Mountain Waves

Mountain waves are atmospheric waves that develop when stable air flows over a mountain range, producing a wave-like airflow pattern.
They are caused by disturbed airflow over mountains, resulting in strong updrafts on the windward side and severe down-drafts on the leeward side.
The intensity of mountain waves depends on wind speed, atmospheric lapse rate, and the length, height, and shape of the mountain range.
Mountain waves can extend to great altitudes, making flight conditions rough and potentially unsafe.
Pilots should avoid flying close to or below ridge level in strong wind and stable atmospheric conditions.

Mountain waves develop when the atmosphere is unstable at lower levels, stable near the mountain crest, and unstable again above the stable layer.
Absolute stability around mountain ridges is often produced by a temperature inversion.
Minimum wind speeds required are approximately 7 m/s for small mountains and 15 m/s for large mountain ranges.
The wind direction should remain steady and within 30° of being perpendicular to the mountain range.

The presence of Lenticular, Cap, and Rotor (Roll) clouds indicates mountain wave activity.
Ragged edges on lenticular clouds are an indication of severe turbulence.
Cap clouds may cover mountain peaks and can extend down the leeward slope as a Föhn Wall.
Rotor and Roll clouds produce the most severe turbulence on the leeward side of mountains.
Rotor streaming clouds are violent rotor clouds formed when strong low-level winds combine with reverse winds aloft.
The strongest rotor is generally found above and near the first mountain wave crest.

Strong vertical air currents between 10 m/s and 25 m/s may occur on both the windward and leeward slopes.
Mountain waves produce abrupt turbulence due to additional wind shear.
Severe turbulence is commonly associated with Cap, Rotor, and Föhn Wall clouds.
Aircraft may experience turbulence loads between 2g and 4g, with values up to 7g possible in rotor clouds.
The severity of bumpiness depends on the size and aerodynamic characteristics of the aircraft.
Closely spaced mountain ranges can produce multiple interacting wave systems, resulting in violent turbulence.
Rapid fluctuations in altimeter readings may occur because of hysteresis effects and temperature deviations from ISA conditions.

Over small mountains, mountain waves are most prominent around sunset due to radiational cooling.
Weak low-level wind conditions associated with evening mountain waves are known as Evening Waves.

Mountain waves are more frequent during winter because of strong, steady winds that increase with height and a stable atmosphere at lower levels.
Mountain waves may extend 100–200 km downstream of a mountain range and can reach into the stratosphere, with wavelengths exceeding 10 km.