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Dear Brian et al.,
It seems reasonable that the built urban environment would have an
enhanced
interaction with the low-frequency component of thunder. Taking the speed
of sound to be about 350 m/s, the approximate wavelength of various sound
frequencies is:
400 Hz 1 m
40 9 m
20 17 m
10 35 m
It seems clear that these lower frequencies (below 50 Hz or so) begin to
have wavelengths comparable to building sizes and separations. In some
cases, for some locations and some restricted frequencies this might lead
to resonant enhancement (through constructive interference), but I would
guess that more often these low frequencies would be largely damped by the
surrounding structures and spaces.
In any case, though I am unable to give a rigorous discussion in this
regard, at least from this argument we should not be surprised that there
is
something special going on at these lower frequencies in urban
environments.
Perhaps someone else can give a more rigorous discussion and better
insight.
Thunder with a french accent? Perhaps if the barrels were made of wood...
- Thunder effects on precipitation -----------------
I note Brian's comment regarding the sudden change in precipitation
intensity associated with nearby strikes/thunder claps. I also have
noticed
this, I think, and am curious to know 1) if anyone has ever tried to
verify this phenomenon (i.e. show that it is a real effect, not just the
effect of "selective memory" where we note and remember the cases where
the
rain increases just after a strike, but do not note or forget the cases
where it doesn't), and 2) if anyone has found a reason for the effect, or
has
themselves an idea about it. Can an intense sonic field passing though
rain-filled air cause water drops to coalesce? Is is possible for the
pressure fluctuations to condense more water drops out of the saturated
air
and grow the droplets large enough to perceptibly affect the precipitation
in a fwe seconds?
Certainly, if real, the effect must be quite local and very fast - there
is
no time for drops formed high in the air to change the precipitation
intensity near the observer in such close temporal proximity to the
thunder...
To begin with, of course, and to reiterate, it would be smart and it would
seem fairly simple to verify the effect by measurement. There is not much
point in fantasizing explanations for hypothetical phenomena...
Chris Luginbuhl
U.S. Naval Observatory Flagstaff Station
Flagstaff AZ
cluginbuhl_at_nofs.navy.mil
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