From: Randall Pratt 
To: psnlist 
Sent: Sat, 13 Apr 2013 20:17
Subject: Re: Seismic Signature of Tornadoes


Hi Randy,
=C2=A0
****Their comment, =E2=80=9CThe seismograms show a strong,low-frequency pul=
se
beginning [before the tornado=E2=80=99s touchdown] around 4:45 a.m. on Febr=
uary
29.=C2=A0Our preliminary interpretation, based onother seismic records of
tornadoes, suggests that we were recording not the tornado itself, but=20
a
large atmospheric pressure transient related to the large thunderstorms=20
that
spawned the tornadoes.=E2=80=9D=C2=A0
     This is in line with my thoughts. The tornado is a very small=20
portion of the
total energy of the storm.=C2=A0 Many times down flows causing microbursts=
=20
do
more local damage than tornados.=C2=A0 I don=E2=80=99t think looking at a f=
ew known
tornado records and noting noise changes can be used as evidence.=C2=A0I
would need to see specifics of how these records are different from a=20
severe
storm with no tornado on the same instruments and area.=C2=A0
=C2=A0
    I appreciate your point, but I suggest that comparing Infrasound
recordings with the seismic recordings could be a more fruitful test ?=20
It is
not disputed that cold fronts and storm cells can produce seismic=20
signals.
I am not familiar with the frequency / period ranges of their signals,=20
but
the cloud bases are several thousand feet above ground. Tornadoes
generate severe turbulence, but, critically, they are in direct contact=20
with
the ground. Consider a tornado of 220 yards diameter moving at 40 mph
-- it will cross a given point in about 11 seconds. The frequency of=20
signals
due to the pressure eddies is likely to be higher.
     Are there any figures for the sort of pressure reduction that is=20
observed
at the centre of a tornado ?

****I'll volunteer to increase my sample rate and listen to some storms=20
and
tornados over the next few months.=C2=A0 May and June generally give me mor=
e
opportunity than I desire. I have SUSD to the NW and ECSD about 80 miles
East of me so spacing is rather large.=C2=A0
=C2=A0
    Great. Whereabouts are you located, please ?

    Regards,

     Chris Chapman
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