Timelapse footage of a mountain wave cloud over the Rocky Mountains. The sharp edge of the cloud appears stationary despite the obvious effect of the wind on other clouds and contrails. This is because the cloud forms in the crest of a standing wave, established as the wind blows over the peaks. At the peak of the wave it is cool enough for the cloud to form, and as the air descends it evaporates. Filmed in Colorado, USA, in autumn.