THUNDER

sound caused by a lightning discharge. The discharge heats and expands the air around it and forms a compression wave that is heard as thunder. The return stroke of the lightning flash produces a much louder sound than does the leader, because it transmits a more powerful electric current. Because the lightning channel is usually quite long, the thunder from the more distant portions arrives later than that from the nearer portions; this accounts for its duration. The distance to the flash may be roughly estimated by measuring the time lag between it and the thunder and allowing about five seconds per mile (three seconds per kilometre). Thunder is seldom heard at distances greater than about 15 miles (24 km). See also thunderstorm.