Here's why:
* Cumulonimbus means "heaped rain cloud" and describes their towering, vertical structure.
* These clouds are associated with thunderstorms and are often called thunderheads.
* Their anvil-shaped tops, which resemble rocks and towers, are formed by the powerful updrafts that carry water vapor high into the atmosphere.
* The anvil top often spreads out horizontally as the cloud reaches the tropopause, the boundary between the troposphere and stratosphere.
So, when you see clouds that look like rocks and towers, you're likely looking at a cumulonimbus cloud, which is a sign of potentially severe weather.
