Airflow over a ridge or mountain range may set up an undulating flow downstream provided certain preconditions are met. This undulating flow is called mountain waves and can be the source of strong turbulence, rotors and high vertical wind speeds. Mountain wave activity is often evidenced by lenticular clouds.

For mountain waves to form the following conditions must exist:

• wind flow almost at right angles to the mountain;
• wind strength of at least 25 knots near the mountain top;
• wind speed increasing with height;
• an upper stable layer (inversion) sandwiched between an unstable layer at the surface and another unstable or weakly stable layer above.

If the air stream is sufficiently moist at any level affected by the wave activity, cloud may form in the ascending sections and produce almond or lens type ‘lenticular’ cloud formations. Mountain waves can exist without any visible indication.

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Technical data content credited to Mr Steve Griffin

Mountain Waves, Aviation Turbulence, Turbulence Weather