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Mountain Waves

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Article Information
Category: Weather Weather
Content source: Skybrary skybrary
Content control: Eurocontrol Eurocontrol

Lee Waves or Standing Waves

Definition

Oscillations to to the lee side (downwind) of a mountain caused by the disturbance in the horizontal air flow caused by the high ground.

Description

The wavelength and amplitude of the oscillations depends on many factors including the height of the high ground above the surrounding terrain, the wind speed, and the instability of the atmosphere.

Formation of Mountain Waves can occur in the following conditions:

  • Wind direction within 30 degrees of the perpendicular to the ridge of high ground and no change in direction with height.
  • Wind speeds at the crest of the ridge in excess of 15 knots, increasing with height.
  • Stable air above the crest of the ridge with less stable air above and below that stable layer.

Verticle currents within the oscillations can reach 2000 feet per minute. The combination of these strong vertical currents and surface friction may cause rotors to form beneath the mountain waves causing severe turbulence.

Effects

  • Loss of Control & Level Bust. The vertical currents in the waves can make it difficult for an aircraft to maintain altitude leading to Level Busts and can cause significant fluctuations in airspeed potentially leading, in extremis, to Loss of Control.
  • Turbulence. Aircraft can suffer structural damage as a result of encountering severe clear air turbulence. In extreme cases this can lead to the break up of the aircraft. In even moderate turbulence, damage can occur to fittings within the aircraft especially as a result of collision with unrestrained items of cargo or passenger luggage. If caught unawares, passengers and crew walking around the aircraft cabin can be injured.

Defences

  • Awareness.
    • Forecasting. Local knowledge of the conditions which tend to cause the formation of mountain waves enables forecasting of potential wave propogation.
    • Cloud Formation. Lenticular Clouds (lens shaped clouds) can form in the crest of the mountain waves if the air is moist. Roll Clouds can also occur in the rotors below the waves if the air is moist. These clouds are a good indication of the presence of mountain waves but, if the air is dry, then there may not be any cloud to see.
  • Restraint Systems. Passengers and crew should fit seat belts and harnesses when seated to protect them in the event of unforeseen turbulence.

Scenarios

An aircraft flying at low level over a mountain ridge experiences sudden loss of height on the lee side of the ridge, followed by a 50kt reduction in airspeed and encounters severe turbulence. The aircraft struggles to maintain height and airspeed untill several miles beyond the ridge.


Solutions

  • Airspeed. Reducing the aircraft speed reduces the risk of structural damage and reduces vibration making instruments easier to read in turbulence BUT beware the effect of vertical currents on airspeed and the risk of stalling the aircraft.
  • Strap in. Notify the crew/illuminate seat belt sign. All passsangers and crew should immediately sit down and fit seat belts/harnesses.
  • Inform ATC. Notify ATC of mountain wave activity.

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