A thunderstorm is a rainstorm with lightning and thunder. During a thunderstorm, electricity builds up inside tall clouds until it suddenly flashes as lightning, heating the air so fast it expands explosively, creating the booming sound we call thunder.

Thunderstorms form when warm, moist air rises quickly into cooler air above. The rising air creates towering clouds that can stretch miles high. Inside these clouds, water droplets and ice crystals bump into each other, creating static electricity (the same force that makes your hair stand up when you rub a balloon on it, but much more powerful). When enough electricity builds up, it discharges as a lightning bolt, which can strike the ground or jump between clouds.

You always see lightning before you hear thunder because light travels almost instantly while sound moves much slower: about one mile every five seconds. If you count the seconds between the flash and the boom and divide by five, you'll know roughly how many miles away the storm is.

Thunderstorms can be exciting to watch from a safe place indoors, but they can also be dangerous. Lightning can strike people, trees, and buildings. Strong thunderstorms sometimes bring heavy rain, hail, strong winds, or even tornadoes. That's why weather forecasters track thunderstorms carefully and why you should head inside when you hear thunder rumbling in the distance.