I normally see the word mercurial in the idiom mercurial rise. A mercurial rise is something I never saw as fickle or volatile. However heard of a mercurial fall, even though it seems just as possible with that definition. Either way the definition makes more sense than just a surging mercury in a thermometer I guess. Can't help but wonder if the person who first coined the phrase mercurial rise didn't grasp the meaning of the word.
We are what we repeatedly do - Aristotle