The existence of rotors associated with mountain waves has been known by the aviation and atmospheric science communities for many decades. Yet many aspects of rotors remain unclear. In both communities, knowledge is often limited to the fact that severe or extreme turbulence can be associated with mountain waves, without knowledge of rotor location or spatial extent. Indeed, one dangerous aspect of rotors is the fact that flow can transition from completely laminar to severely turbulent over a horizontal distance O(100m). Despite their significance, rotors have until recently been largely ignored in studies of mountain waves.

Herein we briefly review what is known about rotors. We begin with a historical overview of observational rotor studies from gliders and powered aircraft, as well as the signals that clouds provide. Observed rotor characteristics are described, including our current state of knowledge about their spatial dimensions, location in the mountain wave system, and turbulence intensity. Of particular interest is the distinction between a relatively benign type of rotor, and the more destructive rotors that form in conjuction with mountain waves, displaying characteristics of a hydraulic jump. In addition, a brief review is given of theoretical hypotheses proposed in the literature and by the authors.