At ground level gases outside and helium inside are in static equilibrium with balloon as separating surface. Both balloon and atmospheric forces are compressing to maintain that state.
But as balloon moves upward because of less density of helium gas in balloon internal force of helium gas increases. At high altitude atmospheric pressure decreases due to gravitational pull and fluid nature of gas. So equilibrium reestablish and it start to expand at high altitude.
Answer:As you can see from the very different numbers in the boxes above, your age changes (sometimes quite a lot) ... Mercury is the closest planet to the Sun and so has a smaller orbit path – it takes just 88 Earth days
