Photograph copyright Charles Doswell III
As mentioned in the Clouds section, thunderstorms are caused by cumulonimbus clouds. They are characterized by lightening, thunder, heavy rain, and sometimes hail.Lightning occurs when electricity builds up in thunder clouds, and then is released as lightning. It is very hot, and is shaped like a zig-zag. Lightning does not always go from the cloud down to Earth, it can also run from cloud to cloud. Lightning takes the shortest path between the sky and the ground, which often causes it to strike trees.
Thunder makes a huge booming sound. Thunder is caused by the expansion of air at high speeds. As the air expends, it causes the booming noise. Thunder can be heard from as far as ten miles away. Thunderstorms can be scary because of the loud boom.