Timelapse footage of a foehn wall of clouds over the Rocky Mountains. A foehn wall is a cloud formation forming just downwind of a mountain range. Warm moist air is forced up by the mountains, causing its moisture to condense into cloud, and producing precipitation on the windward side. Once the air has passed the peaks it is dry and cold, and flows down the downwind side of the mountains as a foehn wind, whose temperature rises sharply due to the increasing pressure. This effect causes an abrupt end to the cloud formation, and the edges of the clouds appear as a wall over the downwind tops of the peaks. Filmed in Colorado, USA, in autumn. Mastered from originals, frame size 1280x1080.