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A survey of methods of sizing andcounting water droplets in clouds
Item Type text; Thesis-Reproduction (electronic)
Authors Allard, Frederick Charles, 1943-
Publisher The University of Arizona.
Rights Copyright © is held by the author. Digital access to this materialis made possible by the University Libraries, University of Arizona.Further transmission, reproduction or presentation (such aspublic display or performance) of protected items is prohibitedexcept with permission of the author.
Download date 17/05/2021 16:17:36
Link to Item http://hdl.handle.net/10150/318334
A SURVEY OF METHODS OF SIZING AND COUNTING
WATER DROPLETS IN CLOUDS
by
F red e ric k C harles A lla rd
A T h esis Subm itted to th e F a c u lty o f th e
COMMITTEE ON OPTICAL SCIENCES
In P a r t i a l F i l f i l lm e n t o f th e Requirem ents • For th e Degree o f
MASTER OF SCIENCE
In th e G raduate C ollege
THE UNIVERSITY OF ARIZONA
1969
STATEMENT BY AUTHOR
This th e s i s has been subm itted in p a r t i a l f u l f i l lm e n t o f r e quirem ents fo r an advanced degree a t The U n iv e rs ity o f A rizona and i s d e p o sited in th e U n iv e rs ity L ib ra ry to be made a v a ila b le to borrow ers under ru le s o f th e L ib ra ry .
B rie f q u o ta tio n s from t h i s th e s i s a re a llow ab le w ithou t s p e c ia l p e rm iss io n , p rov ided th a t a c c u ra te acknowledgment o f source i s made. Requests fo r p e rm issio n fo r extended q u o ta tio n from o r rep ro d u c tio n o f t h i s m anuscrip t in whole o r in p a r t may be g ran ted by th e head o f th e m ajor departm ent o r th e Dean o f the G raduate C ollege when in h is ju d g ment th e proposed use o f th e m a te r ia l i s in th e i n t e r e s t s o f s c h o la r sh ip . In a l l o th e r in s ta n c e s , however, pe rm issio n must be o b ta in ed from th e au th o r.
SIGNED: 'jw L u u x J r C . r f A ' t U
APPROVAL BY THESIS DIRECTOR
This th e s i s has been approved on th e d a te shown below:
"fe. S m u Uk____________ orw. /k..B. R. FRIEDEN Date
A s s is ta n t P ro fe sso r o f O p tica l Sciences
ACKNOWLEDGMENT
The a u th o r would l ik e to ex p ress h is g ra t i tu d e to h is a d v iso r .
Dr. B. R. F ried e n , and to Mr. C. Blenman fo r t h e i r guidance and encour
agement d u rin g th e p re p a ra t io n o f t h i s t h e s i s . Thanks a re a lso due to
Dr. P. N. S l a t e r , f o r h is su g g es tio n s and a s s is ta n c e in p re p a rin g th e
f in a l d r a f t o f t h i s t h e s i s .
In a d d it io n , th e au th o r would l ik e to thank th e v a rio u s o th e r
members o f th e f a c u l ty and s t a f f o f th e O p tic a l Sciences C en ter who co
o p e ra ted in t h i s v e n tu re . .
The au th o r g r a te f u l ly acknowledges th e U. S. N. U nderw ater Sound
L aboratory fo r i t s su p p o rt o f th e a u th o r 's program o f s tu d y le ad in g to
th e M. S. deg ree . . Thanks a re e s p e c ia l ly due to Mr. L. J . Free and v a r i
ous o th e r members o f th e s t a f f o f th e L abora to ry fo r t h e i r c o n tin u a l- X
support and fo rb e a ra n c e .
TABLE OF CONTENTS
LIST OF ILLUSTRATIONS . . . . . . . . . . . . . ............................... v i
ABSTRACT . . . . ' ................................................ v i i
I . INTRODUCTION . ............................................ 1
I I . IMPACTION SAMPLING .......................... 4
Problem a reas . . . .................. ...................... . . . . . . . . . . . 4C o lle c tio n E f f i c i e n c y .................. .... . . . . . . . • ' ..........................4S h a tte r in g and C oalescence . . . . . . . ................................... 6E vapora tion ............................................ ■ . . . 6
C apture Methods . . . . . . . . . . . . . . . . ............................... 7
Development o f C apture Methods .................. 8Method o f Fuchs and P e t r j a n o f f ............................................ 8M o d ific a tio n Due to M a y ...................... 8C oating Developments . . . ....................................................................... 9" I s o k in e t ic Flow" and May's Cascade Im pactor .................. . . . 9D iscu ssio n o f May's Technique . . . . . . . .................. . . . 10
R e p lic a tio n Methods ̂ 12
Comparison w ith C apture Methods . . . . . . . . .................. 12
Development o f R e p lic a tio n Methods ........................... 13Magnesium Oxide Methods ...................... . ........................13
May's C a l ib ra t io n . .............................. 13Method o f F r i th .................. .... , . . •................................ 14Method o f S qu ires and G il le s p ie .................. . . . . . . . 14
. M o d ifica tio n Due to S q u ires . . . . . . . . . . . . . . 15C a lib ra t io n o f S in g le to n and Smith . . . ............................16Method o f Clague . . . . . . . . . . . . . . . . . . . 17
Formvar M ethods. . ......................................................... 17Method o f MacCready and Todd ...................................................... 17O ther Developments ............................... . . . . . . . . . . 18
M iscellaneous R e p lic a tio n Methods . . . . . . . . . . . . 18D rop le t Charge Methods » . ............................18H ygrophotographic Method .................. . . . . . 20D y e -in -G e la tin Method ...................... 20Phase C o n tra s t Method . . . . . . . . . . . . . . . . . 20.
Summary o f Im paction Sampling . . . . . . .......................... 21
iv
V
TABLE OF CONTENTS--Continued ,
I I I . OPTICAL SAMPLING............................................ 22
Comparison w ith Im paction Sam pling. . .......................... 22Corona M e th o d ............................................................. 22D ire c t Photography . . . . ....................................................................... 23
An E le c tro -O p tic a l Approach . . . . . . .. . . . ............................24
F raunhofer D if f r a c t io n Methods . . ...................... 24The "L aser Fog D i s d r o m e t e r " ............................ •......................................24Method o f Oura and H o r i .............................. 25Method o f Thompson and O t h e r s ...................... 27
S p e c tra l A tte n u a tio n Method . - ........................................................................ 27Shadow Method .............................................................. 30
S in g le S c a t te r in g Methods . . ' ................... 31
The P a r t i c l e C ounter ...................... 32M ik iro v ’s Method . . . . . . . . ' . ■. . . . . . . . . . . .. .. . 34L ak tio n o v 's E x tension o f M ik iro v 's Method ...................... 36K azas1 Refinem ent o f L ak tio n o v 's Method . . . . . . . . . 36F u rth e r Work by Kazas and O thers . 37Method o f Konyshev and Laktionov . . . . . . . . . . . . . 37
T h e o re tic a l Developments . . . . . . . . . ................... . . . . 40
Work o f S h if r in and K olm akov.............................. . 40Work o f H odkinson 'and G re en fie ld ....................... 40F raunhofer A pproxim ation o f Hodkinson . . . . . . . . . . 40Q u enze l's Work w ith Mie Theory . . . . . . . . . . . . . . 41
IV. DISCUSSION . . . . . . . . . . . . . . 43
REFERENCES . . . . . . . . . . . . . . . ......................................46
LIST OF ILLUSTRATIONS
F igure Page
1. Block Diagram o f S teps in T y p ica l Im paction Methods . . . . . . 5
2. Schem atic Diagram o f May's Cascade Im pactor (May 1945) . . . .11
3. Schem atic Diagram o f Coherent Background A pparatuso f S ilverm an and O thers (1964) .......................................................... 26
4 . Schem atic Diagram o f Readout Device o f Thompson andO thers (1966)................................................ 28
5. Schem atic Diagram o f Shadow A pparatus o f N a tio n a l C en ter •fo r A tm ospheric R esearch (1968) . .................. . . . . . . . 28
6. Schem atic Diagram o f M ik iro v 's Method (M ikirov 1957) . . . . 35
7. In tak e Arrangement o f Kazas (1963) . . . . . . . . . . . . . . . 35
8 ., Schem atic Diagram Of th e Method o f Konyshev andLaktionov (1966) . . . . . . . . . . . . . . ...................... 39
» •
/ ■
v i
ABSTRACT
V arious methods fo r sam pling n a tu r a l cloud d ro p le ts are review ed
w ith p a r t i c u l a r re fe re n c e t o - t h e i r s u i t a b i l i t y fo r use on a i r c r a f t . For
convenience th e methods a re d iv id e d in to two c a te g o r ie s : im paction sam
p lin g and o p t ic a l sam pling .
