Genesis 9:12-15. Noah & The Sign of the Covenant.
God created the rainbow as a sign of the covenant He made with Noah and every living creature for all generations. This rainbow is a reminder to God Himself that He will never again destroy the earth by flood. God, when we see the rainbow, we will honor You and thank You for the covenant You have made.
"God said, “This is the sign of the covenant which I am making between Me and you and every living creature that is with you, for all successive generations; I set My bow in the cloud, and it shall be for a sign of a covenant between Me and the earth. It shall come about, when I bring a cloud over the earth, that the bow will be seen in the cloud, and I will remember My covenant, which is between Me and you and every living creature of all flesh; and never again shall the water become a flood to destroy all flesh."
Genesis 9:12-15 New American Standard Bible (NASB)