Cumulonimbus

Cumulonimbus (Cb) is a dense, towering vertical cloud extending from near the surface to the tropopause (up to 60,000 ft). It is the cloud type associated with thunderstorms and contains the most violent weather phenomena encountered in aviation.

Contains severe turbulence, heavy precipitation, hail, lightning, and icing
Can produce microbursts, wind shear, tornadoes, and waterspouts
Anvil-shaped top indicates mature stage
Aircraft should maintain at least 20 NM lateral separation
Embedded Cb (hidden within other cloud layers) is particularly dangerous