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Mountain Waves

Mountain Waves is defined as oscillations 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 kts, increasing with height.

- Stable air above the crest of the ridge with less stable air above and below that stable layer.

Vertical currents within the oscillations can reach 2,000 ft/min10.16 m/s. The combination of these strong vertical currents and surface friction may cause rotors to form beneath the mountain waves causing severe turbulence.

Effects
Mountain Waves are associated with severe turbulence, strong vertical currents, and icing.

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 extremes, 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 unaware, passengers and crew walking around the aircraft cabin can be injured.

Icing
Severe icing can be experienced in the wave clouds.

Defenses

- Awareness.

- Forecasting.
Local knowledge of the conditions which tend to cause the formation of mountain waves enables forecasting of potential wave propagation.

- 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.

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