Nimbus clouds generally bear rain, and they appear flat and dark or towering with white sides with rain below them. Rain appears as thick, gray mist from a distance which makes seeing the horizon difficult.
Nimbostratus clouds are low, thick and dense clouds that produce steady precipitation. These clouds spread out from one end of the horizon to another as weather moves through an area.
Cumulonimbus clouds reach high into the atmosphere and feature heavy precipitation and strong gusts of wind. These clouds, when seen from a distance, have white tops and may contain an "anvil" shape at the pinnacle as warm air quickly rises within the cloud. Cumulonimbus clouds produce thunderstorms under the right conditions.