The skies above the traditional Campaign World are filled with more than just clouds.
There are powerful mages who have lifted their castles up into the air and Cloud Giants that have always been part of Grandfather's stories. Creatures of the air like Pegasi, Ki-rin and Dragons live somewhere "up there" and for the most part people living normal lives on the ground ignore the the details. If anyone considers what a population is like in the sky they disparage the idea with "how smart can they be to live that close to the sun? Their brains must be fried." In fact this attitude is mirrored by those that live in the clouds. "Life on the surface? Don't be silly, all of the air is up here; what would they breathe?"
The truth lies between the extremes of course. There is a full society living up in the clouds that for the most part ignores those living on the surface just like those on the surface ignores those in the sky. There is a small group of people call Sky Captains that control most of the access between the two worlds.
Working notes on what the players know so far:
From Norjak: Sky Captain Jack Harkness of the Javelin 5# of metal per 100# of lift The Javelin can carry up to 200# of metal (ie total lift of 4,000#)
There is a "Gemmae Mobilis Device" that is the extra installation that allows the vessel to use three dimensions. That Device consumes gems to accelerate or ascend. Using wind power alone or descent does not use "fuel".