What are rotors?

Rotors are a small-scale overturning circulations that cause localised changes in wind direction and speed. They are associated with high-amplitude mountain or lee waves.

They are sometimes visible through a roll of cloud that seems to rotate on itself, and they can cause problems for aircraft.

For rotors to form we need a mountain range to provide the conditions for lee waves to form, a temperature inversion sitting above the mountain top and winds blowing across the range at a speed of around 30 mph.

Rotors develop during mountain wave conditions because of changes of pressure and wind at the surface that accompany lee waves.

Small areas of low and high pressure exist respectively with the troughs and crests of the waves.

Wind at the surface accelerates beneath the troughs and decelerates beneath the crests.