Meteorologists use weather balloons to carry weather instruments high into the atmosphere. When it is first released at Earth's surface, a weather balloon typically appears underinflated. What happens to the balloon's volume as it rises into the atmosphere? Explain your answer in terms of pressure.

The balloons volume as it rises increases because at higher altitude external pressure is lower, so the air molecules inside the balloons tends to expand more to attain mechanical equilibrium, where the external pressure and internal pressure are equal. Unlike when the balloon is on the ground, external pressure is great so the gas molecule cannot push the walls of the balloon so it is like underinflated.