[...] hiding behind the thick, white fog that the mountain valley provided. [...] hiding amidst the thick, white fog that the mountain valley provided. [...] hiding amid the thick, white fog that the mountain valley provided. I thought of "amidst" because it just sounds better, but I don't know if it's correct.
"Behind" suggests that the person/thing is not actually in the fog, but the fog is between the looker and the look-ee(?). "Amidst" suggests the person/thing is inside of the fog which has obscured the view of the looker. My understanding of amidst vs amid is that they have very close to the same meaning. Amidst is older and isn't used as often (and maybe British English vs American English). I think it's nearly the same difference between "whilst" and "while". This is just off the top of my head so I'm sorry if I've given you bad advice. If you like amidst better than amid, use it. But be sure that it fits the tone of the narrator.
I think that I'll go with "amidst" after all, since it makes sense. ("Behind". Hm. I was thinking of a veil... but forgot to put it into words... I'm tired). I'm writing a Chinese tale in English, so I'm not sure yet with what I'm going for here, but it's somewhat formal, so I think that some old English words might fall into place. Good advise. Thanks.
Depending on what or who is hiding... I would let it read, [...] cloaked in the thick, white fog that the mountain valley provided. I never thought I'd develop a love affair with Thesaurus, but I'm guessing very shortly I'll be using it with reckless abandon.
"Amidst" suggests a crowd of similar things, while the fog is one entity. I'd go with "in" or "within".