In the Noah story, a rainbow is the sign of God’s covenant after the flood.
Genesis 9:12–17 describes the rainbow as the sign of the covenant between God and Noah, his descendants, and every living creature. The covenant promises that a flood will not again destroy all life on earth. The rainbow is presented as appearing in the clouds and prompting remembrance of that promise.
The passage places the covenant after Noah leaves the ark and offers a sacrifice. It broadens the agreement beyond Noah’s family to include animals and the earth itself. This is why the rainbow’s meaning in the story is more specific than a general sign of good weather.
The rainbow is also a natural optical phenomenon caused by sunlight interacting with water droplets. Genesis uses that visible phenomenon as a theological symbol. The text does not identify an eclipse, comet, or fire as the covenant sign, though those images appear in other biblical scenes.