A Nimbostratus cloud is characterized by a formless cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 layer that is almost uniformly dark gray. "Nimbo" is from the Latin word "nimbus", which denotes precipitation. It is generally a stratiform cloud of moderate vertical development (family D1) that produces precipitation, developing cloud base

Cloud base

This article refers to meteorology. For the airborne base of the TV series Captain Scarlet and the Mysterons, see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...

s between the surface and about 10000 ft (3000 m). This cloud typically forms from altostratus in the middle altitude range then subsides into the low altitude range during precipitation.