The roaring sound of thunder strikes fear in the hearts of some people and excitement in others. But the best policy always is if you hear thunder, get inside the closest building because lightning is nearby. In fact, lightning and thunder occur simultaneously, but since the speed of light is faster than the speed of sound, many people think that lightning happens first. Thunder is a sound caused by a shockwave during a thunderstorm. The shockwave is created when a lightning bolt hits and the air is rapidly heated and expanded around the lightning bolt. When lightning strikes, the air is heated up to 30,000 degrees Celsius. This produces acoustic shock waves. You can guess how far away the lightning bolt is by counting the seconds between the lightning flash and the following roll of thunder. The distance of lightning can be calculated by counting one mile for every 5 seconds.