The o ld e r m ethods, which a re s t i l l in u se , in v o lv e th e im paction
o f w ater d ro p le ts on s u i ta b ly p rep a red t a r g e t s . Such t a r g e t s may cap
tu re th e d ro p le ts in an o i l c o a tin g o r may r e p l ic a t e th e d ro p le ts by
means o f p i t s in a so o t c o a tin g . The a c tu a l d ro p le t spectrum may be de
term ined by exam ining th e t a r g e t s under a m icroscope.
W ithin th e p a s t decade and a h a l f , o p t ic a l methods have been de
veloped to re p la c e th e im paction m ethods. These m ethods, n o ta b ly th o se
in v o lv in g s c a t te r in g o r d i f f r a c t i o n , a llow la rg e numbers o f d ro p le ts to
be s iz e d in much le s s tim e than i s p o s s ib le w ith th e im paction methods.
CHAPTER I
INTRODUCTION
A cloud i s n o t a s t a t i c grouping o f w ater d r o p le t s , b u t a dy
namic , ev er-chang ing s e t o f m inute w ater p a r t i c l e s . The fo llo w in g
review o f th e "dynamic" concept o f "c loud" w i l l be h e lp fu l in under
s tan d in g why th e m e te o ro lo g is t i s in te r e s te d in cloud d ro p le t d i s t r i
b u tio n s . A p p rec ia tio n o f t h i s concept w il l a lso a id in u n d e rs tan d in g
th e problem s encoun tered in th e a c tu a l o b se rv a tio n o f c loud m icro
s t r u c tu r e .
B attan (1962) d e fin ed a c loud as a v i s ib le s e t o f m inute w ater
p a r t i c l e s in th e a tm osphere . The in d iv id u a l w a ter p a r t i c l e s may momen
t a r i l y e x i s t as m o is tu re d ro p le ts o r as ic e c r y s ta l s . I t i s im portan t
to r e a l i z e t h a t th e s e t i s n o t a s t a t i c one , and h e re in l i e s th e b a s is
fo r th e "dynamic" c loud con cep t. The e lem en tary m o is tu re d ro p le ts mark
ing th e b i r t h o f a cloud a re g e n e ra lly formed about p a r t i c u l a t e conden
s a t io n n u c le i . V arious f a c to r s de term ine subsequent tra n s fo rm a tio n s o f
th e d ro p le ts . The d ro p le ts may, f o r in s ta n c e , grow by c o l l i s io n and
coalescence w ith o th e r d ro p le ts . They may become so la rg e th a t they
drop out o f th e c loud as r a in , o r th ey may, a t any s ta g e , f r e e z e . A l
te r n a t iv e ly , th ey may grow sm a lle r by e v a p o ra tio n , perhaps v a n ish in g •-
a l to g e th e r . Because th e se t r a n s i t io n s a re f a i r l y c o n tin u o u s , th e cloud
i s seen as a s e t o f w a ter p a r t i c l e s c o n s ta n tly ev o lv in g . Borovikov and
i
o th e rs (1963 t r a n s l a t i o n , p . 1, 71) co n sid e red a cloud as a "p ro cess"
in v o lv in g a s e t o f w a te r p a r t i c l e s , where th e p ro p e r t ie s o f th e s e t are.
determ ined by te m p e ra tu re , h u m id ity , a i r v e lo c i ty , and o th e r f a c to r s .
As th e cloud p a r t i c l e s undergo t r a n s i t i o n , th e cloud evolves
and th e number and s iz e d i s t r ib u t io n o f c loud d ro p le ts vary c h a r a c te r i s
t i c a l l y in a g iven s e c to r o f th e c loud . D uring cloud fo rm a tio n , th e
f i r s t e v o lu tio n a ry s ta g e , th e c loud c o n s is ts o f r e l a t i v e l y sm all d rop
l e t s w ith a narrow s iz e d i s t r i b u t io n . With f u r th e r developm ent, th e
d ro p le ts grow la r g e r and th e s iz e d i s t r ib u t io n w idens. And f i n a l l y , as
th e cloud d i s s ip a t e s , by ev ap o ra tio n o r p r e c ip i t a t i o n , th e mean d ro p le t
s iz e d ecreases and th e s iz e d i s t r ib u t io n narrow s. At any s ta g e o f de
velopm ent, th e mean d ro p le t s iz e may be expected to in c re a s e w ith an i n
c rease in r e l a t i v e h e ig h t w ith in a c lo u d , ex cep t n e a r th e to p . I n t e r
e s t in g ly , d ro p le t d i s t r ib u t io n s do n o t u n iq u e ly s p e c ify th e cloud type
i f th e r e l a t i v e h e ig h t w ith in th e c loud a t which th e sample was taken
and th e le v e l o f c loud developm ent a re ig n o red (Borovikov and o th e rs 1963
t r a n s l a t i o n , p . 70; Mason 1957, p . 9 9 ). In o th e r w ords, to sp e c ify th e
le v e l o f cloud developm ent in term s o f d ro p le t d i s t r ib u t io n s i t i s n ec
e ssa ry to know th e type o f cloud in which th e d ro p le t sam ples were tak en
and to know th e r e l a t i v e h e ig h t w ith in th e c loud a t which th e d ro p le t
samples were tak en . '
D rop le t d i s t r i b u t io n s , th e n , a re dependent on th re e p r in c ip a l
f a c to r s : r e l a t i v e h e ig h t w ith in a c lo u d , c loud ty p e , and s ta g e o f cloud
developm ent. The f i r s t two f a c to r s may r e a d i ly be determ ined by d i r e c t
o b s e rv a tio n , b u t th e t h i r d f a c to r , i t seem s, may be q u a n t i ta t iv e ly de
term ined on ly in term s o f thq d e n s ity and r e l a t i v e s iz e d i s t r ib u t io n o f
3
cloud d r o p le t s , e s p e c ia l ly fo r in te rm e d ia te s ta g e s o f c loud developm ent.
Then, i f th e f i r s t two f a c to r s a re known, th e d ro p le t d i s t r ib u t io n may
serv e as an in d ic a to r o f th e s ta g e o f developm ent o f a c lo u d . M eteo ro l
o g is ts s tu d y in g w eather m o d if ic a tio n te ch n iq u es a re th u s in te r e s te d in
d ro p le t d i s t r ib u t io n d a ta as an in d ic a to r o f th e e f f e c t s o f exp erim en ta l
changes in d ro p le t environm ent on th e le v e l o f c loud developm ent. In a
c lo u d -seed in g experim en t, fo r exam ple, th e s ta g e s o f developm ent b e fo re
and a f t e r seed in g may be compared by means o f d ro p le t d i s t r ib u t io n s in
o rd e r to determ ine th e e f f e c t s o f th e seed in g chem icals and seed ing
method.
