After the biblical flood, God placed a rainbow in the clouds as the sign of the covenant with Noah.
Genesis 9:12–17 presents the rainbow as a visible reminder of God’s promise never again to destroy all living beings with a flood. The covenant is made not only with Noah and his descendants but also with every living creature associated with the Ark.
The passage uses the rainbow as a theological sign rather than offering a scientific explanation of atmospheric optics. Rainbows are produced when sunlight is refracted, reflected, and dispersed by water droplets, but Genesis focuses on their covenant meaning within the story.
A common mix-up is to identify the dove or its olive leaf as the covenant sign. Those appear earlier, when Noah uses birds to test whether the waters have receded. The rainbow comes after Noah leaves the Ark and builds an altar. It is therefore the canonical answer to the question about the post-flood sign in the clouds.