When you heat the gases inside a hot air balloon it makes more sense (for myself at least) to say that it rises due to the velocity and kinetic energy of the molecules in their relation to the gases outside. Outside the balloon the molecules are obviously cooler, more dense, and moving slower (less kinetic energy) than inside. The way I imagine it is to think of how a bullet is less affected by gravity until it loses it's energy over time. The more energy in the bullets motion, the less gravity can pull it down in relation to someone throwing the bullet... or maybe just a weaker caliber :tongue2:.... So with all these molecules being heated and moving so quickly inside the balloon, outside of it there is far less motion and energy, but the gravity remains a constant, leaving the balloon to essentially be pushed upwards by the 'pile up' of slower molecules forming a fluid like cushion below. Once higher into the atmosphere it levels out due to there being fewer molecules outside the hot air balloon. Making the heated molecules motion inside of the balloon less significant concerning the constant pull of gravity... given that there are, at greater heights, fewer molecules outside the balloon (to my knowledge) to present that 'pushing upwards by settling below' effect on the balloon at those higher altitudes. So, regardless of the outside molecules being significantly cooler, the balloon will eventually reach the point where the gases inside it can't be heated enough to override that difference. It also seems like this logic makes sense with gases (or liquids) in their natural state being trapped in basic rubber balloons. They just don't require the manually applied energy. Lastly, I apologize if this is sort of confusing or unclear. I've never actually taken a physics class. High School or College. I've taken some college chemistry at a tech school, however nothing ever goes into great detail.