V arious techn iques--som e o f them q u ite in g e n io u s—have been de
veloped fo r th e s iz in g and coun ting o f c loud d ro p le ts . In t h i s t h e s i s ,
th e methods a re d iv id ed in to two c a te g o r ie s . The 'impaot'Lon methods a re
f a i r l y d i r e c t , in v o lv in g th e cap tu re o r r e p l ic a t io n o f d ro p le ts subse
quent to t h e i r im paction on a s u i ta b le t a r g e t . The non-irrrpaotion methods
a re le s s d i r e c t , u t i l i z i n g o p t ic a l tech n iq u es to rem o te ly count and s iz e
d ro p le ts p a ss in g th rough a p ro b e . B r ie f d e s c r ip t io n s o f some o f th e
o ld e r methods o f s iz in g and coun ting c loud d ro p le ts may be found in Bor
ovikov and o th e rs (1963 t r a n s l a t i o n , c h a p te r X I), and in Mason (1957,
c h ap te r I I I ) . The su rvey p re se n te d in t h i s th e s i s n e c e s s a r i ly in c lu d e s
more re c e n t methods th an found in e i t h e r o f th e above re fe r e n c e s - - th e
in te n t o f t h i s su rvey be in g th e c r i t i c a l comparison o f th e v a rio u s meth
ods re p o r te d in th e l i t e r a t u r e .
CHAPTER II
IMPACTION SAMPLING
The d i r e c t methods re q u ire th a t th e d ro p le ts im pact on a s p e c ia l
t a r g e t where th ey may be cap tu red o r o th e rw ise c o n tac te d long enough fo r
a re c o rd o f t h e i r s iz e s to be o b ta in e d . For reaso n s o f convenience, me
te o r o lo g is t s p r e f e r to sample clouds from a i r c r a f t , and th e d ro p le t im
p a c t v e lo c i t i e s encoun tered a re g e n e ra lly in excess o f 50 m /sec: This
th e s i s w i l l , th e r e f o r e , concern i t s e l f m ainly w ith th e advantages and
d isad v an tag es o f th e v a rio u s methods p e r t in e n t to th e sam pling o f d rop
l e t s from fix ed -w in g a i r c r a f t .
I t w i l l be seen th a t th e im paction m ethods--alm ost w ith o u t excep-
t io n - - r e q u i r e te d io u s coun ting and s iz in g o f th e d ro p le ts o r t h e i r r e p
l ic a t io n s under a m icroscope. With Borovikov and o th e rs (1963 t r a n s l a
t io n , p . 76) c la im in g th a t about 10,000 d ro p le ts would be a reaso n ab le
minimum number fo r th e a cc u ra te r e p re s e n ta t io n o f a re g io n o f a c lo u d ,
t h i s job can be q u ite te d io u s in d eed . The procedure i s shown in F ig . 1.
Problem Areas
C o lle c tio n E f f ic ie n c y
One o f th e f a c to r s th a t a d v e rse ly a f f e c t th e accuracy o f th e se
methods i s " c o l le c t io n e ff ic ie n c y ,V which Mason (1957, p . 85) d e fin e s
as th e f r a c t io n o f th e d ro p le ts ly in g in th e p ro je c te d c ro ss s e c tio n o f
th e t a r g e t which a re a c tu a l ly im pacted th e re o n . I d e a l ly , c o l le c t io n
. 4
5
COUNTING AND SIZING (from photom icrograph)
DROPLET CAPTURE
DROPLET IMPACTION
UNEXPOSED TARGET
DROPLET REPLICATION
RETRIEVAL OF EXPOSED TARGET
COUNTING AND SIZING (under m icroscope)
DROPLET SIZE SPECTRUM
PHOTOMICROGRAPHY OF TARGET
F ig . 1. Block Diagram o f S teps in T yp ica l Im paction Methods
6
e f f ic ie n c y would be u n i ty f o r a l l d r o p le t s , b u t in a c tu a l i t y c o l le c t io n
e f f ic ie n c y i s a fu n c tio n o f d ro p le t s i z e , t a r g e t c o n f ig u ra t io n , and
a irsp e e d .
S h a tte r in g and C oalescence
The accuracy o f th e im paction methods f u r th e r depends on p re v e n t
ing ev ap o ra tio n o f th e d ro p le ts b e fo re s iz in g i s accom plished and on
avo id ing s h a t te r in g o r co a lescence o f th e im pacted d r o p le ts . S h a tte r in g
o f d ro p le ts on im pact does n o t u s u a lly p re s e n t a problem fo r d ro p le ts
sm a lle r than 50 pm in ra d iu s a t ty p ic a l a irsp e e d s (Mason 1957, p . 85 ).
C oalescence o f d ro p le ts , can occur when a d ro p le t im pacts on o r n e a r a
p re v io u s ly , im pacted d ro p le t and i s u s u a lly a problem on ly w ith th e m eth
ods in v o lv in g d ro p le t c ap tu re . T a rg e ts can be exposed fo r a s u f f i c i e n t ly
s h o r t tim e th a t th e l ik e l ih o o d o f co a lescence i s sm a ll. Mason (1957,
p . 85) c o n sid e rs an exposure tim e a llow ing th e d ro p le ts to cover 10% o f
th e t a r g e t a re a as s u f f i c i e n t l y sm a ll.
E vapora tion
t A ccording to C lausse and Facy (1961, p . 6 1 ), th e f in e r d ro p le ts
w ill ev ap o ra te quick ly i f th e a i r tem p era tu re i s r a is e d more than a te n th
o f a degree o r i f th e vapor te n s io n c h a r a c te r i s t i c s o f th e environm ent
change due to th e aerodynam ic desig n o f th e sam pling probe. A ir p re s su re
u s u a lly in c re a se s as th e d ro p le ts approach th e ta r g e t , w ith s ta g n a tio n o f
th e a ir f lo w o c cu rrin g a t th e c e n te r o f th e t a r g e t , j u s t above th e surface
o f th e t a r g e t . The in c re a s e in p re s su re accompanying s ta g n a tio n i s gen
e r a l ly n o t s u f f ic ie n t , c o n s id e rin g th e amount o f tim e a d ro p le t i s in th e
s ta g n a tio n re g io n , to cause any s ig n i f i c a n t ev ap o ra tio n o f d ro p le ts
7
b e fo re im p ac tio n . The in c re a se in a i r p re s s u re , however, has an adverse
e f f e c t on th e c o l le c t io n o f sm all d r o p le ts , which ten d to w aft around
th e t a r g e t as a l e a f w i l l w aft over th e w in d sh ie ld o f a c a r . L arger
d r o p le ts , w ith r a d i i g r e a te r than 10 pm, a re c o l le c te d w ith f a i r l y u n i
form e f f ic ie n c y (S qu ires and G il le s p ie 1952). I t i s im p o rtan t th a t th e
f in e r d ro p le ts be cap tu red in a m inim ally d is tu rb e d environm ent l e s t
th ey be lo s t o r d im in ished in s iz e due to ra p id e v a p o ra tio n . May (1945,
1961) w il l be c i te d l a t e r in t h i s re g a rd .
