Thunder is the sound made by lightning during a thunderstorm. Lightning is a huge discharge of electricity. As a lightning bolt shoots through the air, the intense heat causes the air around it to expand extremely quickly. The air particles vibrate violently, creating shock waves that make the loud noises we call thunder. If you are near a lightning strike, you hear thunder as a really loud crack. This is the direct sound of the lightning. Farther away, thunder sounds more like a loud, long rumble. This happens when thunder bounces off objects—common in towns and cities where there are lots of buildings, but the echoes can also be heard off bare ground in the countryside. The shock waves from the different forks of lightning may also bounce off each other, as well as the clouds, to add to the rumbling noise of thunder.

Thunderstorms happen when the atmosphere is unstable. This is when a mass of warm air collects underneath much colder air, often after a bout of hot, dry weather. As the warm air rises, it cools and condenses forming small droplets of water. If the updraft of warm air is rapid, towering cumulonimbus clouds quickly form. The water droplets grow larger and freeze at high altitudes. Eventually they become too heavy to stay in the air and they fall as hail. As the hailstones shower down through the cloud, they collide with smaller ice crystals and become charged with electricity. A negative charge forms at the base of the cloud where the hail collects.

​​​​​​​The electricity builds up until it is suddenly released as a great flash of lightning—sometimes known as a lightning strike or lightning bolt. It leaps from the clouds (negatively charged) to the ground (positively charged) in a few thousandths of a second.