Imagine a rock in a stream just below the surface, with the water flowing over it. Downstream of the rock are waves in the water which don't flow downstream, but remain "stationary" in relation to the rock. In certain conditions, air will do the same when flowing over hills/mountain ranges.

Glider height record attempts used to be done in CuNims :eek: before wave was discovered. The current record is 50,699' by Steve Fosset.
Steve Fossett Does it Again - Glider World Altitude Record - Technology Evangelist

Here's a wave flight over Hawaii. Sadly the pilot was killed on another flight the following year.


Lots of pictures of wave clouds here:
wave clouds - Google Search

Well I did remember basic cable break when many years later gaining a PPL the instructor did not appreachite the hard stick forward with practice engine failure guess the first training can catch you out later. All was forgiven in bar later when instructor said he had completly forgotten my early training on gliders.

Gliding - a question to the military guys
 

I'm fairly sure the current UK glider absolute height record still stands at a tad over 38000'. This was set in the mid 90's by a guy called Chris Rollings flying out of Aboyne in Scotland IIRC . He also managed the first 1000Km flight earlier the same year. Pretty good year for him I'd say