Here’s an interesting article:
Some Christians claim that there was no rain before the Flood. Many of them make this statement quite dogmatically as if it were obvious from a reading of the biblical text. However, a close examination of Scripture does not bear this out.
A Biblical Analysis
Proponents of the “no-rain” view refer to Genesis 2 to support their position. Genesis 2:5–6 states that “the LORD God had not caused it to rain upon the earth, and there was not a man to till the ground. But there went up a mist from the earth, and watered the whole face of the ground” (empashsis added).
From this passage, all that can really be said is that no rain had fallen up to that time—that is, prior to the creation of man. Remember, Genesis 2 is primarily a detailed recap of Day Six of Creation Week. The passage describes the environment before Adam was created. This mist may have been one of the primary methods that God used to hydrate the dry land He created on Day Three. Furthermore, while this mist was likely the watering source for that vegetation throughout the remainder of Creation Week, the text does not require it to be the only water source after Adam’s creation.
Some argue that this mist eliminated the need for rain until the time of the Flood. However, presence of the mist prior to Adam’s creation does not preclude the existence of or the need for rain after he was created.
Genesis 2:5–6 reveals that before the Sixth Day of Creation Week, God had watered the plants He made with a mist, but had not yet caused rain or created a man to till the ground. To demand that rain didn’t happen until after the Flood from this passage has no more logical support than to claim, from the passage, that no one farmed until after the Flood.
Read more here – http://www.answersingenesis.org/articles/2010/10/19/rain-before-flood#