ANSWER: The scripture that mentions no rain before the flood is found in Genesis 2.
When the Lord God made the universe, 5 there were no plants on the earth and no seeds had sprouted, because he had not sent any rain . . . 6 but water would come up from beneath the surface and water the ground. (verses 5 - 6)
According to some, this condition 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 rain and this condition was only short-lived. On the other hand, there are other factors to consider that lead one to believe there were no showers before Noah's flood.
The timing of the formation of rainbows offers evidence that it did not rain on the earth until after the deluge.
12 As a sign of this everlasting covenant which I am making with you and with all living beings, 13 I am putting my bow (rainbow) in the clouds. It will be the sign of my covenant with the world. 14 Whenever I cover the sky with clouds and the rainbow appears, 15 I will remember my promise to you . . .
If it had not rained until after the deluge then rainbows "in the cloud," would be a new phenomena. They would not have existed pre-deluge 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 made right after the deluge.
The earth before the flood 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 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 cause unequal regions of hot and cold on the globe which produce the various winds. The pre-flood earth was warmer, had more vegetation, had smaller oceans, and had smaller mountains compared to today.
The atmosphere of the earth, before the flood, had less particles for rain to condense around than what we find today. Particles called condensation nuclei are absolutely essential to the formation of rain drops.
"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). Lewis Publishers - CRC Press. Washington DC. page 267)
If the earth before the flood had smaller oceans, 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. After the waters receded, the change in weather patterns, the formation of condensation nuclei from oceans, increased volcanic activity and the increased wind systems would have produced rain.
The lack of rain, however, did not mean the world before the flood was dry. Since the earth 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 drop in temperature would have caused a very large condensation cycle. This would mean the planet would have been watered, not by water drops falling from the sky, but by a very heavy mist or fog each day (possibly at sunset and sunrise).