Lenticular (lens-shaped), or lee wave, clouds are a type of formation usually associated with an air disturbance known as a lee wave, which forms downwind from an obstacle in the path of a strong air current. Since air warms as it falls and cools as it rises, moisture condenses on the upwind side at the peak of the wave, and clouds form. In the Boulder, Colorado, area the obstacle is the Front Range of the Rocky Mountains, seen at the bottom of the picture, and lee wave clouds form on the eastern side of the mountains. Lenticular clouds frequently form one above another, like a stack of pancakes. In the foreground of this image is the Mesa Laboratory of the National Center for Atmospheric Research.
