Which cloud type is known for being continuous rain clouds that are grey and thick?

Nimbostratus clouds are characterized by their thick, grey appearance and their ability to produce continuous and steady precipitation. These clouds cover the sky in a uniform layer, leading to overcast conditions. The persistent rain associated with nimbostratus clouds occurs because their structure allows moisture to be released continuously over large areas, making them primarily the type of cloud associated with prolonged rain events.

Cumulus clouds, on the other hand, are typically fluffy and indicate fair weather, while stratocumulus clouds appear as low, lumpy clouds that may produce light rain but not the continuous rain associated with nimbostratus. Stratus clouds, while also grey and capable of light precipitation, do not have the same thickness or continuous rain capability as nimbostratus clouds. Thus, nimbostratus is the correct cloud type that fulfills the criteria of being thick and associated with continuous rain.