[zeropoint46]

sure but these aren't really weather balloons. the last two are basically tethered dirigibles. the first one is the closest and it's more like what project loon is doing. Yes, it's true, balloons can stay up for long periods of time, but this is not how weather balloons are made or what they are designed for. Weather balloons are designed to pop at a specific pressure and fall. As the balloon rises, the volume of gas expands due to there being less atmospheric pressure. At a point the balloon pops and it falls back to earth. The loon and the balloon in your first link probably use a system that adjusts the pressure in the balloon by compressing it into a storage container. This allows it to "hold" an altitude and keep the balloon from ascending until it bursts. Also, at least in loon's case, it uses a different material than a standard weather balloon. This probably also plays a roll in it's expandability, resistance to long term UV and keeping itself from popping.