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Tuesday, 13 June 2017

Thunder

Thunder is the sound caused by lightning .
Depending on the distance and nature of the
lightning, thunder 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 surrounding and within a
bolt of lightning. In turn, this expansion of air
creates a sonic shock wave, similar to a sonic
boom , which produces the sound of thunder,
often referred to as a clap , crack , peal of
thunder, or boom .
Cause
The cause of thunder has been the subject of
centuries of speculation and scientific inquiry .
The first recorded theory is attributed to the
Greek philosopher Aristotle in the fourth
century BC, and an early speculation was that
it was caused by the collision of clouds .
Subsequently, numerous other theories were
proposed. By the mid-19th century, the
accepted theory was that lightning produced a
vacuum .
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. [1] 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). [2] 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, [3] similar in principle to the
shock wave formed by an explosion , or at the
front of a supersonic aircraft .
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. [4][1] Other causes have also been
proposed, relying on electrodynamic effects of
the massive current acting on the plasma in
the bolt of lightning. [5] The shockwave in
thunder is sufficient to cause injury, such as
internal contusion, to individuals nearby. [6]
Inversion thunder results when lightning
strikes between cloud and ground occur
during a temperature inversion. In such an
inversion, the air near the ground is cooler
than the higher air. The sound energy is
prevented from dispersing vertically as it
would in a non-inversion and is thus
concentrated in the near-ground layer.
Inversions often occur when warm moist air
passes above a cold front; the resulting
thunder sound is significantly louder than it
would be if heard at the same distance in a
non-inversion condition.

Etymology
The d in Modern English thunder (from earlier
Old English þunor) is epenthetic, and is now
found as well in Modern Dutch donder (cp
Middle Dutch donre , and Old Norse þorr , Old
Frisian þuner , Old High German donar
descended from Proto-Germanic *þunraz ). In
Latin the term was tonare "to thunder". The
name of the Germanic god Thor comes from
the Old Norse word for thunder. [8]
The shared Proto-Indo-European root is * tón-r̥
or *tar- , also found Gaulish Taranis and Hittite
Tarhunt.
Calculating distance
A flash of lightning, followed after some time
by a rumble of thunder, illustrates the fact
that sound travels significantly slower than
light . Using this difference, one can estimate
how far away the bolt of lightning is by timing
the interval between seeing the flash and
hearing thunder. 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). [9]
This translates to approximately 5 seconds
per mile (or 3 seconds per kilometer).
The speed of light is high enough that it can
be taken as infinite in this calculation because
of the relatively small distance involved.
Therefore, the lightning is approximately one
kilometer distant for every 3 seconds that
elapse between the visible flash and the first
sound of thunder (or one mile for every 5
seconds). In the same five seconds, the light
could have traveled the Lunar distance four
times. (In this calculation, the initial shock
wave, which travels at a rate faster than the
speed of sound, but only extends outward for
the first 30 feet, is ignored.) [10] Thunder is
seldom heard at distances over 20 kilometers
(12 mi). [3] A very bright flash of lightning and
an almost simultaneous sharp "crack" of
thunder, a thundercrack , therefore indicates
that the lightning strike was very near.

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