A thundercloud is a tall, dark cloud that produces thunder, lightning, and heavy rain. These massive clouds tower high into the sky like giant mountains, sometimes reaching heights of 40,000 feet or more. The technical name for a thundercloud is cumulonimbus, but most people just call them thunderclouds or storm clouds.

Inside a thundercloud, powerful winds rush up and down, carrying water droplets and ice crystals that crash into each other. This collision creates static electricity, the same force that makes your hair stand up when you rub a balloon on it. When enough electricity builds up, it releases as a lightning bolt, and the sudden heating of air creates the booming sound of thunder.

You can often see a thundercloud approaching from miles away. The bottom appears dark and menacing, sometimes with a greenish tint, while the top spreads out like an anvil. When you spot one building on a summer afternoon, it's wise to head indoors, since these clouds can unleash dangerous lightning, heavy rain, strong winds, and sometimes hail or tornadoes.