C apture Methods
C apture methods in v a r ia b ly in v o lv e a ta r g e t th a t i s composed o f
a s u b s t r a te coated w ith an o i l o r v a se lin e m ix ture w ith in which im pacted
d ro p le ts are com plete ly en ca p su la te d . Such t a r g e t s , a f t e r s u i ta b le ex
posure. to th e d ro p le t-b e a r in g a ir s tr e a m , a re p laced under a m icroscope
fo r immediate exam ination o r a re photographed fo r l a t e r s tu d y . Because
th e d ro p le ts ten d to ev ap o ra te more o r le s s q u ic k ly in to th e c o a tin g ,
d i r e c t v is u a l exam ination o f th e exposed t a r g e t s must occur in a con
t r o l l e d environm ent as soon as p o s s ib le a f t e r exposure. Counting and
s iz in g d ro p le ts under a m icroscope in a bouncing a i r c r a f t can , o f c o u rse ,
be q u ite dangerous, so photom icrography i s p r e fe r re d because o f th e min
im al h azard inv o lv ed in sim ply en su rin g p ro p e r focus on th e t a r g e t .
Many d i f f e r e n t c o a tin g s have been t r i e d in an e f f o r t to c o l le c t
d ro p le ts w ith o u t s h a t te r in g them and a t th e same tim e r e t a r d th e evapo
r a t io n o f cap tu red d ro p le ts . I f s h a t t e r in g o c cu rs , e i t h e r a t th e t a r g e t
o r a t some p reced in g m echanical p a r t o f th e p ro b e -- th e e n tran ce p o r t ,
fo r exam ple--the s iz e d i s t r ib u t io n would be d is to r te d in fa v o r o f th e
sm a lle r d ro p le t s iz e s . I f ev ap o ra tio n occurs b e fo re th e d ro p le ts a re
photom icrographed , th e s iz e d i s t r ib u t io n would be p a r t i c u l a r ly d e f ic ie n t
in p o r tra y in g th e a c tu a l number d e n s ity o f th e f i n e s t d r o p le ts . Of th e
d i f f e r e n t co a tin g s t r i e d over th e p a s t few decades in a ttem p ts to o v e r
come th e above p rob lem s, some have se rv e d , perhaps u n in te n t io n a l ly , to
d e fin e th e problem s much more c le a r ly th an th ey had been d e fin ed
p re v io u s ly .
Development o f C apture Methods
Method o f Fuchs and P e tr j a n o ff
One o f th e e a r ly developm ents was th a t o f Fuchs and P e t r j a n o f f
(1937), who c o l le c te d fog d ro p le ts on s l id e s coated w ith a m ix ture o f
v a se lin e and m inera l o i l . The p ro p o r tio n o f each was a d ju s te d acco rd ing
to a i r te m p e ra tu re . They claim ed th a t no p e rc e p t ib le d ro p le t evapora
t io n o ccu rred f o r s e v e ra l hours a f t e r c o l le c t io n i f th e s l id e s were
s to re d in a damp atm osphere a t a tem p era tu re n e a r o r below th a t a t which
th e d ro p le ts were c o l le c te d .
M o d ifica tio n Due to May
May (1945) re p o r te d u s in g a c o a tin g s im i la r to th a t o f Fuchs and
P e tr ja n o f f , w ith th e in te n t o f red u c in g d ro p le t ev ap o ra tio n a f t e r cap
tu r e . As a r e s u l t o f a r a th e r awkward p rocedure w ith t h i s c o a tin g p re p
a r a t io n , May claim ed th a t h is d ro p le t sam ples were p e r f e c t ly p re se rv ed
fo r s e v e ra l days. Because o f th e lack o f a ccu ra te c a l ib r a t io n methods
a t th a t tim e , th e re may be rea so n ab le doubt as to th e t r u t h in th i s
' c laim . May d id r e f e r to th e c a p tu r e - in - o i l method as th e " a b so lu te "
method in h is 1945 p ap er and in h is 1950 p a p e r , in which he d iscu ssed
d i f f e r e n t im paction m ethods. The d i f f e r e n t methods were c a l ib r a te d
a g a in s t th e s o -c a l le d " a b so lu te " m ethod, which May reg ard ed as a method
th a t p e r f e c t ly re p re se n te d cap tu red d ro p le ts .
C oating Developments
Weickmann and aufm Kampe (1953) took a more c au tio u s approach.
They used a c a s to r o i l c o a tin g and slowed a 145 mph a irs tre a m to 90 mph
in o rd e r to p re c lu d e d ro p le t s h a t t e r in g . R ather th an t r y in g to e l im i
n a te d ro p le t ev ap o ra tio n a f t e r cap tu re in t h e i r m ethod, th ey accounted
fo r i t by a c a l ib r a t io n f a c to r , adding 1 .0 pm to each ra d iu s measured
from a photom icrograph tak en app rox im ate ly 20 sec a f t e r exposure o f th e .
t a r g e t . Brown and W ille t (1955) used a s i l i c o n o i l c o a tin g , ta k in g p r e
cau tio n s to photograph th e samples as soon as p o s s ib le a f t e r exposure
and in an atm osphere c o n tro l le d to have th e same tem p era tu re and humid
i t y as th e n a tu r a l d ro p le t environm ent. .
W ithout m entioning e v a p o ra tio n . Mason (1957, p . 85) s t a t e s th a t
S h e ll "S p irex 230" o i l i s s a t i s f a c to r y in red u c in g th e s h a t t e r in g o f th e
la rg e r d ro p le ts encoun tered in c lo u d s.
" I s o k in e t ic Flow" and May's Cascade. Im pactor
Regarding th e aerodynamic desig n o f th e sam pling p ro b e , May
(1945) d iscu ssed th e im portance o f i s o k in e t ic flow o f th e d ro p le t-b e a r in g
a irs tre a m th rough th e e n tran ce p o r t o f th e sam pling p ro b e . I s o k in e t ic
flow i s th a t s p e c ia l case in which a ir s tre a m v e lo c i ty i s th e same w ith in
th e e n tran ce p o r t o f th e sam pling probe as o u ts id e th e sam pling probe in
th e u n d is tu rb e d a ir s tre a m . With i s o k in e t ic flow , s tre a m lin e s e n te r ' th e
sam pling probe w ith o u t a change in d i r e c t io n . The p resen ce o f an ob
s t r u c t io n w ith in th e p ro b e , such as th e t a r g e t , n e c e s s a r i ly a f f e c t s th e
10
upstream a ir f lo w so th a t th e i d e a l , i s o k in e t ic a ir f lo w c o n d itio n s a re
never f u l l y r e a l iz e d w ith th e im paction m ethods. At th e t a r g e t , th e
s tre a m lin e s a b ru p tly d iv e rg e in th e v i c i n i t y o f th e s ta g n a tio n p o in t ,
c a rry in g th e s m a lle r , l i g h t e r d ro p le ts around th e t a r g e t . The h e a v ie r
d ro p le ts , because, o f t h e i r i n e r t i a , im pact on th e t a r g e t .
May (1945) has a c tu a l ly used t h i s e f f e c t to advantage in a c a s
caded im pacto r. In th e Cascade Im pacto r, th e d ro p le t-b e a r in g a irs tre a m
i s d ire c te d a t fo u r t a r g e t s in su ccess io n (F ig . 2 ) . A vacuum pump i s
a tta c h e d a t th e r e a r o f th e dev ice so th a t th e a irs tre a m i s a c c e le ra te d
as i t p ro g re s se s from s tag e to s ta g e . The n e t e f f e c t o f t h i s a rra n g e
ment i s a rough s iz e g rad in g from ta r g e t to t a r g e t . The la r g e s t d rop
l e t s a re cap tu red by th e f i r s t t a r g e t , in te rm e d ia te s iz e d ro p le ts a re
cap tu red by th e in te rm e d ia te s ta g e s , and th e f i n e s t d ro p le ts a re cap
tu re d by th e l a s t t a r g e t . As th e a irs tre a m v e lo c i ty in c re a s e s , th e
sm a lle r d ro p le ts in c re a s e in momentum and f i n a l ly overcome th e p u l l o f
th e a irs tre a m n e a r th e s ta g n a tio n p o in t o f th e l a t t e r s ta g e s and a re '
im pacted.
