a dense, towering vertical cloud

Cumulonimbus (from Latin cumulus, 'heaped' and nimbus, 'rainstorm') is a dense, towering vertical cloud, forming from water vapor carried by powerful upward air currents.

a type of cloud that is tall, dense, and involved in thunderstorms and other bad weather

Cumulonimbus is a type of cloud that is tall, dense, and involved in thunderstorms and other bad weather.

thunderstorm clouds

Cumulonimbus are thunderstorm clouds, and are most impressive when not hidden by other cloud types or pulled apart by strong winds.

cumulus clouds that have grown vertically into an anvil-like shape

Cumulonimbus are cumulus clouds that have grown vertically into an anvil-like shape.

a dense and vertically developed cloud that produces thunderstorms

Cumulonimbus are a dense and vertically developed cloud that produces thunderstorms.

the only clouds that produce lightning

Cumulonimbus are the only clouds that produce lightning.

associated with thunderstorm cloud

A cumulonimbus, or thunderstorm cloud, has charges or energy associated with thunderstorm cloud.

tall, towering clouds that can produce lightning, tornadoes, and hail

Cumulonimbus are tall, towering clouds that can produce lightning, tornadoes, and hail.

a rain cloud that has a 'piled' appearance

A cumulonimbus is a rain cloud that has a 'piled' appearance, as opposed to a stratus, or 'flat' cloud.

the cloud that everyone fears

Cumulonimbus – the cloud that everyone fears – portends thunder, lightning, violent downpours, maybe even a tornado.

a thunderstorm cloud which can begin at low levels and reach into high levels

Cumulonimbus is a thunderstorm cloud which can begin at low levels and reach into high levels.