Thunder is produced as a sound wave. Just as friction causes noise when you rub your hands together, massive friction between neighboring air molecules produces the loud noise. The rapid expansion and subsequent contraction of air produces a sound wave that travels out from the source in all directions. Thunder is produced along the entire length of the lightning channel where air expands rapidly. This gives a prolonged thunder that lasts for several seconds. Close lightning strikes don't sound as long because the thunder sound close to you is so loud that it overpowers the sound made when the lightning channel was further away. Also, the sound waves from overhead thunder tend to curve away from a surface observer. Thunder that originates further away has a more rumbling sound to it due to reflection, scattering and damping of the noise as it moves away from the source region into low level terrain.

Sometimes, thunder is not heard altogether. When more than 15 miles from the lightning discharge, the thunder will not make it to a surface observer since sound waves generally refract gradually away from the earth's surface due to the air density structure of the atmosphere.