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Thunder's Voice
Wed May 05, 2004
Listen in RealAudio 
Thunder, weather's bold voice, is a highly complex sound. It cracks and claps, it
explodes, it peals, it rumbles through stormy skies. Hi, I'm Bryan Yeaton for The
Weather Notebook.
Thunder's pitch, loudness and form -- its crack, rumble, boom -- all depend upon the
parent lightning flash and its segment orientation relative to our ears.
Thunder's loudness and duration depend initially on the energy of the bolt's current: the
more energy, the louder and longer the thunder. The lightning channel's radius
determines the thunder's pitch: the more powerful the parent lightning stroke, the wider
the channel and lower the pitch. Thunder generally has a pitch around the lowest note
on a full keyboard piano.
Crackling thunder comes from high-pitch sound waves while long lasting, drawn-out
rumbles come from low-pitch waves.
Thunder sound waves originating from the lightning flash do not radiate with equal
strength in all directions, however. The largest lightning segments emit their loudest
sound in roughly the same direction. The orientation between segments determines
whether we hear thunder as a sudden clap or prolonged rumble.
Thunder's sound waves are also modified by the atmosphere as they travel to the
observer. They may be scattered, attenuated, refracted, or reflected, thus further
altering the volume, pitch and character.
Scattering and attenuation reduce the higher pitched thunder frequencies. Thus, after
several miles of travel from the stroke's epicenter, the predominant sound will be a
low-pitched rumble. High-pitched, cracking thunder dissipates much faster. Thus,
rumbling originating along the whole length of the lightning stroke arrives at our ears
progressively over several seconds. A crack comes from the closest lightning segment
and dissipates quickly.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is funded by the National Science Foundation and Subaru of America. Find all of our
shows online at www.weathernotebook.org.
Today's Links
Thunder: A Child of Lightning
http://www.islandnet.com/~see/weather/elements/thunder2.htm
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