Nimbo is from the Latin word "nimbus" meaning rain. A Nimbostratus is a cloud of the class characterized by a formless layer that is almost uniformly dark gray; it is a rain cloud of the layer type, of medium altitude, usually developing above 6500 ft (2000 m) as defined by the WMO. It may drop below 2000m and is classified in some nations as low level cloud. Nimbostratus block a great amount of sunlight as a result of its characteristic dense structure. Nimbostratus usually has a thickness of 6,500-10,000 ft (2,000-3,000 meters), but can vary from 15,000 ft (4,500 m) to 3,500 ft (1,000 m) in thickness. In rare cases, nimbostratus can be very thin and accompanied by a separate layer of altostratus divided by a cloudless layer.
The base of a nimbostratus base cloud is dimmed by precipitation and is usually not clearly visible. In all cases, nimbostratus is accompanied by pannis clouds, which develop under layer of nimbostratus. If pannus layer is completely opaque, presence of precipitation indicates presence of nimbostratus. The pannus movement is slow and uniform under nimbostratus.
Nimbostratus, stratus, altostratus and stratocumulus clouds all have a smooth gray appearance. Usually, nimbostratus is a sign of steady moderate to heavy precipitation. However, precipitation does not occur at ground level in case of virga and accompanies other cloud types. There are a number of features allowing the observer to distinguish nimbostratus from other clouds:
Moreover, large and low cumulonimbus cloud covering most of sky can be mistaken for nimbostratus. In this case, nimbostratus can be distinguished by lighter, constant precipitation.
Other typical precipitation-bearing clouds are cumulonimbus.