Additional :;!etai]~s of the ~perature and wind structure can
be obtained by placing ~crophones near the tropopause where the velocity of sounc
is at a minimum. To our .knowledge, no one has ever tried such an experiment, ~nd
in ord~r to do this new equipment had to ·~ developed, sinca wind produces strong
nois.a in any microphone it was obvious ti•.!: t tlia detectors could not be us~d on an
aircraft 1 It was further believed that the noise level of .:on instrwn~nt. }:J:·lete anal~·sis, but tt can be stnted tho.t.
observed results ~enerally ~gree Nitt p~cdictions ~as~ct upon theory.
Observations of the t:-'avel tiuss of 'Aaves frc.u en explosive source has
yielded a considerable amount of jata on tne te;nr.erlltu!'e an1 wi..nd structare of the
at·t.osrh re u,~: to altitude of 11bour. )~ :{31 ( 160,CcC t'e~t). The i.nterpret.aticn of the
data l':as :so far been oe.sed on .seo.::1etrical ·.cr.·~·e theory, and leafis to a ·1ariation of
prora:sation velocity ·Hitt altit:.1de 'lft.ich is in reasonable &1reer.ten.t with other
lines of Q.Vidence. Th~re are, howecer, seve:al observed facts which cannot be ex-
rlained on the basis of the ele;tlentary ~eorr..et.rical ray theory, and .require a more
con11:.lete analysis in ter,~.s of ,4~.e theory. They are: --(1) the 11zones of silence",
tlu.t follow ;..ccordi!lJ to 5eoJ,,~tri~c..l ray theory fro..n tti~ initial dec1•ease of velocity
with altitude, wtdch do not .bav.: 3harply Qt#fin~d bound,e.rie S j ( 2) the SaJn8 an::o.rent
o.n~le of a.·~ival is ·:-fter: observed ovel' e± considerable range of distance from the
source, wht:reas on t.i:id ra:; tl.eo:!'y c. ~iven .::.n.:;l~ of arrival was associated with one
p&.rticuL r distance only; U) at lc.r~e d:.stances, the total duration of the si~nals re
is very much ~reater t.t.~n co.n be ex! lained by ray theory, and the character of tild
signal received is tn~;.t of a lon · train of waves ;;,f v.aryin:; a:nplitude. Wll.l !re~uency rc.
a limited numbt!r oi' well defined transient fulses.
frelilninary studies indicc.te the.t all of these facts may be eXJ::lained
qualatatively by more complete ifeve theoretical analysis of the diffraction of wave
enerzy into .the regions thr.t are zones of silence in th~ .dement< ry ray tlieory, ~nd 1'u.
work, aimed at quantative treatment is in 1-ro~ress. UntU an analysis of t.i.is Kind ha
been· carried through, one ctn not feel too IJlUCh conf'idence in att·ampts thc..t h&.ve been
made to nse lq distance sonic and microuarea1etric wave propagation d&.ta to ...teuuuce
at~os· eric temreratures at levels a~ve the second inversion.
In additiun to the theoretical aJ,• roach to tl.:.is problem, cvnddert.t i .m is
bein~ ~iven to the use of s!lrface we.ves on s.h< llo'll water as a .aode.L of Wl:.Vd pro-
pa~ation in the atmos:rh rd. Th<J velocity of surface waves whose ·r1c..ve len~th is gr~c..t~.
than the depth o1' the water is a t'unctilln of the depth, so that t.h.a variation o1 valoc
with c:ltitude in the atmos·hcre co.n be si.nulated on a thin sheet of watar by suitable
contourin · of the bottom. Surface tension and visosity s~t at a lower limi.t of (?_bout
4 em. to the wave len~ths that can ba used in such a model. tith ~ water table a~out
four fet!t wide si.tul; tin?; the at!uos~here up to 50 km. a four C<3nti:nater wave length
·.vo'.lld rer resent a wave len~th in the at!Ilosph re or about 1 mile, or c period of aoout
seconds.
2