Nimbostratus

A cloud of the 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 feet (2400 meters).

Nimbostratus (Ns) A principal cloud type (cloud genus), gray colored and often dark, rendered diffuse by more or less continuously falling rain, snow, sleet, etc. of the ordinary varieties and not accompanied by lightning, thunder, or hail. Precipitation in most cases reaches the ground. Low, ragged clouds frequently occur below the layer, with which they may or may not merge.

(abbrev. NS)- A cloud of the 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 (2400 m).