The photo above showing cloud trains over the southern Appalachian Mountains was snapped from cruising altitude (approximately 30,000 ft or 9,144 m) while on a flight from Pittsburgh, Pennsylvania to Atlanta, Georgia. Image © Seth Adams
Notice how the clouds are parallel with the ridges below them. Wind near the surface blowing up the western slopes forms waves in the atmosphere. At the crest of the wave, over the ridge tops, the air has cooled sufficiently to condense into clouds. As this air descends toward the wave trough, it becomes slightly warmer and drier, inhibiting condensation. Photo was taken with my phone from west-facing window seat midafternoon on January 14, 2014.