Rotor cloud

A rotor (cloud) is a closed, vertical circulation that develops in the lee of high mountain barriers, or in the valley between two mountain ranges, when conditions are appropriate. The rotor is a form of lee eddy. At the surface the wind is in the opposite direction to the gradient wind. Rotors are often associated with extreme and turbulence an are often marked by a rotor cloud such as the helm bar. Rotor clouds belong to the altocumulus lenticularis family.

The Helm Cloud and Helm Bar of Cross Fell, northern Pennines