And God said, 'This is the sign of the covenant I am making between 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.'
God even gave us a sign in the sky to help us to remember His promise. God knew that, over time, the covenant would be forgotten by man and so He puts a rainbow in the sky to remind us of the promise when the rain comes.