Question: Why does helium balloons rise and not come down?

Alice Harpole answered on 16 Mar 2016:

Helium is the second lightest element in the periodic table. This means that helium gas is much less dense than air (which is mostly made up of nitrogen and oxygen) if both are at the same pressure. As Archimedes is said to have discovered back in Ancient Greece, less dense things float on top of more dense things. This is known as buoyancy.

A helium balloon on the Earth’s surface is less dense than the air around it, and so it rises. Assuming the air is perfectly still and no helium leaks out of the balloon, it will not come back down again until it reaches a point where the air around it is the same density as the helium inside it. The Earth’s atmosphere gets less dense as you go up, towards space, so if you let a helium balloon go on a still day it may go quite high up before it reaches a region of low enough density that it begins to fall!