What is a defining feature of Cumulonimbus (CB) clouds?

Cumulonimbus clouds are characterized by their large vertical development and distinctive anvil-shaped tops. These clouds can extend from low altitudes up to the higher levels of the atmosphere, often developing towering structures that can reach impressive heights, sometimes even extending into the stratosphere. The anvil shape forms as the rising air within the cloud reaches the tropopause—the boundary between the troposphere and stratosphere—where the cooler temperatures cause the water vapor to condense into ice crystals, spreading outward at the top of the cloud. This feature is a key indicator of severe weather potential, as Cumulonimbus clouds are associated with thunderstorms, heavy rainfall, and other extreme weather phenomena. The other characteristics mentioned in the other options do not accurately describe Cumulonimbus clouds.