thunder, sound produced along a path of a lightning flash, caused by the rapid heating and expansion of the adjacent air; lightning can heat air to temperatures as much as five times hotter than those at the surface of the sun. Rolling thunder occurs either as a result of the time difference between sounds from the far and near end of a flash, or when mountains, layers of air, or other obstructions cause reverberations. Since sound travels about 1 mi in 5 sec, the distance between a lightning flash and an observer may be determined by counting the seconds between the flash and the thunder. Thunder as far distant as 10 to 15 mi (15 to 25 km) from an observer is usually not heard, even though lightning is often seen. See thunderstorm.