in the beginning the balloon filled with helium rieses in air because weight of the air displaced by balloon is more than the weight of the balloon and helium gas filled inside balloon. We know that the density of air and the value of ac celeration due to gravity decreases with height. due to it, the weight of air decrease at greater height. The balloon halts at such a height where the weight of the air displaced just equal to the weight of helium gas and the balloon.
