Comment Re:Ozone Hole (Score 4, Informative) 23
That is correct. Ozone can (and is) measured in microwave spectral bands. For example, the Microwave Limb Sounder (MLS) on the AURA satellite retrieves Ozone around 240 GHz. Actually, every Microwave sounder that I know of can measure Ozone, so I have no doubt that Ozone signatures in the upper atmosphere (just as from other trace gases) could affect microwave space observation.
But it's not the main reason why they fly there I believe. If they want to do long duration flights, everywhere else, they will have to cross large water masses and cross various airspaces. I believe it would be difficult to do the same in the north hemisphere (crossing Russian airspace). Furthermore, in the polar summer, you do not need to worry about day-night cycles, which makes power supply system simpler. If they need sun for power (always the case I guess over a 48h float), a flight in the polar winter cannot work. The only alternative could be equatorial flight, but getting the overflight permits is complex and there are, to my knowledge, no active balloon bases in equatorial/tropical regions these days.