MOUNTAIN WAVE
Pilot/Controller Glossary with JO 7110.65BB, Change 3, effective July 9, 2026 · Official text on faa.gov
Mountain waves occur when air is being blown over a mountain range or even the ridge of a sharp bluff area. As the air hits the upwind side of the range, it starts to climb, thus creating what is generally a smooth updraft which turns into a turbulent downdraft as the air passes the crest of the ridge. Mountain waves can cause significant fluctuations in airspeed and altitude with or without associated turbulence.
Refer to AIM.