God created the earth, but rather that there was a spring that came up out of the ground that watered the whole earth.

I suspect this changed with the flood, that the atmosphere began to work differently. The rainbow was probably something man had never seen before, otherwise it wouldn't have been much of a sign from God. But the rainbow indicated that now moisture was going to come from the sky, unlike before the flood.

This is what i've heard before.  Before the Flood, there was no rain, because the firmament had not opened.  Seems all the waters of the earth were below ground.  Springs would water the earth. 

I've also heard the temperature was near constant (some theologians speculate it was a "perfect" 72 degrees, year round). If there was no rain, there'd be no clouds, then the sun would always shine, which would make farming super easy and massively productive.  The entire earth would be like a tropical rainforest, with no winter, or "off season".  Sounds like paradise.

Then, the flood started with rain.  First time in history.  The firmament being opened and earthquakes causing the 'waters of the deep' to gush forth. 

Seems like before the Flood, with the Firmament being closed, the earth's temperature/weather were in a small, contained "closed system" which reduced fluctuations.  After the Flood, with the Firmament open, this caused the entire system to change, and become larger.  Instead of water only having to travel from below the ground to a few hundred feet in the air in springs, now it had to travel from below ground, all the way up into the atmosphere, 1000s of miles above.

Then, one wonders if the opening of the firmament introduced the colder air, which caused summer, winter, etc.  Further, it's been proven that pre-Flood, the land mass of earth was all connected together.  After the flood, the major continents were formed, due to the massive earthquakes, tsunamis and water that pushed the lands 1,000s of miles apart.  This separation of land would also help cause the seasons because the sun would necessarily have to travel further to give it's light/heat everywhere.  I'll bet that before the Flood, the sun's movement was more fixed, or operated different entirely, because all the land was centralized.

The Flood definitely caused massive changes to all aspects of earthly life.  You could probably write an encyclopedia full of changes, just as the changes from Adam/Eve in the Garden to post-Original Sin were innumerable.

Indeed.  And we know that God sent the rainbow as a sign that He would not destroy the world by Flood anymore.

So, we can presume that something changed at the Flood to cause rainbows, which presumably didn't exist before the Flood?

As to what, who knows?  If one believes the scientific explanation that rainbows are caused by raindrops or moisture in the atmosphere, perhaps the overall humidity of the earth's atmosphere increased after the Flood, since there was definitely a net increase of water on earth.  Or perhaps it has something to do with how the sun shines from within the firmament onto the earth and rainbows are actually caused by some kind of prism effect through the firmament.

I don't think rainbows nor any non rainbow ring of light need a prism effect of the firmament to form. I think the physics of light can accomplish such an effect just from water droplets or ice particles.