Meaning of nimbostratus

nim•bo•stra•tus

Pronunciation: (nim"bō-strā'tus, -strat'us), [key]
— pl. -tus.
  1. a cloud of a class characterized by a formless layer that is almost uniformly dark gray; a rain cloud of the layer type, of low altitude, usually below 8000 ft. (2440 m).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.