Refer to figure.
Mountain waves, may also be referred to as standing waves or lee waves, and develop on the lee, or downwind, side of mountains. The most severe turbulence can occur in the rotor zone lying beneath the crests of lee waves and is often marked by roll clouds. The most powerful rotor lies beneath the first wave crest (one wavelength downwind). Flight in waves can be smooth, but severe turbulence may occur. Occasionally violent turbulence will occur, due to wave ‘breaking’.

The resultant waves can extend for hundreds of miles downwind of the range, if suitable conditions prevail.

If sufficient moisture is available in the atmosphere, Rotor/Roll clouds can be observed, indicating the presence of a rotor phenomenon - which occurs at the same height or under the crests of strong waves downwind of the ridge.