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Thunder: Voice of the Heavens
Thunder: the natural (non-human) sound most recognized by people across the planet. It lends its name to one of the most common weather phenomenon on Earth -- the thunderstorm -- and its associated elements: the thunderhead, thunder cloud, thunder bolt, thundershower and thunder clap.
Early humans believed thunder came from the deities -- the voice and expression of their god(s). The roster of thunder gods includes: Thor of the Scandinavians, Donar of the Germans, Zeus of the Greeks, Jupiter of the Romans, Taranis of the Celts, Perkunis of the Slavs, Indra of the Indians and Shango of the Nigerian Yoruba. Each is know to throw thunderbolts or bundles of lightning at the earth while their voices reverberates across the heavens.
Many early cultures believed thunder was an omen. For example, the Greeks thought thunder on the right was a good omen; however, the Romans regarded thunder on the left as favourable. Both agreed that thunder in the east was more favourable than thunder in the west -- perhaps because, since weather generally moves from west to east, thunder to the east means the tempest has passed.
Various other cultures recognize a thunderbird as responsible for thunder and lightning. The power of the thunderbird is frequently found in the legends of native American nations and African tribes. For example, the Bantu of south Africa believed that thunder resulted from the beating of the wings of Umpundulo as the bird dove toward the earth.
Early Scientific Theory
Eventually, people began to realize that thunder (and lightning) had natural causes that could be explained through observation and logical deduction. The earliest know scientific theories of thunder were proposed by the Greeks: Anaximander (ca 611-547 BC) and Anaximenes (ca 585-528 BC), both followers of the great Greek natural philosopher Thales (ca 600 BC). They both believed that air smashing against the clouds caused thunder and, as the air struggled its way through the clouds, it kindled a flame which was lightning.
Anaxogoras (ca 499-427 BC), however, believed that thunder resulted when fire flashing through clouds (lightning) was quenched by the water in the cloud. Democritus (ca 460-370 BC), on the other hand, suggested that thunder and lightning were due to the unequal mixing of particles within clouds which caused violent motions, the resulting sound of which was thunder.
Aristotle (384-322 BC), in his series of essays entitled Meteorologica (written around 334 BC), wrote that thunder occurs when
"the dry exhalation that gets trapped when the air is in the process of colliding is forcibly ejected as the clouds condense and in its course strikes the surrounding clouds, and the noise caused by the impact is what we call thunder...similarly the windy exhalations in the clouds produces thunder when it strikes a dense cloud formation. Different kinds of sound are produced because of the lack of uniformity in the clouds and because hollows occur where their density is not continuous."
He also wrote that, in general,
"the ejected winds burns with a fine and gentle fire and it is then what we call lightning, which occurs when the falling wind appears to us to be coloured. Lightning is produced after the impact and so later than thunder, but it appears to us to precede it because we see the flash before we hear the noise."
The Roman poet Lucretius in his classic treatise ( written around 60 BC) on the universe, Of the Nature of Things, could not arrive at a single cause for thunder. For one of the causes of thunder, he supposed that:
With thunder are shaken the blue deeps of heaven,
Lucretius felt that thunder could also be caused by the sound of hail and ice within the cloud crashing together or by the bursting of a cloud when the wind trapped inside escaped, like the popping of a balloon. He reasoned that lightning occurred simultaneously with thunder, the result of sparks thrown from the collision or bursting of clouds.
The Riddle Solved
"First let me talk with this philosopher -- what is the cause of thunder?" William Shakespeare's King Lear
Those early theories of thunder held for over two thousand years. Even such astute observers of the weather and nature as Rene Descartes claimed thunder was the resultant sound of clouds colliding. Descartes suggested in 1637 that thunder resulted when higher clouds descended onto lower ones, the sound of their collision reverberating in the air space between the cloud layers.
In the mid-seventeenth century, British physicist Robert Hooke deduced that the duration of thunder was dependent upon the distance between the lightning stroke and the observer. J.N. De L'Isle measured the time delay between observing the lightning flash and hearing the thunder. His 1738 paper noted that thunder was rarely heard from lightning discharges occurring more than 25 km (15 miles) away. Over 150 years later, L.C. Veenema would observe nearly every thunderstorm occurring in his area from 1895 to 1916 to determine how far thunder could be heard. His studies confirmed the work of De L'Isle that thunder generally could not be heard more than 25 km from the lightning flash although in certain instances, it could be heard up to and beyond 100 km (60 miles).
Benjamin Franklin, who proved in his famous kite experiment that lightning was an electrical discharge, reasoned that if a spark produced in the laboratory produced a loud snap, then lightning should also produce a sound. "How loud must be the crack of 10,000 acres of electrified cloud?" he wondered.
By the end of the nineteenth century, the true relationship between lightning and thunder had finally been established, although the exact process of cause and effect was still being debated in scientific circles. These debates distilled into four common principle theories.
One prevailing theory proposed that thunder was produced when lightning, passing through the air, caused a vacuum to form. When this vacuum collapsed, the air rapidly rushing back in produced a thunderous explosion.
The steam theory of R.V. Reynolds (1903), held that the lighting bolt caused water in its path to be rapidly transformed to steam at enormous pressure and it was the rapid expansion of that steam which caused the thundering sounds.
R.S. Mershon hypothesized a third process in 1870. He believed that when lightning passed from one cloud to another, the electrical discharge decomposed water into its atomic components: hydrogen and oxygen. These elements then explosively recombined in the great heat of the lightning stroke. The explosion, of course, would be the sound of thunder.
J.A. Lyon in 1903 disagreed with the theories of both Mershon and Reynolds. He pointed out that electrical sparks produced in the laboratory through atmospheres devoid of water or explosive gases still produced a sound. Lyon's theory for thunder was similar to that proposed previously (1888) by Hirn.
In a 1888 article in Scientific American, M. Hirn advanced the theory that
"thunder is due simply to the fact that the air traversed by an electrical spark, that is, a flash of lightning, is suddenly raised to a very high temperature, and has its volume, moreover, considerably increased. The column of gas thus suddenly heated and expanded is sometimes several miles long, and as the duration of the flash is not even a millionth of a second, it follows that the noise burst forth at once from the whole column, though for a observer in any one place it commences where the lightning is at the least distance....the beginning of the thunderclap gives us the minimum distance of the lightning, and the length of the thunder clap gives us the length of the column."
Here at last we have a nearly complete answer to the thunder riddle (although the duration of a lightning flash is longer than a millionth of a second). Atmospheric studies in the twentieth century would broaden Hirn's work into the true nature of thunder.
Part 2. The Modern Theory: Thunder: A Child of Lightning
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