Below cloud base, the air is very disturbed and can be extremely
turbulent. Once above cloud base, things normally calm down nicely.
Indeed, the glider pilot struggling to get into the wave proper
realises that the battle has been one once the turbulence is replaced
by amazingly smooth (laminar) air.

But don't forget that an aircraft, even in the "smooth air", if
it is flying at say 400 knots would pass through the up and down
currents in a very short time and would experience the effect as
"turbulence". However an aircraft flying beneath cloud in mountain
wave conditions would experience an altogether much more extreme form
of "chop". When approaching to land at airfields such as Aberdeen,
Newcastle, Teesside and Leeds/Bradford for example could be quite an
entertaining affair in the right (wrong?) conditions.

Glider pilots become interested when the vertical speeds of the wave
exceeds the rate of descent of the glider, roughly 200 ft/min. This is
about the threshold that would become an irritation to the airline
pilot.

Jack