In the Genesis Flood story, the covenant sign God placed in the clouds was a rainbow.
After Noah left the ark, Genesis describes God making a covenant with Noah, his descendants, and every living creature. God promised that a flood would not again destroy all life on Earth. The rainbow in the clouds was presented as the sign of that covenant.
The passage gives the rainbow a theological meaning within the story: it serves as a reminder of the promise. It is not described as something created for the first time after the Flood. The text instead says that it would appear in the clouds and be seen as a sign of the covenant.
Genesis connects the promise with the whole Earth and with future generations. The rainbow therefore became one of the best-known images associated with Noah. In modern science, a rainbow is an optical phenomenon produced when light interacts with water droplets; that scientific explanation is separate from its symbolic role in Genesis.