Crepuscular rays: shafts of light which are seen just after the sun has set and which extend over the western sky radiating from the position of the sun below the horizon. They form only when the sun has set behind an irregularly shaped cloud or mountain which lets the rays of the sun pass through a cloud in bands.

In this case, I think the light is making it look like the cloud is dark bands, when in fact the entire cloud is probably all dark but being lit up by the rays. (And yes, this affect can be seen both at sunrise and sunset.)