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?