Zero-pressure balloons

Because lift gas expands as a balloon ascends, they are always smaller at launch then they are at higher altitudes.

The ducts allow excess gas to escape when the balloon gets full during ascent.

As the balloon approaches the altitude at which it will float, it will continue to vent the lift gas that is causing it to rise.

The more lift gas the balloon expels the less free lifting force exists and the slower the balloon will ascend until eventually it will stop ascending and start to "float", or remain at an altitude in the sky neither going up nor down.

The natural balloon shape typically roughly resembles an elipsoid atop a cone.