Cumulonimbus

Height: These clouds build in a vertical direction.

Shape: These are flat, dark puffy clouds. These are the worst clouds you can have.

Color: Cumulonimbus clouds are dark and black clouds.

Weather: The worst weather happens when there are cumulonimbus clouds in the sky. The weather is severe.

Source for picture: http://www.scienceclarified.com/images/uesc_03_img0136.jpg