From Academic Kids
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 low altitude, usually below 8000 ft (2400 m). Compare nimbostratus clouds with stratus, altostratus and cirrostratus.
Other typical precipitation bearing clouds are cumulonimbus.
- National Science Digital Library - Nimbostratus (http://www.nsdl.arm.gov/Library/glossary.shtml#Nimbostratus)nl:nimbostratus