D iscussion o f M ay's Technique
L a te r , May (1961) re p o r te d th a t f u r th e r ex p erien ce w ith th e fo u r
s tag e im pactor in d ic a te d th a t.so m e o f th e sm a lle r d ro p le ts evaporated
a f t e r th e second s ta g e o f th e Cascade Im pactor. He th e re fo re m odified
h is tech n iq u e and used on ly two s ta g e s in h is l a t e r work, c la im ing accu
r a te sam pling o f d ro p le ts as sm all as 1 .0 urn in d iam eter w ith h is new
tech n iq u e .
11
S lid e 1
S u ction tubeIn tak e
S lid e 4
c / / / / / r-rz-x
S lid e 2
S lid e 3
V s / / / 7~/ y / s\
F ig . 2. Schem atic Diagram o f May’s Cascade Im pactor (May 1945)
12
There i s l a t e r ev idence th a t i s o k in e t ic flow may n o t be q u ite so
im portan t to accuracy as May and o th e rs had supposed. An a c tu a l compar
iso n o f samples o b ta in ed under c o n d itio n s o f i s o k in e t ic and a n is o k in e tic
flow was d isc u sse d by Lodge ( in Weickmann and Smith 1957, pp . 182-183).
The sam ples compared fa v o ra b ly .
R e p lic a tio n Methods
Comparison w ith C apture Methods -
P r im a rily due to ev ap o ra tio n lo s se s o f th e f i n e r d r o p le ts , th e
c a p tu r e - in - o i l methods have la rg e ly been re p la ce d by r e p l ic a t io n m ethods.
In th e r e p l ic a t io n m ethods, d ro p le ts a re im pacted on t a r g e t s o f v a rio u s
com position w here, by m echanical deform ation o r chem ical change o f th e
t a r g e t c o a tin g , a mark i s l e f t by each d ro p le t . I t i s e s s e n t ia l t h a t
th e s iz e o f th e mark be r e l a t e d to th e s iz e o f th e d ro p le t th a t caused
i t . For t h i s re a so n , th e s p e c i f ic r e p l ic a t io n method must in c lu d e a
s u i ta b le c a l ib r a t io n o f th e m easuring tech n iq u e used to s iz e th e o r i g i
n a l d ro p le ts from th e marks l e f t by th e d ro p le ts . S ince th e marks a re
made e s s e n t i a l ly on im p ac t, th e subsequent ev ap o ra tio n o f th e d ro p le ts
i s o f l i t t l e concern . One o f th e main advantages o f th e r e p l ic a t io n
methods i s in th e s im p l i f ic a t io n o f th e a c tu a l sam pling p ro c e d u re . No
lo n g er i s i t n e ce ssa ry to ru sh an exposed t a r g e t to th e m icroscope, fo r
th e r e p l ic a t io n s may, in g e n e ra l , be s to re d in d e f in i t e ly w ith o u t damage
to th e sam ple.
Development o f R e p lic a tio n Methods
Magnesium Oxide Methods
One o f th e more w idely a p p lie d r e p l ic a t io n methods in v o lv es th e
c r a te r in g o f a th in la y e r o f magnesium oxide smoke p a r t i c l e s , which have
been coated on th e t a r g e t s u b s t r a te by h o ld in g i t over a b u rn in g magne
sium rib b o n . The c o a tin g i s u s u a lly a p p lie d in a la y e r th ic k e r than th e
d iam eter o f th e la r g e s t d ro p le t ex p ec ted . Because th e magnesium oxide
smoke p a r t i c l e s have a g ra in s iz e o f about 0 .5 pm, o n ly d ro p le ts la rg e r
than about 5 .0 pm in ra d iu s w i l l be a c c u ra te ly r e p l ic a te d (May 1945).
The d iam eter o f th e c r a te r l e f t by an im pacted d ro p le t i s a fu n c tio n o f
d ro p le t s iz e and im pact v e lo c i ty and may be gauged under a m icroscope by
u s in g s tro n g t ra n s m itte d l i g h t , assuming a t r a n s p a re n t s u b s t r a te , o r by
u sin g s tro n g su rfa c e i l lu m in a tio n from th e s id e (May 1945). An advantage
o f th e magnesium oxide m ethods, over th e c a p tu r e - in - o i l m ethods, i s th a t
o v erlap p in g im pacted d ro p le ts do n o t c o a le sc e , bu t leave o v erlap p in g
c r a t e r s .
M ay's C a l ib r a t io n . May (1950) d e sc rib e d th e magnesium oxide
method in d e t a i l , d e sc r ib in g th e range o f d ro p le t s iz e s in v e s t ig a te d and
h is method o f c a l i b r a t io n . In t h i s c a l i b r a t io n . May r e l i e d on the
c a p tu r e - in - o i l method fo r an a b so lu te s ta n d a rd . As m entioned p re v io u s ly ,
i t was assumed th a t th e o i l- e n c a p s u la te d d ro p le ts p re se rv e d t h e i r o r i g i
n a l s iz e f o r 24 hours o r more. May measured th e d iam eters o f th e p i t s
formed by d ro p le ts im pacting a t d i f f e r e n t v e lo c i t i e s . He found th a t th e
r a t i o o f o r ig in a l d ro p le t d iam eter to c r a t e r d iam eter was about 0.86 fo r
d ro p le t d iam eters g r e a te r than 20 pm. This r a t i o decreased as th e
14
d ro p le t d iam e te r decreased from 20 to 10 ym, 10 ym be in g th e s m a ll-s iz e
l im i t to th e range o f th e method. The c a l ib r a t io n was openly accep ted
by s e v e ra l m e te o ro lo g is ts (F r i th 1951, S q u ires and G il le s p ie 1952), who
took a c tu a l c loud sam ples w ith magnesium oxide c o a tin g s and su b seq u en tly
r e la te d c r a t e r s iz e s to d ro p le t s iz e s by means o f th e c a l ib r a t io n pub
lis h e d by May (1950). S in g le to n and Smith (1960), however, chose to
perform t h e i r own c a l ib r a t io n o f a magnesium oxide tech n iq u e and a r r iv e d
a t a d i f f e r e n t r a t i o o f d ro p le t s iz e to c r a t e r s i z e . .
Method o f F r i t h . S ince th e l a t e 1940’s , v a rio u s experim en ta l
tech n iq u es have evo lved , p a r t i c u l a r ly w ith reg a rd to th e d esign o f th e
probe h o ld in g th e co a ted t a r g e t . F r i th (1951) designed a probe 'w ith a
m echanical s h u t te r th a t allow ed a c o n tro l le d exposure, o f a magnesium ox
id e t a r g e t . With t h i s c o n f ig u ra tio n F r i th claim ed a volume o f 250 ml o f
cloud a i r was sampled p e r exposure w ith ho s ig n i f i c a n t lo s s o f d ro p le ts
g re a te r than 4 .5 ym in d iam e te r . The a c tu a l count o f c r a te r s under a
m icroscope encompassed a narrow s t r i p o f th e s l i d e , r e p re s e n tin g only 2
ml o f c loud a i r , which F r i th claim ed as r e p re s e n ta t iv e over d is ta n c e s o f
100 m w ith in th e c lo u d . Borovikov and o th e rs (1963 t r a n s l a t i o n , p . 75)
cau tio n ed a g a in s t e x tra p o la t in g from such sm all sam ples to th e c h a rac
t e r i s t i c s o f whole reg io n s o f a c loud . That F r i th r e l i e d on th e c a l i
b ra t io n due to May (1950) i s an o th er f a c to r in th e assessm ent o f th e
accuracy o f F r i t h ’s d a ta . To F r i t h ’s c r e d i t , h is method seems to be th e
e a r l i e s t capab le o f g a in in g in fo rm a tio n on th e v a r ia t io n o f th e d ro p le t 1
spectrum w ith in a g iven c loud .
