Some of the longest-running Science goals here at S Pole involve observing the weather and atmosphere. Much of that research is done using sondes launched with large helium balloons. Depending on what is being measured by a particular launch, different sizes and types of balloons are used.
Rubber balloons stretch out as they rise in the air, due to the contained helium expanding to balance with the lower air pressure high up, and pop when they reach their maximum altitude. Plastic balloons don’t stretch, and can be used to keep sondes at their peak altitude for longer by using vents at the bottom of the balloon. I think the bigger balloons tend to be plastic, so plastic balloons tend to go higher.
The launch this morning got “up to” 16 millibars of pressure, which works out to about 80,000 feet.