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| Cumulonimbus clouds
often form thunderstorms. |
Thunder
Thunder is the sound caused by lightning. Depending on
the distance from and nature of the lightning, it can
range from a sharp, loud crack to a long, low rumble (brontide).
The sudden increase in pressure and temperature from
lightning produces rapid expansion of the air within and
surrounding the path of a lightning strike. In turn,
this expansion of air creates a sonic shock wave, often
referred to as a "thunderclap" or "peal of thunder". The
study of thunder is known as brontology. |
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Cause
The cause of thunder has been the subject of centuries
of speculation and scientific inquiry. Early thinking
was that it was made by deities, but the ancient Greek
philosophers attributed it to natural causes, such as
wind striking clouds (Anaximander, Aristotle) and
movement of air within clouds (Democritus). The Roman
philosopher Lucretius held it was from the sound of hail
colliding within clouds.
By the mid-19th century, the accepted theory was that
lightning produced a vacuum; the collapse of that vacuum
produced what is known as thunder.
In the 20th century a consensus evolved that thunder
must begin with a shock wave in the air due to the
sudden thermal expansion of the plasma in the lightning
channel. The temperature inside the lightning channel,
measured by spectral analysis, varies during its 50 μs
existence, rising sharply from an initial temperature of
about 20,000 K to about 30,000 K, then dropping away
gradually to about 10,000 K. The average is about 20,400
K (20,100 °C; 36,300 °F). This heating causes a rapid
outward expansion, impacting the surrounding cooler air
at a speed faster than sound would otherwise travel. The
resultant outward-moving pulse is a shock wave, similar
in principle to the shock wave formed by an explosion,
or at the front of a supersonic aircraft. In close
proximity to the source, the sound pressure level of
thunder is usually 165-180dB, but can exceed 200dB in
some cases.
Experimental studies of simulated lightning have
produced results largely consistent with this model,
though there is continued debate about the precise
physical mechanisms of the process. Other causes have
also been proposed, relying on electrodynamic effects of
the enormous current acting on the plasma in the bolt of
lightning. |
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Consequences
The shock wave in thunder is sufficient to cause
property damage and injury, such as internal contusion,
to individuals nearby. Thunder can rupture the eardrums
of people nearby, leading to permanently impaired
hearing. Even if not, it can lead to temporary deafness. |
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Perception
The most noticeable aspect of lightning and thunder is
that the lightning is seen before the thunder is heard.
This is a consequence of the speed of light being much
greater than the speed of sound. The speed of sound in
dry air is approximately 343 m/s or 1,127 ft/s or 768
mph (1,236 km/h) at 20 °C (68 °F). This translates to
approximately 3 seconds per kilometre (5 seconds per
mile); saying "one thousand and one... one thousand and
two..." is a useful method of counting the seconds from
the perception of a given lightning flash to the
perception of its thunder (which can be used to gauge
the proximity of lightning for the sake of safety). |
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 Kiddle: Thunder
Wikipedia: Thunder |
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