What is the interpretation of the bow in the cloud after the Flood? After the Flood abates, God reassures Noah that it was a one time event:

Genesis 9:13 (ESV)


  I have set my bow in the cloud, and it shall be a sign of the covenant between me and the earth.


The most obvious interpretation is that this is a rainbow, and that it was something new.  But rainbows are a physical consequence of having rain and sun.

God spells out the significance of the sight: the earth will not be flooded again.  But should we interpret this as an already-familiar sight being used as a reminder, or did He somehow change things then, and rainbows had not appeared before the Flood?  Or was it not actually a rainbow at all, but some other phenomenon with which we are no longer familiar?
Jesus spoke in parables that were full of very familiar imagery to his hearers, turning them into spiritual lessons.

The Lord commanded Abraham to circumcise all his male descendants, a custom which was already being practiced before the time of Abraham, but God made it a token of his covenant with Abraham.

Likewise, there's no reason to believe that rainbows, being a natural phenomenon, did not exist before the Flood.  But as a part of God's covenant with Noah, he gave them a new, special significance.