Ask A Scientist Weather Archive

    
name         Tom
status       other
grade        other
location     VA

Question -   This question refers to David R. Cook's explanation of
thunder. (Please refer to question #170 of the weather category archives,
titled: "Making Thunder Clouds"
http://www.newton.dep.anl.gov/askasci/wea00/wea00170.htm ).  Several
times, I have been so close to a lightning strike that I heard an
electrical "zap" sound just prior to hearing the thunder. I assumed this
was the sound of the electrical discharge, and the delay in hearing the
thunder was that it took a brief time for the air column surrounding the
lightning path to be heated, expand, then collapse. If thunder is created
by the splitting of air molecules and not by the air getting heated, then
that process, and the sound it makes, should happen almost
instantaneously. Does this mean that the zap sound is caused by something
that occurs just prior to the main discharge?
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First, the phenomenon of "lightning", which causes "thunder" IS NOT
completely understood. It is a tough "apparatus" to do controlled
experiments. What is known is that lightning is due to an electrical
discharge between a cloud and the ground, or between two clouds. It is
several billion times the strength of the "zap" you get when you touch a
door knob after sliding across a rug on a dry day, or the crackle you get
sometimes when you stroke a cat.
  Second, there appears to be several atmospheric mechanisms that result
in lightning.
     Third, there is a "leader" pre-discharge. These partial breakdowns have
been observed in high speed photos of lightning. Just why and how these
occur is not known, but they appear to initiate a discharge path for the
primary lightning discharge.
     Fourth, in some cases there is a "return" stroke that runs in the
opposite direction as the initial discharge. That may be what you heard, but
I cannot be sure. The physics of how "thunder" is produced is also not fully
understood. The lightning produces a shock wave due to the rapid heating,
ionization and expansion of air in the discharge. This is called a plasma.
The implosion (collapse) of the plasma column no doubt produces another
shock wave as the ionized air is re-compressed.
     Fifth, what we "hear" is only a part of the acoustics of thunder, just
as light we "see" is only a portion of the electromagnetic spectrum. There
are no doubt sub-sonic and ultra-sonic acoustic waves produced by lightning.
I cannot find any studies on these inaudible sound frequencies.
     If you do a "Google" search on "physics of lightning" or similar search
term you can find many sites on the topic. In addition, Chapter 9, Volume II
of Richard Feynman's Lectures on Physics is devoted to lightning, and is a
concise place to start researching the topic.

Vince Calder
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Tom,

The sound produced by the explosion of the molecules travels
at the speed of sound, which, if you are at least 50 feet away,
is faster than the expansion rate of the air column.  So, thunder
reaches you faster than the expanding air column.

The zap that you heard before the lightning flash is most likely
the result of the charging and subsequent ionization of the air
column as the upward leader forms from the ground.  The upward
leader connects with the stepped leader coming down from the cloud.
The leaders have little flow of current in them, just enough to
ionize the air molecules (stripping off electrons) and produce a
path of reduced resistance for the lightning current to flow in.

Only at the time of the connection of the leaders is there significant
current flow, which explodes the molecules, producing the
lightning flash and thunder.

The leaders are only very slightly luminescent and normally can
only be detected with sensitive high speed film; you normally cannot
see them with the naked eye. However, that stripping of electrons may
produce enough sound that you hear a zap before the lightning strike
itself.

I could sometimes detect this sound when we were performing our outdoor
simulated lightning tests with a huge spark generator in Florida.
Even though I was within 50 feet of the spark in my Faraday cage, it was
often hard to distinguish the zap from the spark discharge itself,
perhaps because the spark was only 8 feet long. An upward
leader is usually about 400 feet long and would produce more sound.


David R. Cook
Atmospheric Physics and Chemistry Section
Environmental Research Division
Argonne National Laboratory
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