Timelapse footage of a classic foehn wall of clouds over mountains. The foehn wall is formed on the downwind side of a mountain range. Air on the windward side of the range is forced to rise by the mountains, which cools it and causes its moisture to condense into clouds, which rain on the windward side. On the lee (downwind) side of the mountains, the now-dry air sinks down the slope, warming rapidly as it does so, evaporating any remaining moisture and leaving the region free of cloud. The boundary appears as a wall of cloud above the mountains. The descending dry air warms quickly as it descends, and the downslope wind, called a foehn wind, can be very strong, warm and dry. Filmed in Colorado, USA, in winter.