What causes thunder to follow lightning during storms?

Thunder is caused by the rapid heating of air surrounding a lightning bolt. When lightning strikes, it superheats the air along its path to around 30,000 Kelvin almost instantaneously, causing the air to expand explosively. This rapid expansion creates a shockwave that we hear as thunder. The reason thunder follows lightning is simply the difference in speed between light and sound. Light travels at about 300,000 kilometers per second, so we see the flash almost instantly. Sound travels much slower, roughly 343 meters per second, so the thunder takes time to reach us. You can estimate how far away a storm is by counting the seconds between the lightning flash and the thunder, with every five seconds representing about one mile of distance.