A cumulonimbus is a towering, powerful storm cloud that creates thunder, lightning, and heavy rain. These massive clouds can stretch from close to the ground all the way up to where airplanes fly, sometimes reaching heights of 10 miles or more.

A cumulonimbus looks like a giant cauliflower or a mountain made of cotton, often with a flat, anvil-shaped top where it hits the upper atmosphere and spreads out.

These clouds form when warm, moist air rises rapidly and keeps climbing higher and higher. As the air rises, it cools and the water vapor condenses into a cloud. This process releases energy, which makes the air rise even faster, building the cloud bigger and more powerful. Inside a cumulonimbus, strong updrafts and downdrafts create the conditions for lightning, hail, and sometimes tornadoes.

When you see dark cumulonimbus clouds building on a summer afternoon, that's nature's warning that a thunderstorm is coming. Pilots avoid flying through them because of dangerous turbulence, and meteorologists track them carefully because they can produce severe weather.