The photo above showing two contrasting cloud layers was taken while flying from New York City, New York to San Diego, California on the morning of April 5, 2011. Photographer: Bob Kelly, Bob’s Website
I was somewhere over the Middle Atlantic States (at approximately 30,000 ft or 9,000 m, looking north) and noticed the deck of altocumulus wave clouds casting shadows on the continuous stratocumulus deck below. It’s interesting that the wave clouds, typically an indicator of turbulence, are in proximity to the “solid” stratocumulus deck, generally associated with stable atmospheric conditions. However, wave clouds may form when stable air is forced to rise over elevated terrain, the ridges of the Appalachian Mountains for instance.