According to Genesis, God gave Noah a rainbow as the visible sign of the covenant. Genesis 9:13 says that God set the bow in the clouds as a reminder of the promise made after the Flood.
The covenant concerns the future of the earth and its living creatures. God promises that the waters will not again become a flood destroying all flesh. The rainbow therefore functions in the story as a public sign of divine commitment, not merely as a weather observation.
The rainbow appears after Noah leaves the Ark and builds an altar. This detail distinguishes it from the dove, which Noah used earlier to test whether the waters had receded. The passage also speaks of the covenant as involving every living creature, making the sign broader than a private agreement between God and Noah alone.