Method o f S q u ires and G i l l e s p ie . S q u ires and G il le s p ie (1952)
t r i e d to improve on F r i t h ’s method. They used a magnesium oxide c o a tin g
15
on a c y l in d r ic a l s u b s t r a te to improve th e c o l le c t io n e f f ic ie n c y f o r sm all
d ro p le ts . The c o a tin g was con fined to a 1 .0 mm wide s t r i p , th e long
dim ension o f th e s t r i p b e in g p a r a l l e l to th e long dim ension o f a 3 .0 mm
d iam eter ro d . With t h i s c o n f ig u ra t io n . S qu ires and G il le s p ie claim ed
s a t i s f a c to r y c o l le c t io n o f c loud d ro p le ts as sm all as 2 .0 pm in d iam e te r .
There was some d i f f i c u l t y in in te r p r e t in g c r a t e r s iz e s fo r d ro p le ts le s s
than 3 .0 pm in d iam eter due to th e g ra in s iz e o f the magnesium o x id e .
The sam pling a p p ara tu s enclo sed 10 co ated g la s s rods which could be ex- ,
posed a t 3 -sec i n t e r v a l s . The a c tu a l volume o f c loud a i r sampled de
pended on th e a re a o f each t a r g e t th a t was examined under th e m icroscope.
G en era lly , enough o f each t a r g e t was examined th a t th e minimum cloud
volume re p re se n te d was 1 .0 ml p e r rod . Because th e rods were exposed by
poking th e co ated rod in to th e a irs tre a m and im m ediately r e t r a c t in g i t ,
th e exposure tim e ac ro ss th e c o a tin g was n o t uniform . An advantage o f
th i s method o f exposure i s th a t th e o u te r t i p o f th e rod was exposed
m ostly in th e u n d is tu rb e d a ir f lo w f u r th e r ou t from th e n o zz le o f th e de
v ic e . Because o f th e probe d e s ig n , d ro p le t s p l a t t e r from th e n o zzle was
i n s ig n i f i c a n t .
M o d ifica tio n Due to S q u ire s . S q u ires (1958) l a t e r re p o rte d some
d ro p le t sp e c tra , determ ined w ith a m o d if ic a tio n o f th e dev ice used by
S qu ires and G il le s p ie (1952). Soot c o a tin g s were used in p la c e o f th e
form er magnesium oxide c o a tin g s . S q u ires a lso a tta c h e d e r r o r f ig u re s to
h is d a ta . Exposure tim e was found to f lu c tu a te by 3% and a irsp e e d was
co n sid ered a cc u ra te to about 5%. These f a c to r s a f f e c t th e measurement
o f d ro p le t number d e n s i ty , w hich, th e re fo re , , was assumed to have an ac
curacy o f about 10%. E rro rs in s iz in g were co n sid ered to vary from 5%,
16
fo r d ro p le t d iam eters g r e a te r than 20 ym, to 15% fo r d ro p le t d iam eters
o f about 5 ym.
C a lib ra t io n o f S in g le to n and Sm ith . S in g le to n and Smith (I960)
went to some tro u b le to ensu re th a t t h e i r d ro p le t sam ples were r e p re
s e n ta t iv e o f f r e e -c lo u d d r o p le t s . S ince t h i s was n o t e n t i r e ly p o s s ib le ,
c o r re c t io n s were a p p lie d w ith in th e l im i ta t io n s o f th e method. Thus,
im pression s iz e s were c a l ib r a te d and c o r re c t io n f a c to r s were a p p lie d th a t
took in to account c o l le c t io n e f f ic ie n c y as a fu n c tio n o f t a r g e t geom etry,
a ir s p e e d , d ro p le t s i z e , and lo c a t io n o f th e sam pler on th e a i r c r a f t . As
w ith most c loud d ro p le t in v e s t ig a to r s d u rin g th e p reced in g decade,
S in g le to n and Smith o b ta in ed v a lu es f o r c o l le c t io n e f f ic ie n c y from th e
work o f Langmuir and B lo d g e tt (1946). These c a lc u la te d v a lu es in d ic a te d
th a t th e minimum d ro p le t s iz e l ik e ly to be cap tu red by th e t a r g e t s used
corresponded to th e minimum s iz e d e te c ta b le w ith th e magnesium oxide
la y e r : th a t i s , 3 to 4 ym in d iam e te r. By a c a re fu l c a l ib r a t io n o f im
p re s s io n s i z e s , i t was found th a t May’s (1950) va lu e o f 0 .86 fo r th e r a
t i o o f d ro p le t d iam eter to im p ression d iam eter was to o h ig h , so a va lue
o f 0.713 was a p p lie d to th e c r a te r s th a t S in g le to n and Smith o b ta in ed .
Measurements were made from two d i f f e r e n t types o f a i r c r a f t , w ith s u i t
ab le care in c a l i b r a t io n tak en fo r each s e t o f m easurem ents. Comparing
th e r e s u l t s from each a i r c r a f t c le a r ly in d ic a te d th e u n c e r ta in t ie s in
th e whole p ro ced u re . S in g le to n and Smith co n sid ered t h e i r d a ta p e r t i
n en t on a r e l a t i v e s c a le and were ab le to make u se fu l g e n e ra l iz a t io n s
about th e e v o lu tio n o f c louds in term s o f r e l a t i v e changes in cloud
m ic ro s tru c tu re .
Method o f C lague. F u r th e r developm ent o f th e r e p l ic a t io n meth
ods has been in th e d i r e c t io n o f p ro v id in g s h o r te r exposures o f more
ta r g e t s a t s h o r te r in te r v a l s between ta rg e ts ^ Clague (1965) developed
a tech n iq u e p a tte rn e d a f t e r th a t ' o f S q u ires and G i l le s p ie (1952).
Clague used a magazine o f 18 s o o t-c o a te d s l i d e s , each o f which was ex
posed fo r 2 .5 msec, ±0.25 msec. The in te r v a l between exposures was as
sh o r t as 1 sec d u rin g a c tu a l t r i a l s . No s ig n i f i c a n t sp la sh in g e f f e c t s
were observed . Clague a s s e r te d t h a t h is tech n iq u e allow ed b e t t e r re c k
oning o f th e v a r ia t io n in d ro p le t spectrum a long a p a th th rough a cloud
than d id p rev io u s r e p l ic a t io n te ch n iq u es u s in g so o t o r magnesium oxide
co ated t a r g e t s .
Formvar Methods
A re c e n tly -d e v e lo p e d method fo r th e con tinuous sam pling o f cloud
d ro p le ts u ses a Formvar ( tra d e name) c o a tin g in p la c e o f o i l o r magne
sium ox id e .
Method o f MacCready and Todd. MacCready and Todd (1964) coated
a f r e s h s o lu t io n o f Formvar on a moving s t r i p o f t r a n s p a re n t 16-mm film .
