weather balloons are typically not up for that long, 90 or so minutes on average. So I don't think that's solving the extended air time. In fact they are pretty much designed to go up and pop and come back down. Lighter than air in general can be used to solve extended air time, and we're back to dirigibles, not weather balloons.
The Stanford Student Space Initiative's valbal is an interesting project where they add a lightweight set of ballast and a valve to a weather balloon so it can maintain an altitude. They're limited by power, the amount of ballast, and how much gas they can bleed but they can get flight times of days. https://stanfordssi.org/blog/ssi-52-breaks-world-record-agai...
Well, I'm disappointed that the majority of their flights are this short. I thought there were some of them which were permanent atmospheric weather stations.
But short flight times don't really have to be the case, because it seems that they can fly for extended periods of time: one of NASA's balloons stayed up for 46 days [0]. The US air force also uses balloons as surveillance stations, though I don't know the actual flight time of these [1][2]. JLENS was supposed to stay up for 30 days at a time.
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.
