I'm quite sure there WAS rain before the Flood. Noah never asked God, "What's rain?"
In addition, water evaporates, even when it's not very warm. It then condenses when it gets cooler and falls. There is no reason to think things were different before the Flood.
There is a clue after the Flood when God puts His rainbow in the clouds. It is the 'in the clouds.' This clue fits in with another which is not translated correctly in any of our modern versions. God states that "As long as the earth endures, seedtime and harvest, cold and heat, summer and winter shall not cease by day or night." This is from the ancient Alexandrian Septuagint, translated about 300 years before Christ. Thus, this is the way they understood this passage. If this is correct, what it is saying is that just as day and night have not ceased up to this point, now something new will not cease as well -- the seasons.
This is a clear indication that there was an axis tilt of the earth during the Flood. Physically, that makes sense, but that is another post. An axis tilt means the earth now has poles which get very cold for part of each year, and this not only indicates seasons will now occur, but this sets off the jet streams which will carry the clouds over land during the day, for daytime rain. It is that daytime rain which was not in existence before the Flood.
There is no reason to suspect that water did not evaporate from the seas, condense into clouds, and then fall when the night brought the coolness. But these clouds would have moved very little from where they formed. Thus rain over the seas during the night would have been quite normal, but would never have shown a rainbow in the clouds. There may have also been some showers over land, but they also would have been after sunset when the air cooled.
After the Flood, however, with the axis tilt, not only seasons were initiated, but wind currents would begin driving clouds over the land and rain during the day would have been started. This would have been a new and VERY scary thing to those first people off the Ark. So the Promise. It is not going to be a repeat of what you just went through. That won't happen again. Here is proof -- the rainbow in the clouds. That means the clouds are breaking up during the day and showing the breaking up of the sunlight as rainbows.
There is a possibility of a vapor canopy having existed which would have protected us from radiation. But if that were all there was to the long ages, it would not explain the very SUDDEN drop in ages not only after the Flood, but at the time of Peleg as well. There was about a 50% drop in life expectancy after the Flood, and then another 50% drop after Peleg, followed by a slow decline to about 120 years maximum, which is what we see today.
The one thing which can to something like this is exposure to radiation.
It is standard geological knowledge today that, regardless of which model you choose to work with, there were no radioactive elements in the crust of the earth to begin with. They were buried deep into or even under the mantle.
But they were decaying, causing enormous heat. In Genesis 2 we read that water was seeping up from the ground, watering the earth. Water does not move up on its own. It moves down. Up means pressure and pressure means heat. We read in Genesis 7:11 that ALL the great fountains exploded at once, and it is this which probably precipitated the most incredible rain the world has ever experienced. Ancient legends from other cultures say this rain was hot, and that would fit with the idea of exploding waters which had finally reached a critical point of heat and pressure (very probably triggered by a series of asteroid impacts). This exploding water would have carried up with it massive amounts of pulverized debris -- carring with it the first radioactive materials to reach the surface of the earth. Eight people on the Ark -- they were all exposed by the gamma radiation, for the Ark was made of wood and while wood will stop Alpha and Beta radiation, it will not stop Gamma radiation -- the deadliest of all.
If there were some kind of genetic mutation along some kind of hot spot (an area of the genetic material which is very easily mutated), or several, then we have the possible reason why ages decreased so suddenly after the Flood -- within one generation.
We see the same effect during Peleg's catastrophe, when the continents divided. This would not have been a quiet event. When the radio decay in the interior of the earth had driven out so much water, which had exploded, initiated the Deluge of Noah, the radio decay did not stop. Gradually the very rocks themselves were melting. When rocks melt, they gain 10% in volume, or, in a sphere, 18% circumference.
The Atlantic Ocean is 4,700 miles across near the equator -- and that is 18% of the earth's circumference. During the time of Peleg another series of asteroid hits (we still can see their craters stretching from Europe across to the Caribbean), triggered a massive 'unzipping' of the earth along the Atlantic Rift. This continued for one or two hundred years. The people who survived would have again been exposed to new amounts of radioactivity from the upwelling materials.
Again the lifespans drop, just as God promised in Genesis 6.
Job is probably Jobab, the thirteenth son of Joktan, Peleg's brother. If you look at the book of Job, you will find references to massive wind and firestorms, mountains being torn up, the ice age which followed, volcanism, the cave men, and watching the sea. Why watch the sea? Because the unzipping would have produced enormous tsunami action and the waters pull back before the tsunami hits, exposing the seabed, which is also mentioned in Job.
At any rate, that is one probable explanation for the change in lifespans of the human race.