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. Wind blows most types of clouds across the sky, but lenticular clouds seem to stay in one place. Air moves up and over a mountain, with the lenticular cloud forming just past the mountaintop. The cloud evaporates on the downwind side, so it appears stationary even though air is moving through the cloud. Lenticular clouds are lens-shaped and often look like flying saucers.