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 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.