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Thunder: Voice of Summer Clouds

© Keith C. Heidorn

For much of North America, thunder is the main voice of summer weather. Thunder lends its name to some of the most common weather phenomenon on Earth: the thunderstorm and its associated elements: the thunderhead, thunder bolt, thundershower and thunder clap.

Do you remember the first time you heard thunder and asked what that loud noise was? I recall being told it was the sound of "God bowling." Although I accepted the answer at the time, I am not sure I really knew then what "bowling" was! Today, I know the cause of thunder, but still the thrill and excitement remain, perhaps more so because by watching the lightning bolt spawning the thunder, I can anticipate the subtle nuances of the thunder peal to come.

Thunder's Cause

What causes thunder? The thunderous process starts with two ingredients: lightning and the surrounding air. Each lightning flash, an electrical current, superheats the surrounding air, which has electrical resistance, to temperatures hotter than the sun's surface, up to 30,000^o Celsius. Such rapid heating causes an instantaneous pressure increase to 10 to 100 times normal atmospheric pressure forming a shock wave radiating at speeds exceeding the speed of sound.

The shock wave quickly "relaxes," and the air pressure surrounding the lightning channel returns to normal. In relaxing, the shock wave produces an acoustic, or sound, wave that moves perpendicular to the lightning segments (mesotortuous segments)-- those kinks, twists and jags seen in the lightning bolt -- that produced it.

Thunder is one of nature's loudest sounds. A nearby thunderclap may reach 120 decibels, equivalent to being within 200 feet (60 metres) of a jet aircraft during take-off. Sounds of 140 decibels are considered painfully loud. (Note, each 10 decibel increase in noise level sounds twice as loud.)

Thunder Variations

Thunder, however, is more than a simple, loud explosive sound. Thunder cracks and claps. Thunder rolls and rumbles through a stormy sky. Thunder peals; it explodes.

What accounts for this wide variety of sounds that we call thunder? Thunder's pitch, loudness and form (crack, rumble, etc.) all depend upon the nature of the parent lightning flash and the orientation of its segments relative to our ears.

The loudness and duration of the resulting thunder depend on the energy of the lightning bolt's current: the more energy available, the louder and longer the thunder. The radius of the initial shock wave channel determines the resulting thunder's pitch: the more powerful the parent lightning stroke, the wider the channel and the lower the thunder's pitch. Thunder generally has a pitch either between 15 and 40 hertz or between 75 and 120 hertz. (The lowest note on a full keyboard piano has a tuned pitch of 66 hertz.)

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