Mountain Wave

Mountain Waves are defined as oscillations to the lee side (downwind) of high ground resulting from the disturbance in the horizontal airflow caused by the high ground. The wavelength and amplitude of the oscillations depend on many factors including the height of the high ground relative to the surrounding terrain, the wind speed and the instability of the atmosphere.

Formation of Mountain Waves can occur in the following conditions:

Wind direction that is perpendicular, or roughly perpendicular to the ridge of high ground.
Wind speeds at the crest of the ridge in excess of 15 knots.
Stable air above the crest of the ridge with less stable air above and a stable layer below the ridge.

Vertical currents within the oscillations can reach 2,000 feet/min. The combination of these strong vertical currents and surface friction may cause rotors to form beneath the mountain waves causing severe turbulence, strong vertical currents and icing conditions