Here's why:
* Cumulonimbus clouds are the thunderhead clouds associated with thunderstorms. They are massive, towering clouds that can reach heights of 13 km or more.
* Their vertical development is driven by powerful updrafts and downdrafts, which can create strong winds, heavy rain, lightning, and even tornadoes.
* The top of a cumulonimbus cloud can spread out into a flat, anvil-shaped structure called a cumulonimbus capillus. This is formed when the rising air reaches the tropopause (the boundary between the troposphere and stratosphere), and spreads out horizontally.
While other cloud types can reach high altitudes, they typically don't exceed 13 km:
* Cirrus clouds are high-altitude clouds made of ice crystals, but they are thin and wispy, and usually found below 13 km.
* Altocumulus clouds are mid-level clouds, also composed of ice crystals, but they are generally found between 2 and 7 km in altitude.
Therefore, the Cumulonimbus cloud is the only one that can reach altitudes of over 13 km.
