Weather balloons when filled rise to up. As they rise the atomspheric pressure changes as all the gases aren't so compressed due to gravity so the balloon starts to expand.
There are also temperature changes as they get higher, eventually the Helium loses it's lighter than air capacity and the waether balloon eventually drops back to earth, although they can get blown for many miles with the winds higher up.
You're suppose to return them if you find them to the meteorological centres, although one badly damaged one was given to a school I once attended.
So I could see the polystyrene module that would have been attached to the base of the balloon. There is actually a report out there about a so called "UK UFO Bunker" in the south west of England, but the Americans that tired to infiltrate it didn't understand that their "Tall Garage" Was actually where a weather balloon is filled with helium before it's let loose.
