Definition

Cumulonimbus is a heavy and dense cloud of considerable vertical extent in the form of a mountain or huge tower, often associated with heavy precipitation, lightning and thunder. The mature Cumulonimbus cloud has a distinctive flat, anvil shaped top.

Description

The Cumulonimbus cloud (Cb) forms when three conditions are met:

1. There must be a deep layer of unstable air.
2. The air must be warm and moist.
3. A trigger mechanism must cause the warm moist air to rise:
   • Heating of the layer of air close to the surface.
   • Rising ground forcing the air upwards (orographic uplift).
   • A front forcing the air upwards.

The way in which a Cb develops is covered in a separate article: Lifecycle of the Thunderstorm.