The air around a balloon supplies the lift
A helium balloon rises because it displaces surrounding air. That air would occupy the space the balloon takes up, and its weight creates an upward buoyant force—much like water pushing up on a floating object.
Gravity still pulls the balloon downward. The outcome depends on a simple comparison: if the displaced air weighs more than the balloon, helium, and attached string or ribbon combined, the upward force is greater and the balloon accelerates upward.
Helium makes the difference
At the same pressure and temperature, helium is less dense than ordinary air. So a balloon filled with helium contains less gas mass than the air that would otherwise fill its volume. The balloon’s material adds weight, but if that weight does not use up the difference, the surrounding air can support it and provide extra lift.
This is why a helium balloon does not rise indefinitely. As it climbs, the surrounding air becomes thinner, so each volume of balloon displaces less air and produces less lift. The balloon eventually stops rising when the forces balance—or bursts first as the expanding gas stretches its skin.
A balloon can fall without changing helium
Add enough weight, such as a heavy ribbon or a small object, and the total weight can exceed the lift. Then the same air that once carried the balloon upward cannot support it. The decisive factor is not that gravity switches off, but whether the balloon displaces air heavy enough to outweigh everything being lifted.
