Timelapse footage of rotor clouds, a complex air motion induced by mountain lee wave activity. As wind blows over a mountain it cools, and swiftly travels down the lee side of the mountain. This often sets up a rotating vortex perpendicular to the wind direction and parallel to the mountain range, in which air moves in a tight circle, forming clouds at its crest as it does so. Filmed in the USA in the late afternoon in winter.