Is it an idiom that clouds disperse?

Not an idiom. Idioms are fixed phrases formed by long-term use and tempering of language. It is a language unit with richer meaning and grammatical function equivalent to words, with rich and profound ideological connotation, short and pithy, and easy to remember and use. Often accompanied by feelings, there are derogatory meanings and justice, and of course there are neutrals. Clouds and clear skies are just words to describe the weather.