Timelapse footage of iridescence in an altocumulus cloud. The rainbow colours are caused by the diffraction of sunlight by the tiny water droplets or ice crystals forming the cloud. This is an altocumulus lenticularis cloud, formed at the crest of a standing wave of air downwind of a mountain range. Moisture in the air condenses at the altitude of the wave's crest, but evaporates again at lower, warmer levels. This gives the appearance of the wind blowing through the cloud. Filmed in Colorado, USA, in winter. Mastered from originals, 1280x1080.