The coated f ilm then moved p a s t a s l o t , c ap tu r in g im pinging d ro p le ts and
p a r t i c u l a t e s . The m echanical arrangem ent was such th a t th e Formvar so
lu t io n com plete ly en cap su la ted th e p a r t i c l e s and hardened b e fo re th e ex
posed p o r t io n reached th e takeup r e e l . This method f a i l s to r e p l ic a t e
th e d ro p le ts e x a c tly , and i t i s n e ce ssa ry to c a l ib r a te th e method in o r
d e r to r e l a t e th e r e p l ic a s iz e s to th e o r ig in a l d ro p le t s iz e s . The
Formvar tech n iq u e i s u sab le down to a low er d ro p le t s iz e l im i t dependent
on c o l le c t io n e f f ic ie n c y . I t has a decided advantage in th e ex ac t
r e p l ic a t io n o f im pacted ic e c r y s t a l s . Readout i s accom plished w ith th e
a id o f a s to p -m otion movie p r o je c to r .
O ther D evelopm ents. A v e r i t t and Ruskin (1967) re p o r te d an im
proved method th a t in c lu d ed a method o f d ry ing th e exposed Formvar,
which was dyed befo rehand fo r b e t t e r c o n tra s t d u ring re a d o u t . S pyers-
Duran and Braham (1967) re p o r te d on a Formvar method d i f f e r in g from th e
above method p r in c ip a l ly in th e f a c t t h a t th e f ilm was p re c o a te d --a
s o f te n in g p ro c e ss o c cu rrin g in f l i g h t im m ediately b e fo re th e sam pling
was to be done. As w ith th e o th e r Formvar m ethods, ic e c r y s ta l s were
r e p l ic a te d a c c u ra te ly . Snowflakes and p e l l e t s were d is t in g u is h a b le from
d ro p le ts . Small d ro p le ts were r e l i a b ly r e p l ic a te d w ith in an upper s iz e
l im i t o f 50 to 70 pm in d ia m e te r , w ith s p la t t e r in g from la r g e r d ro p le ts
ob scu ring th e re c o rd when th e se s iz e s were p re s e n t in th e sampled v o l
ume. The con tinuous re c o rd produced by th e Formvar te ch n iq u es i s b e s t
u t i l i z e d in th e s tu d y o f ic e c r y s t a l s , r a th e r than l iq u id d ro p le ts .
MacCready and Todd (1964) e x p l i c i t l y m entioned th e p o te n t ia l o f
o p t ic a l methods to overcome some o f th e b a s ic problem s o f th e im paction
m ethods. The above developm ents do n o t r e a l l y change th e p ro sp e c ts fo r
an a l t e r n a t iv e method o f sam pling d r o p le t s .
M iscellaneous R e p lic a tio n Methods
D rop le t Charge M ethods. K eily and M illen (1960) d e sc rib e d a
method fo r s iz in g d ro p le ts w herein th e m agnitude o f th e a r t i f i c i a l
charge accep ted by a sampled d ro p le t s p e c i f ie d th e s iz e o f th e d ro p le t .
Each d ro p le t sampled was removed from th e a irs tre a m by an a s p ir a t in g
probe w ith h igh and f a i r l y uniform e f f ic ie n c y ac ro ss th e d ro p le t
19
spectrum . D ro p le ts accep ted by th e probe were a c c e le ra te d a c ro ss a gap
o f 750 pm, w herein a p o te n t ia l g ra d ie n t o f 5000 v o lts /c m e x is te d , and
im pacted on a t a r g e t a t 400 v o l t s above th e le v e l o f th e r e s t o f th e
p robe . Each d ro p le t im pact r e s u l te d in a 60 psec p u lse in th e t a r g e t
charg ing c i r c u i t . S ince i t was b e lie v e d th a t th e p u lse h e ig h t was r e
la te d to th e amount o f d ro p le t ch arg e , which i s a fu n c tio n o f d ro p le t
s i z e , th e au th o rs c laim ed th a t th e d ro p le t s iz e co u ld .b e in f e r r e d from
th e h e ig h t o f th e v o lta g e p u ls e . They claim ed a u s e fu l s iz e range o f
2 to 60 pm in d iam e te r . Scarce m ention o f t h i s tech n iq u e in th e re c e n t
jo u rn a l l i t e r a t u r e in d ic a te s th a t th e p o te n t ia l o f t h i s method has n o t
been r e a l iz e d . Because th e v o lta g e p u lse from a 2 -pm -diam eter d ro p le t
was o f th e o rd e r o f a few pV, th e n o ise v o lta g e in th e c i r c u i t may w e ll
have exceeded th e s ig n a l v o lta g e a r i s in g from sm all d ro p le ts . Then,
to o , th e opening in th e probe was on ly 200 pm in d ia m e te r , p o s s ib ly con
t r i b u t in g to sp u rio u s coun ting and s iz in g . This e f f e c t may be v i s u a l
iz ed by im agin ing th e p e rcen tag e o f sampled d r o p le ts , in te r c e p te d a t th e
boundaries o f th e opening o r by th e in s id e w a ll o f th e p ro b e , th a t would
s t i l l reach th e t a r g e t a f t e r s h a t te r in g ,
A r e la te d tech n iq u e was p o s tu la te d by Winn (1968), The change
in c a p a c ity as a d ro p le t approached and c o n tac te d one o f th e e le c tro d e s
o f a c a p a c ito r was r e la te d to d ro p le t s i z e . The th e o ry has been te s te d)
by an experim ent in which a s t e e l b a l l approached one o f th e e le c tro d e s ,
changing th e v o lta g e in agreem ent w ith th e p re d ic t io n o f th e th e o ry .
Winn made su p p o rtin g re fe re n c e to K eily and M illen and e s p e c ia l ly to th e
l a t e r work o f K eily (1964, 1965).
H ygrophotographic Method, In an independent e f f o r t to overcome
th e problem s o f ev ap o ra tio n and co a lescen ce o f d ro p le ts cap tu red in o i l ,
S iv a d jia n (1957) employed a hygropho tograph ic method f o r th e sam pling o f
c loud d ro p le ts . In t h i s method th e t a r g e t s l id e i s co a ted w ith a sp e
c i a l em ulsion th a t darkens lo c a l ly where w ater has been absorbed . For
sam pling from a i r c r a f t , th e em ulsion i s overcoated w ith a th in la y e r o f
o i l to minimize f a ls e counts due to s p la t t e r in g o f th e l a r g e r d ro p le ts .
The o i l la y e r a lso a c ts as a p ro te c t iv e c o a tin g a f t e r exposure o f th e
s l i d e , p re v e n tin g a c c id e n ta l damage to th e re c o rd . C e r ta in drawbacks
e x i s t , how ever, in th a t S iv a d jia n re p o r te d e x c e lle n t r e s u l t s in sam pling
cloud d ro p le ts o f 20 to 30 pm d iam e te r o n ly . S ince c loud d ro p le ts e x is t
in a much w ider range o f s i z e s , say 1 to 100 pm in d ia m e te r , th e method
o f S iv a d jia n i s o f r a th e r l im ite d u se .
D y e -in -G e la tin Method. L id d e ll and Wootten (1957) re p o r te d on a
method s im i la r in p r in c ip le to th a t o f S iv a d jia n (1957). T h e ir method
invo lved th e use o f a green dye in c o rp o ra te d in to a g e la t in c o a tin g .
D ro p le ts im pacting on th e c o a tin g would d is p la c e th e dye, le av in g a dark
green r in g fo r each im pacted d ro p le t . On th e av erag e , th e r in g d iam eter
was 2 .5 tim es th a t o f th e d ro p le t t h a t produced i t . L id d e ll and Wootten
claim th a t th e s m a ll- s iz e l im i t to t h e i r method i s le s s th an 1 .0 pm in
d iam eter.
