Here's why:

* Rising Air: Cumulonimbus clouds form when warm, moist air rises rapidly. This rising air creates an area of low pressure below the cloud.

* Convection: The upward movement of air within the cloud is driven by the instability created by warm, buoyant air parcels.

* Storms: These clouds are responsible for thunderstorms, which are characterized by strong updrafts (rising air) and downdrafts (falling air).

Think of it this way: Imagine a balloon. When you blow air into it, the pressure inside increases. The opposite happens with cumulonimbus clouds. As warm air rises, it creates a low-pressure area below the cloud, allowing more air to be drawn in and fuel the storm.