Okay, I just looked it up, and I realize my mistake: I've lived on O'ahu for years, where we have no active volcanoes. "Vog," apparently, "volcanic fog," or a form of smog formed from volcanic vapors. I guess I should have known that after all the troubles on the Big Island this year.
I lived on Maui in the 80s but didn't know the word for this, but know exactly what it is.
I love how Duolingo incorporates aspects of the language's culture into their lessons.
I was going to post a cynical and sarcastic comment, but now I know that "vog" is a thing!!