Genesis 9:12-13

 "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.'"  Genesis 9:12-13

Now that God has made his pledge to Noah and his descendants and all the living land creatures, not to destroy the whole world by flood again, he is putting up a sign as a reminder.  Every time we see a rainbow, we know that God promised us.  And he has kept that promise - never since then has there been a global flood that wiped out all life on the land.