And God said, “This is the sign of the covenant I am making between me and you and every living creature with you, a covenant for all generations to come: I have set my rainbow in the clouds, and it will be the sign of the covenant between me and the earth. Whenever I bring clouds over the earth and the rainbow appears in the clouds, I will remember my covenant between me and you and all living creatures of every kind. Never again will the waters become a flood to destroy all life. Whenever the rainbow appears in the clouds, I will see it and remember the everlasting covenant between God and all living creatures of every kind on the earth.” Genesis 9:12-16

According to Genesis and the way God introduces it, the rainbow appears to be a new concept.

Rainbows are multi-colored reflections of the sun rays through water condensation in the sky. Many suggest that before Noah it never rained. That means plants were watered from the ground up, not the sky down. When Noah’s flood strikes, the passage reads “the fountains of the great deep were opened.” 
Also, in a perfect world as described in Eden, there are no catastrophic weather incidents like floods, tornadoes, hurricanes caused by the abundance of precipitation in the sky. If water flowed freely under the earth, then more water existed underground than in the sky.

The earth existed in a perfect environmental terrarium, with an equal balance of light and moisture. It could explain why people lived so long. Then, after the flood, the water balance shifted more to the sky causing precipitation events like rainbows.