Why does a hot air balloon rise when nothing is pushing it up?
Explored it? Here's the recap
Why does a hot air balloon rise when nothing is pushing it up?
The short answer
Hot air rises because heating air makes its tiny bits (molecules) spread farther apart, so the same amount of space holds fewer of them and weighs less than the cooler air around it. Anything lighter than the surrounding air floats up, which is why a balloon full of hot air lifts off.
How it works
Air is made of countless tiny molecules bouncing around. When you heat the air inside a balloon, the molecules move faster and push farther apart, and because the balloon stays the same size some are pushed out the open bottom. Now the balloon holds fewer molecules than the same-sized scoop of cooler outside air, so it weighs less than that surrounding air. The cooler, heavier air pushes up underneath the lighter balloon harder than the balloon's weight pulls down, so the balloon floats upward.
What people get wrong
A common mistake is thinking the heat or the flame physically pushes the balloon up, or that hot air weighs the same as cold air. Heat does not add an upward shove. It simply spreads the same air apart so the balloon holds fewer molecules and becomes lighter than its surroundings, and being lighter than the air around it is what makes it float.
The catch
Hot air floats but cools down quickly, so a balloon needs the burner fired again and again or it crowds back together and sinks. Cool air is heavy and settles low, which is steady and predictable, but a balloon full of cool air weighs about the same as the air around it and will not lift at all. There is no free ride without heat, and no staying up without more heat.
Questions kids ask
Does heating the air add more air to the balloon?
No. In an open-bottomed balloon, heating actually lets some air escape because the warmed molecules spread out and need more room. The balloon ends up holding fewer molecules than before, which is exactly why it gets lighter than the cooler air around it.
Why does the balloon come back down?
Hot air cools off fairly quickly. As it cools, its molecules slow down and crowd back together, so the balloon holds more of them again and gets heavier. Once it is no longer lighter than the surrounding air, it sinks, which is why pilots fire the burner over and over to stay up.
Is this the same reason a boat or a balloon of helium floats?
Yes, it is the same idea, called buoyancy. Anything that weighs less than the same volume of the stuff around it floats. A boat is lighter than the water it pushes aside, helium is lighter than air, and hot air is lighter than cooler air, so all three float up or stay up.
Why does warm air near the ceiling and cold air near the floor happen indoors?
The same rule works without a balloon. Warm air in a room has its molecules spread out, so it is lighter and drifts up toward the ceiling, while cooler, heavier air sinks to the floor. That is why upstairs rooms feel warmer and a cold draft pools at your feet.
For grown-ups
Heating a gas at roughly constant pressure makes it expand (the ideal gas law, PV = nRT), lowering its density so fewer molecules occupy the same volume. A balloon of lower-density warm air weighs less than the equal volume of cooler, denser air it displaces, so the upward buoyant force exceeds the balloon's weight and it rises. It is Archimedes' principle, the same effect that floats a boat, with air acting as the fluid.