Timelapse footage of two very tightly-wound lee wave rotor vortices, made visible by clouds within them. The rotors are rotating vortices that lie parallel to the mountain range. They form as air passing over the mountains sets up standing waves of air downwind of the mountains. The fast flow of the air combined with friction from the ground leads to the formation of the rotor. Such airflow is a serious hazard to aircraft, and is avoided where possible. Filmed in Colorado, USA, in autumn.