Definition
Turbulence produced when strong wind blows across a mountain range and is forced upward over the ridge, then descends on the leeward side in a series of waves. These standing waves can extend for many miles downwind and to high altitudes, generating areas of severe up- and downdrafts, rotor activity near the surface, and sharp, sometimes violent jolts to aircraft passing through them.
Plain English
When strong wind hits a mountain, it doesn't just stop — it ripples up, over, and down the other side like water flowing over a rock in a stream. Those ripples in the air can shake an aircraft hard, sometimes for many miles past the mountain.
Context Anchor
Seen in instrument weather flying, weather briefings, and pilot reports for flights near or downwind of mountain ranges.
Derivation
"Mountain wave" describes the wave-like pattern the air takes after crossing the ridge. "Turbulence" comes from Latin turbulentus, meaning disturbed or agitated. Together: disturbed air caused by the wave pattern downwind of a mountain.
Why Pilots Care
Can produce vertical currents strong enough to exceed aircraft climb or descent capability and create structural loads or loss of control.
Analogy
Picture a fast stream flowing over a submerged rock. The water doesn't go flat again right away — it forms standing waves downstream. Air does the same thing past a mountain, and an aircraft flying through those waves gets the same kind of shaking a small boat would get crossing them.
Grounding Statement
Picture strong wind crossing a ridge and then rising and sinking in invisible waves on the far side.
Intuition Check
Do not assume mountain wave turbulence only happens directly over the mountain peaks. It often occurs downwind of the mountains and can extend far from the ridge line.
Example Sentence 1
The pilot chose a higher cruise altitude after the briefer warned of mountain wave turbulence on the lee side of the Rockies.
Example Sentence 2
The pilot descended below the wave layer to escape the mountain wave turbulence reported on the arrival route.