In Genesis's account of Noah's Ark, God placed a rainbow in the clouds as the sign of the covenant. Genesis 9:13 describes the rainbow as the visible sign of the covenant between God, Noah, and every living creature after the Flood.
The covenant promises that a flood will not again destroy all life on earth. Genesis presents the rainbow as a reminder whenever clouds cover the earth and the bow appears in them.
The Hebrew word in the passage is also the ordinary word for a bow, creating a vivid image of a weapon set in the clouds. English translations commonly render the sign as “rainbow.”
The rainbow appears after Noah leaves the ark, so it is part of the post-Flood covenant scene rather than a construction detail or a method of finding the vessel.