How can the Perlan II glider climb to 90,000 feet?
Perlan II will climb to extreme altitudes on wave lift, not thermal lift. Wave lift is created when strong winds blow approximately perpendicular to a mountain range, and the wind speed increases steadily with altitude. This creates a standing wave in the atmosphere, kind of like the ripple behind a rock in a fast moving stream of water. On the leading edge of the wave, air is moving up, and that's where gliders fly. Standing waves can reach very high into the atomosphere, which is why Perlan II is expected to reach 90,000 feet.

Perlan II is different from normal gliders in that its wing is optimized for minimum sink, instead of best glide ratio. It's meant to float along while losing a minimum of altitude, which makes sense if it's going to fly in nothing but wave. Normal gliders are usually optimized more for gliding long distances, for which glide ratio is more important than minimum sink. Interestingly, Perlan II's wing looks a lot like RC model glider wings, which are also optimized for minimum sink, since RC gliders don't typically fly cross country.