Thunder is the sound caused by a nearby flash of lightning and can be heard for a distance of only about 10 miles from the lightning strike. The sound of thunder should serve as a warning to anyone outside that they are within striking distance of the storm and need to get to a safe place immediately! Thunder is created when lightning passes through the air. The lightning discharge heats the air rapidly and causes it to expand. The temperature of the air in the lightning channel may reach as high as 50,000 degrees Fahrenheit, 5 times hotter than the surface of the sun. Immediately after the flash, the air cools and contracts quickly. This rapid air expansion and contraction creates the sound wave that we hear as thunder.

Source: National Weather Service