In technical terms, the relevant concept is Archimedes’ principle: the upward buoyant force on an object is equal to the weight of the fluid it displaces. This, in turn, is an immediate consequence of the deeper principle that is a bedrock of physics: the universe is a giant energy minimization machine. If the universe can lower its potential energy by displacing your hot air balloon upward to make room for heavier (colder) air, then it will do so. All you have to do is hold on.
In this way, “flight by flotation” is a simple, natural, and almost timeless idea. It feels clear and intuitive at every conceptual level.
We just found it impractical for some reason.