Zero-pressure balloon

From Glossary of Meteorology

zero-pressure balloon

A balloon that is not sealed.

The base of the balloon may be open or it may have a duct high on the balloon that is attached at or near the base of the balloon. The opening is used to vent excess gas when the balloon reaches ceiling altitude. The large plastic balloons used to carry scientific payloads to altitudes above 40 km are zero-pressure balloons.