Definition
A standing wave of air that forms on the downwind (lee) side of a mountain or ridge when stable air is forced up and over the terrain. The wave remains roughly stationary relative to the ground while the air flows through it, often producing strong updrafts, downdrafts, and turbulence at altitudes well above the ridge.
Plain English
When wind blows over a mountain, the air on the far side can rise and fall in a wave pattern, like ripples downstream of a rock in a river. The wave stays in one place while the air keeps moving through it.
Context Anchor
Seen in mountain flying, weather briefings, and forecasts for routes that pass near or downwind of high terrain.
Derivation
‘Lee’ is an old English nautical term meaning the side sheltered from the wind — the downwind side. A lee wave is therefore literally a wave on the downwind side of an obstacle.
Why Pilots Care
Lee waves can produce strong vertical currents that cause sudden altitude changes, loss of control, or structural stress if not anticipated.
Analogy
Think of water flowing over a submerged rock in a stream. The bumps and ripples downstream stay in place even though the water keeps moving through them. Lee waves work the same way in air.
Grounding Statement
Picture strong wind crossing a ridge; after the crest, the air keeps rising and falling downwind instead of immediately becoming smooth.
Intuition Check
Do not assume the lee side is calm just because it is the sheltered side of a mountain. Here, lee means downwind of the terrain, where strong waves and rough air can form.
Example Sentence 1
The pilot climbed to cross the ridge well above the peaks to avoid the strongest turbulence in the lee wave.
Example Sentence 2
The weather briefing warned of lee waves east of the mountains that could affect the planned route.