<Tr> <Th> Appearance </Th> <Td> Dark, widespread, formless layer </Td> </Tr> <Tr> <Th> Precipitation cloud? </Th> <Td> Yes, but may be virga </Td> </Tr> <P> Nimbostratus is a stratiform genus formerly classified as "Family C" low - level, but now considered by the World Meteorological Organization (WMO) to be a middle - or multi-level stratus type. . Although it is usually a low - based cloud, it actually forms most commonly in the middle level of the troposphere and then spreads vertically into the low and high levels . This change in classification would once have made it a "Family D" cloud, but this style of nomenclature was discontinued by the WMO in 1956 . Nimbostratus usually produces precipitation over a wide area . Nimbo - is from the Latin word nimbus, which denotes precipitation . It has a diffuse cloud base generally found anywhere from near surface in the low levels to about 3,000 m (9,800 ft) in the middle level of the troposphere . Although usually dark at its base, it often appears illuminated from within to a surface observer . Nimbostratus usually has a thickness of about 2000 m . Though found worldwide, nimbostratus occurs more commonly in the middle latitudes . It is coded C 2 on the SYNOP report . </P> <P> Downward - growing nimbostratus can have the same vertical extent as most large upward - growing cumulus, but its horizontal extent tends to be even greater . This sometimes leads to the exclusion of nimbostratus from the group of vertical clouds by some independent meteorologists and educators . Classifications that follow this approach usually show nimbostratus as low - level based on its normal base height range . </P>

What is meant by the term nimbus regarding clouds