A "cloud on the horizon" means that something bad is about to happen. Meanwhile, someone with their "head in the clouds" is thoroughly out to lunch. As Gavin Pretor-Pinney points out in today's talk, clouds get a bad rep when it comes to language.
"But I think they're beautiful, don't you?" he says. "It's just that their beauty is missed because they're so omnipresent, so commonplace that people don't notice them … unless they get in the way of the sun."