It's the same phenomenon that sailplane pilots use to reach the stratosphere without an engine. Mountain wave has been used to reach altitudes over 50,000 feet, and is thought to extend to perhaps 90,000 feet or more. I used to fly sailplanes, and wave lift is incredibly smooth (just so long as you avoid the VERY turbulent rotor underneath). You just catch the updraft portion, adjust your speed to stay in it, and up you go on an invisible elevator. Climb rates can reach as high as 6,000 feet per minute, though in practice they're usually more in the range of about 1,000 to 3,000 fpm.
Fascinating that the same effect occurs on Venus.