The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Visibility and Fog
Aircraft Icing
Weather Reports and Forecasts
Practice Exam

Mountain Waves

When the wind blows towards a hill or mountain, it is forced to rise up over the top (updraughts) and down the far side (downdraughts). If the atmosphere is stable, this can form a mountain wave that can extend for hundreds of miles downwind.

These downdraughts can be so strong that light aircraft may not be able to out climb them.

At the crest of the wave(s), lens or saucer-shaped clouds can form, known as lenticular clouds.

On the leeside of the ridge, the air at a low level can form severe turbulence. This may sometimes be associated with rotor clouds if the turbulence causes enough mixing for the lower air to rise and cool to its dew point.

Light aircraft need to keep well away from these areas as the turbulence will exist whether or not the rotor clouds are present.

Turbulence is most severe when the wind is blowing across a hill. The worst turbulence will be found on the leeward side (downwind) at roughly the same height as the top of the hill.
Here we can see a lenticular cloud that has formed over the top of Mont Blanc:

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