We have been thinking of using Aerostats/Balloons for an environmental monitoring project in our university. We have been considering 2 options: Tethered Aerostats (which fly upto 1 km) and Stratospheric Aerostats (upto 20 - 30 km).
As I was exploring the available options, I was surprised to find that Tethered Aerostats (i.e. low-altitude balloons, within 1km) have much less persistence in air than Stratospheric Balloons. E.g. Airstar's tethered balloons have maximum persistence of 15 days while their stratospheric versions can hover for upto 6 months.
What is the reason? Is it the wind speed? Or less density (but how would it affect)? Or maybe it's just they are designing the stratospheric balloons to last longer?