I have an idea for my adventure, but I am unsure of if it fits the setting. What I am thinking of is a winter that is going on too long because of a volcano creating a year without summer. The Lowlanders think that it is caused by mountain folk who still have fresh medicinal herbs and spores they trade with. They think some how the mountain folk have stolen the summer when in reality they have an ancient building that they have turned into a hothouse/greenhouse. 

I was thinking this plays into the ecological themes and themes of desperation and forgotten knowledge. Regardless of what happens there will be a tragedy (famine), but with the players efforts they can blunt or prevent that tragedy. But I am unsure if that fits the Cloud Empress universe.

Connecting the ecological themes of Cloud Empress with the threat of an increasingly long and harsh winter definitely sounds cool. Perhaps even in some way caused by the chalk as a pollutant - directly or indirectly - or nature's defence against the polluters.

Having a worldwide yearlong winter would definitely sound like a campaign-defining feature to me, rather than a local adventure. So if that's not what you're going for you could try making it a more local meteorological phenomenon, perhaps with the threat of its expansion?

However you choose to go about it, I'm looking forward to see what comes out of it!