Which cloud type is most commonly responsible for an intensely dark, threatening sky signaling a classic thunderstorm?

Answer

Cumulonimbus

The cumulonimbus cloud is characterized by immense vertical development, and because of its sheer size and moisture volume, its base is frequently a deep, bruised grey or nearly black when viewed from the ground during a thunderstorm.

Which cloud type is most commonly responsible for an intensely dark, threatening sky signaling a classic thunderstorm?
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