Lenticular clouds are a type of wave cloud that form when a layer of air near the surface is lifted and cools. This usually happens because of orographic lifting over a topographic feature — wave clouds form on the lee side. In the middle latitudes, lenticular clouds typically are found between 6,500 and 15,000 ft (1,981 and 4,572 m) above the surface. Even though it may be windy aloft and at the surface as well, as it was on this April day, lenticular clouds can appear almost motionless with respect to the terrain since they form at waves crests in stationary air currents. Because sufficient moisture was available and the airflow was strong, this wave consisted of layers of lenticulars stacked on top of each other. Photo taken on April 18, 2011.