Which is why there's no "Waterworld 2"By Rabbi Jack Abramowitz
G-d forged a pact with Noach and his descendants, as well as with the animals and birds of the world. He committed not to strike the world with a universal flood ever again. As a sign of this commitment, G-d placed His rainbow in the clouds. The rainbow serves as a reminder that, while He might get angry with us, He will never again flood the entire world.
When we say a rainbow, we recite a blessing that G-d remembers this pact and fulfills His word. It is not our practice, however, to stare at a rainbow or to point it out to others, as it is a sign of G-d’s displeasure with us (i.e., that we would otherwise deserve to be destroyed).