The time-lapse video (converted to animation) above was taken in San Antonio, Texas on February 18, 2008 by Steven Browne. Notice how the clouds are literally rolling toward you, like rolling water pipes. You can see them on satellite, also.
This is caused by extreme wind shear in the atmosphere in a stable environment, and is called Kelvin-Helmholtz instability. The wind is out of the west, at 2 mph at 4,500 feet and 30 mph at 7,500 feet, while the air is stable there, as shown in the model sounding from San Antonio. This often causes turbulence for aircraft at that level.
