The Sound of Thunder

Clap...Peal...Rumble and Roll....
Why Does Thunder Sound So Different From Time-to-Time?

by Steve Horstmeyer, Meteorologist, Cincinnati, Ohio, USA

Cumulonimbus and Lightning

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Fulgura Frango

The inscription on many church bells in Medieval Europe was the Latin Fulgura Frango (I break up the lightning flashes), because the belief was that ringing church bells would dissipate lightning. In a 33 year period, 103 German bellringers were killed doing this, if they had only known what was flowing their way down the lightning channel from the thunderstorm above.

Billions and Billions of Electrons (with apologies to Carl Sagan)

What was surging towards the bellringers was electrons. Moving electrons is what an electric current is and they flow from a thunderstorm to earth in a lightning bolt.

Charge Distribution in a Thunderstorm
Within the thunderstorm when a small in-cloud flash occurs between the small positive region near the base and the negative region above, electrons begin to flow downward and accelerate towards the surface. The progress is not smooth, but surges ahead in straight segments, 10 to 300 yards long in a path that is barely visible. This is called the STEPPED LEADER and it pushes at more than 300,000 miles per hour (150,000 meters per second), rapidly heating the air (to 20,000 to 35,000K) as it goes. Depending on the size of the channel it cools to between 2000 and 8000K.

When the downward surge gets close enough the RETURN STROKE proceeds upward, from the ground or from the top of any close object. When the two meet the channel becomes highly luminous as the air molecules are ionized as the air is again heated to as much as 30,000K. Following the return stroke several DART LEADERS may follow the channel.

The entire process is called a FLASH and takes about .2 seconds. Each flash is made up of 3 to 4 strokes and the strokes are separated by maybe 40 microseconds (40 millionths of a second). Sometimes a flash appears to flicker and in those cases you are seeing the individual strokes.

Why The Crooked Path

All questions do not have answers, this is one of them. The best educated guess scientists have is that unevenly distributed pockets of positive and negative charge push and pull the stepped leader as it heads for the ground.

Back to Thunder

Each straight segment of the strokes of a flash heat the air and produce a shock wave...BUT because the electrical discharge occurs so quickly the shock wave you hear is essentially a single shock wave. Each straight segment produces a shock wave that travels outward. And many things affect the sound you hear.

A clap is a loud sudden sound and is also called a peal. A rumble or a roll is a prolonged sound not quite as loud. Claps usually come from nearby straight segments. Less of the enegry of the shock wave has been absorbed by the atmosphere than shock waves from segments farther away.The individual near the bottom of the diagram hears a clap followed by a rumble. The individual in the upper right quadrant hears a longer, lower rumble and may not hear a clap at all.

Shock Wave Animation

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2008 Steven L. Horstmeyer, all rights reserved

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Cumulonimbus and Lightning