Why do we hear thunder after lightning?

Lightning produces both a bright flash and a rapid expansion of heated air. The light reaches our eyes almost instantly because it travels at about 300,000 kilometers per second. Sound, however, moves through air at only about 343 meters per second at room temperature. That means thunder arrives later, and the delay can be used to estimate storm distance: about 3 seconds per kilometer, or roughly 5 seconds per mile. The thunder itself comes from shock waves generated by superheated air around the lightning channel.