Rashi on Genesis
בענני ענן WHEN I BRING A CLOUD — When I have it in mind to bring darkness and destruction upon the world.
Sforno on Genesis
בענני ענן, a reminder that the rainbow will not be visible unless there are dark clouds in the sky after the atmosphere had been very moist.
Radak on Genesis
והיה בענני ענן, I will place clouds for a variety of reasons.