The composition of a thunderstorm

Many composers have waved their batons to produce a thunderstorm

Many composers have waved their batons to produce a thunderstorm. Beethoven's Sixth Symphony, for example, the Pastoral, describes in vivid detail the progression of a brief, but violent, thunderstorm.

Hector Berlioz prescribes ominous rolls on the tympani in the third movement of his Symphonie Fantastique to the same end; Johann Strauss jnr wrote the Thunder and Lightning Polka; the fourth section of Rossini's William Tell Overture, reminiscent of the "Lone Ranger", is followed by a less well known passage which depicts a violent storm.

The sudden rise in temperature accompanying a lightning stroke causes the air in the immediate vicinity of the incandescent channel to expand rapidly.

This expansion causes a compression wave that has all the characteristics of a common sound wave, and which travels outwards radially from the flash. We hear it as a peal of thunder, but unlike the light which travels almost instantaneously, the sound wave moves through the atmosphere at a mere 700 m.p.h.; it takes five seconds to travel every mile.

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The sound heard by a listener varies depending on their position relative to the thunderstorm. A bolt of lightning follows a jagged zigzag path that is, in effect, a meandering channel of small, straight segments of superheated air. The successive segments can be thought of almost as a string of beads, with sound emanating simultaneously from each bead.

Thunder from the "beads" furthest away is not only slower to reach the listener's ear, but is also modified by the atmosphere. As a result, the noise from the closer beads is heard as a sharp clap of thunder, but that from distant segments is more muffled.

To add to the cacophony, sounds waves travelling perpendicularly to the individual segments of the lightning path in which they had their origins, are of greater intensity - and therefore louder - than those that leave the path obliquely. Finally, sound waves from segments near and far amalgamate while heading towards the listener, so they hear something like a near-continuous roll with a variety of sharp cracks and low rumbles interspersed.

Further acoustic complications are introduced by the fact that a large thunderstorm may consist of several "thundercells", each an independent zone of thundery activity.

One cell may travel past an observer, only to be followed shortly afterwards by another heading in the same direction. This procession often produces the illusion of an oscillating or rotating thunderstorm.