I, as a non-native speaker of English, find it difficult all the time, despite the fact that I live in the UK. But there was this instance of a good piece of advice that I found in one of the textbooks I'd come across - it said that if you're speaking of an example of phenomenon, you often use an indefinite article with the noun referring to it, e.g.
We could see a clear blue sky - a student might ask why not the clear blue sky, since sky is unique and as such should be used with a definite article. Here goes the explanation - in this case, it is an example of the phenomenon called sky, and the sky (the phenomenon, not an example of it) was clear and blue. Use of adjectives often makes a noun go with an indefinite article when we first introduce or put an idea in context. It's just one mere example of such use, and, to be frank, the more I read, and even write my own books, I've noticed that it's become more natural or automatic, without prior reasoning. Obviosuly, there are cases where you don't normally use an indefinite article, even when used with adjectives describing a phenomenon.
