A Nimbostratus cloud is characterized by a formless cloud layer that is almost uniformly dark gray. "Nimbo" is from the Latin word "nimbus", meaning rain. It is a stratiform cloud that produces rain, developing cloud bases between the surface and 10000 ft (3000 m). Nimbostratus usually has a thickness of 6,500-10,000 ft (2,000-3,000 meters), but can vary from 3,500 ft (1,000 m) to 15,000 ft (4,500 m) in thickness. In rare cases, Nimbostratus can be very thin and accompanied by a separate layer of altostratus divided by a cloudless layer. Though found worldwide, nimbostratus is found more commonly in the middle latitudes.
The base of a nimbostratus base cloud is dimmed by precipitation and is usually not clearly visible. In all cases, nimbostratus is accompanied by pannus clouds, which develop underneath of nimbostratus. If the pannus layer is completely opaque, the presence of precipitation indicates presence of nimbostratus. The pannus movement is slow and uniform under nimbostratus.
Nimbostratus will occur along warm fronts where the slowly rising warm air mass creates nimbostratus and stratus clouds, which are preceded by higher-level clouds such as cirrostratus and altostratus. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus.
Usually, nimbostratus is a sign of steady moderate to heavy precipitation, as opposed to the shorter period of typically heavier precipitation released by a cumulonimbus cloud. However, precipitation does not occur at ground level in case of virga and accompanies other cloud types. Precipitation may last for several days, depending on the speed of the occluded front it accompanies.