When the sky is overcast, it has the quality of cloudiness. Your favorite days at the beach might be when cloudiness keeps everyone else away!
You can use this noun for the state of being overcast or foggy, like the cloudiness at the top of a mountain or the cloudiness that makes it impossible to watch a meteor shower. It's also good for something that's unclear or vague, whether it's water or ideas. So the cloudiness of a pond might keep you from diving in, and the cloudiness of a philosopher's writing may leave you more confused than before you opened the book.