What kind of world is so uninhabitable, that humanity had to evacuate? What kind of catastrophe could occur, that wouldn't effect the upper atmosphere?
Could humanity survive in the clouds?
Yes and no.
Humans are natural pirates. The Surface was a lost cause. Everyone knew this. Every fort, bastion, castle and impressive wall collapsed and fell under the weight of the oncoming tide. The Grin. We couldn't stop it. So now it owns the surface.
We couldn't escape to Space, so we are stuck. Purgatory. Limbo. Between a rock and a hard place, or in this case, a dead rock and dead vacuum.
So humans turned to their natural piratey ways. With limited resources, and airships capable of blowing up at the smallest flame, rules were.... Encouraged. If you didn't follow the rules, you were fair game. Which means no one had to follow your rules.
So we raid and plunder Cityships. Then we nest in our own, called Malenar. It used to mean something else in Old German, but no one remembers that anymore.
No one aims for the balloon. If you pop it, all the resources are lost permanently. Surface salvaging is never worth the effort. Never worth the loss of life.
So we aim for the wooden bits, we use weapons that won't burn but will shatter.
But our most important asset are the Riggers. Only the big Mothers can carry biplanes, but a ship our size has something similar. We attached some wire harnesses to a couple brave idiots, then throw them off the side of the ship.
It's so stupid it works. Most of the time. If the wire snaps, you have a very limited time to be saved or splat. Less then a minute in most cases. One guy managed to fall for about 90 minutes but he was impossibly high.
When falling, every one knows the last thing to go through your mind is your feet.