The word nimbus means a cloud that already has rain or snow falling from it. These clouds are dark and seen during a thunderstorm along with thunder and lightning. They can be a combination of two clouds, like a cumulonimbus, which means a puffy black cloud with rain falling out or it, or a stratonimbus, which is a dark blanket with rain falling out of it.
Nimbostratus
Altostratus often spread over thousands of square miles and are strongly linked to light rain or snow. Though they’re not capable of yielding heavy rain it’s common for altostratus clouds to morph into nimbostratus clouds which are packed with moisture and can deliver a pounding. They’re uniformly gray, smooth, and mostly featureless which is why they’re sometimes called ‘boring clouds’. You’ll commonly see this types of clouds in an advancing warm frontal system, preceding nimbostratus clouds.
Cumulonimbus
The thunderstorm cloud, this is a heavy and dense cloud in the form of a mountain or huge tower. The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume. Under the base of this cloud which is often very dark, there are often low ragged clouds that may or may not merge with the base. They produce precipitation, which sometimes is in the form of virga.