What are heavy, dark storm clouds called?

Heavy, dark storm clouds are referred to as "nimbus clouds." The term "nimbus" specifically denotes clouds that are associated with precipitation. These clouds are typically thick and dense, which allows them to hold significant amounts of moisture, leading to rain, thunderstorms, or other types of severe weather.

For instance, cumulonimbus clouds, which are towering and can reach high altitudes, are the primary clouds involved in thunderstorms and severe weather events. While other cloud types such as cumulus, stratus, and cirrus might appear in the sky, they do not carry the same characteristics of heaviness and darkness associated with nimbus clouds that indicate stormy conditions. Cumulus clouds are often fluffy and white, stratus clouds form in flat, uniform layers, and cirrus clouds are thin and wispy, all lacking the stormy attributes that define nimbus clouds.