Yes - and glide ratio is what this is all about. If a glider is in an updraft of say 50 mph vertical and it has a high speed glide ratio of 10:1 it can be doing 500 mph as it corkscrews down - without losing any altitude.
So what would be needed is a craft capable of 10:1 glide ratio at supersonic speeds and an updraft of 80 mph or more.
As much as I hate to disagree with the esteemed GG - I don't think the above explanation is correct. If I understand what you are saying I don't think a glider which is falling due to gravity could achieve anything like 500mph let alone supersonic speeds. A WW11 bomb falling vertically from 20,000ft was usually no faster than about 500mph. - I doubt a glider would dive vertically any faster than maybe 100mph at best. I am also not sure the reference to glide ratio is really valid - glide ratios involve distances not speeds.
I suspect the reason these gliders are so bloody fast is a similar effect to vertical axis wind turbines (VAWT apparently) like this blighter:http://www.platek.com/wind/
Note the glider-like wing size and section. The glider in question on the slope appears to circulate in a near-vertical bank - the overall action of the glider (and its wings) acting like a large (very large) diameter VAWT.
I am not sure if there is at present a generally-accepted theory as to why these gliders are so fast. The only vaguely "official" explanation I have seen involved the glider continually looping from the rising fast-moving air on the slope into the relatively still air in the lee of the slope - apparently picking up a bit of pace each time it completed a circuit - I don't think this theory is correct either.
Edited by Kelpiecross, 25 October 2013 - 08:05.