What’s “Mountain Waves”?

[Ref AC 00-6B Chapter 17, AC 00-57]

Mountain waves are a form of mechanical turbulence which develop above and downwind (Lee side) of mountains ridge.

Risk of Mountain waves?

  1. Range: This mechanical turbulence may extend 600 miles (1,000 km) or more downwind from the mountain range.
  2. Intensity: This mechanical turbulence are classified as severe to extreme turbulence.
  3. Vertical coverage: The effect of this mechanical turbulence can cover up to 200,000 feet (20 times of  FL200)! – far higher than modern airliners’ cruising level.


Clouds related to Mountain waves

When sufficient moisture is present in the upstream flow, mountain waves produce interesting cloud formations:

  1. cap clouds
    – cover the ridge of the mountain
  2. cirrocumulus standing lenticular (CCSL)
    – lee side of the mountain, middle clouds in the stationary group
  3. Altocumulus Standing Lenticular (ACSL)
    – lee side of the mountain, high clouds in the stationary group
  4. rotor clouds
    – lee side of the mountain, lowest cloud in the stationary group

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