Lenticular, or lee wave, clouds form downwind of an obstacle in the path of a strong air current. In the Boulder, Colorado, area, the obstacle is the Front Range of the Rocky Mountains, seen at the bottom of the picture. Since air cools as it rises and warms as it falls, lenticular clouds are most likely to form at the peak of a standing wave, where moisture condenses in the cooled air. The vertical stacks of cloud in this photo resulted from a particularly deep wave or set of waves.
