The scripture that mentions no rain before the great flood is found in Genesis 2. It states, "When the Lord God made the universe, there were no plants on the earth and no seeds had sprouted, because he had not sent any rain . . . but water would come up from beneath the surface and water the ground" (Genesis 2:5 - 6).
According to some, this condition of no rain only lasted until shortly after the creation events mentioned in Genesis 1. If this is true, then this fog and mist began the hydrologic cycle, which eventually led to water falling from the sky. On the other hand, there are other factors to consider that lead one to believe there were no showers before the deluge. The timing of the formation of rainbows offers evidence that such an atmospheric event did not take place on the earth until after the deluge.
If this type of moisture did not exist until after the deluge, then rainbows "in the cloud" would be a new phenomenon. They would not have existed before the flood since they are created by the refraction of light passing through suspended water (water drops). It seems likely that God used the new phenomena known as rainbows as a 'reminder' to both him and man of the promise he made to not kill humans with water (Genesis 9:11 - 16).
Shorter year
Compared to today, the earth before the deluge was a much different place to live. For example, evidence points to the fact that the earth had a year consisting of 360 days as opposed to today's 365 days plus a fractional day. A comparison of several verses in Genesis (Genesis 7:24, 7:11, and 8:4) shows that five months consisted of 150 days. This would mean that the days before the flood were slightly longer than the present 24-hour days.
The earth before the flood, even without regular rain, was a lush tropical forest with large amounts of vegetation. Since the days were longer, the temperature was warmer. After the waters receded, the ice ages began and the earth's ecosystem changed.
Before God brought the waters and then allowed rain to fall, there would have been less wind and atmospheric turbulence. Winds are caused by temperature differences cause by the tilt of the earth's axis, mountain systems and ice caps. All of these caused unequal regions of hot and cold on the globe. The pre-Noah earth was warmer, had more vegetation, and smaller oceans than today.
The atmosphere
The atmosphere of the earth, before Noah, had less particles for rain to condense around than what we find today. Particles called condensation nuclei are essential to the formation of water droplets.
"Some atmospheric particles . . . formed by the evaporation of water from droplets of sea spray, are natural and even beneficial atmospheric constituents. Very small particles called condensation nuclei serve as bodies for atmospheric water vapor to condense upon . . . " (Manahan, Stanley E., Environmental Chemistry 7th edition).
If the earth before the flood had smaller oceans, and no rain, then the production of particles for water drops to condense around would be very small if not non-existent. Less atmospheric turbulence would also have led to few particles put into the air.
No lack of water!
After the waters receded, the change in weather patterns, the formation of condensation nuclei, increased volcanic activity and the increased wind systems would have produced rain. The lack of water dropping from clouds, however, did not mean the world before the great deluge was dry.
Since the earth, before the flood, would have been warmer and laden with moisture that was not condensed, there would have been an extremely efficient condensation cycle. Condensed water droplets would have been bigger and more commonplace. The earth's surface would have cooled very rapidly at night. This would mean the planet would have been watered by a very heavy mist or fog each day instead of rain.