Phase C o n tra s t Method. May (1959) found th a t th e troublesom e
dye used by L id d e ll and Wootten could be e lim in a te d s in c e th e im pacted
d ro p le ts l e f t a t r a c e in th e g a la t in th a t could be d e te c te d under a
phase c o n tra s t m icroscope when no dye w as.u sed . May re p o r te d th a t phase
c o n tra s t i l lu m in a tio n was re q u ire d fo r t r a c e s l e f t by d ro p le ts le s s than
10 pm in d ia m e te r , w hile i t was n o t re q u ire d fo r la r g e r t r a c e s due to
d ro p le ts l a r g e r than 10 pm. .. This method was co n sid e red s u p e r io r to th e
magnesium oxide methods fo r s iz in g d ro p le ts sm a lle r than 10 pm in
d iam e te r.
Summary o f Im paction Sampling
With th e ex cep tio n o f th e d ro p le t charge m ethod, th e r e p l ic a t io n
methods a re q u ite s im i la r to th e c a p tu r e - in - o i l method. In f a c t , i t ap
p ea rs th a t t a r g e t s o f v a rio u s c o a tin g com position cou ld be used i n t e r
changeably in th e same p ro b e .
Most o f th e r e p l ic a t io n methods a re more conven ien t to use th an
th e c a p tu r e - in - o i l method, owing to s im p le r p ro ced u res fo r h an d lin g ex
posed t a r g e t s . These s im p le r p ro ced u res a r i s e from th e f a c t th a t evapo
r a t io n o f d ro p le ts a f t e r im paction i s o f l i t t l e consequence in most o f
th e r e p l ic a t io n methods b u t i s a s e r io u s problem in th e c a p tu r e - in - o i l
method.
S erio u s sam pling problem s a re p re s e n t in a l l o f th e im paction
methods due to th e unavo idab le d is tu rb a n c e o f th e d ro p le t environm ent by
th e p resen ce o f th e t a r g e t in a d ro p le t-b e a r in g a ir s tre a m . D rop le t c o l
le c t io n e f f ic ie n c y , f o r exam ple, i s found to be a fu n c tio n o f d ro p le t
s i z e , a ir s tre a m v e lo c i ty , and t a r g e t and probe c o n f ig u ra tio n . D rop le t
e v ap o ra tio n b e fo re im paction i s a lso encoun tered in th e im paction
m ethods. ■ ' y • - ‘ i.
CHAPTER III
OPTICAL SAMPLING
In p r a c t i c e , o p t ic a l sam pling i s th a t c la s s o f d ro p le t sam pling
th a t does n o t in v o lv e d ro p le t im paction on a t a r g e t . R a th e r , th e p r e s
ence and s iz e o f a d ro p le t a re determ ined by th e e f f e c t o f d ro p le ts on
th e t r a n s p o r t o f r a d ia n t f lu x . Such an e f f e c t may be s c a t t e r in g , r e
f l e c t i o n , o r s p e c t r a l a t te n u a t io n o f r a d ia n t f lu x in a manner th a t i s
r e la te d to th e s iz e d i s t r ib u t io n and number d e n s ity o f th e sampled
d ro p le ts .
Comparison w ith Im paction Sampling
The advantages o f o p t ic a l sam pling a re im p o rtan t ones. The d ro p
l e t environm ent i s g e n e ra lly much le s s d is tu rb e d by an o p t ic a l probe th an
by an im paction p robe . With some o f th e o p t ic a l te c h n iq u e s , th e sam pling
can be c o n tin u o u s , w ith th e d a ta re d u c tio n o ccu rrin g in s ta n ta n e o u s ly .
Thus th e ex p erim en ter can expect a t r u e r sam pling o f th e cloud d ro p le ts
in a d d itio n to th e p o s s i b i l i t y o f au tom ating th e d a ta re d u c tio n . The
e lim in a tio n o f th e te d io u s coun ting and s iz in g p ro ced u res a s so c ia te d w ith
im paction sam pling i s . a most d e s ir a b le fe a tu re from th e e x p e r im e n te r 's
v iew p o in t. '
Corona Method
Not a l l o f th e o p t ic a l methods can be autom ated , however. In
one method th e coronae around th e su n , th e moon, o r any c o n v en ien tly
' ■ : : « : ' ' :
23
sm all l i g h t source a re m easured. By m easuring th e an g u la r d iam eter o f
f i r s t and h ig h e r o rd e r d i f f r a c t io n r in g s , in fo rm atio n on th e r a d i i o f
th e d ro p le ts cau sin g th e coronae may be in f e r r e d . A ccording to Mason
(1957, p . 8 6 ), t h i s method i s s e v e re ly l im ite d in u se . The measured
ra d iu s o f a g iven r in g co rresponds only to th e most f req u e n t s iz e o f
d ro p le t o r ic e p a r t i c l e . As o f 1957, no s a t i s f a c to r y method had been
developed f o r e x tra p o la t in g to an a c tu a l s iz e d i s t r i b u t io n . Sharper
coronae occur f o r narrow er s iz e d i s t r i b u t i o n s , o f c o u rse , so th a t t h i s
method i s most a p p lic a b le in th o se s p e c i f ic cases when th e d ro p le t s iz e
d i s t r ib u t io n i s narrow . Mason a lso s t a t e s t h a t t h i s method i s unsound
fo r p a r t i c l e d iam eters le s s th an 10 pm.
D ire c t Photography
A sim ple method o f o p t ic a l sam pling i s th e d i r e c t photography o f
l ig h t s c a t te r e d by c loud d ro p le ts . D ro p le t im paction i s avo ided , and
th e sample i s co n sid e red to be r e l a t i v e l y u n d is tu rb e d . A ty p ic a l method
i s th a t o f Webb (1956), who sim ply photographed l i g h t s c a t te r e d by d rop
l e t s i l lu m in a te d a t an ang le w ith a c o llim a te d p h o to f la sh beam. Due to
th e dep th o f f i e l d and f i e l d o f view o f th e cam era, th e volume sampled
was 1 .0 ml. S e n s i t iv i ty was adequate f o r s iz in g d ro p le ts la r g e r than
1 .5 pm in d ia m e te r , p ro v id ed th a t th e r e l a t i v e v e lo c i ty o f th e d ro p le ts
was low enough. S ince no image m otion com pensation was u sed , th e method
was employed only on th e ground. Mason (1957, p . 87) n o ted th a t E l l i o t t ,
in 1946, designed a camera fo r sam pling from a i r c r a f t . I n t e r e s t in d rop
l e t photography was shown by I ta g a k i (1966), who d e sc r ib e d a cloud
d ro p le t camera th a t has been te s t e d on fog d ro p le ts . I t a g a k i 's pap er
in c lu d ed i l l u s t r a t i o n s o f th e e f f e c t o f v a rio u s i l lu m in a tio n schemes on
th e image o f th e d ro p le t .
An E le c tro -O p tic a l Approach
The method o f P e tro v (1959) i l l u s t r a t e s th e d iv e r s i ty o f ap
proaches tak en to th e problem o f d ro p le t s iz in g . In t h i s method, th e
d ro p le t-b e a r in g a ir s tre a m flow ing p a s t th e a i r c r a f t was slowed down to
0 .2 m /sec and then e n te re d an e l e c t r i c f i e l d . Some o f th e d ro p le ts c a r
r ie d a charge and were acco rd in g ly d e f le c te d from t h e i r o r ig in a l t r a j e c
to ry . From a photom icrograph o f th e d ro p le t t r a c k s - - t im e exposure was
u s e d - - i t was p o s s ib le to determ ine th e w idth o f th e d ro p le t and th e
charge on th e d ro p le t . The d ro p le t d iam eter was determ ined from th e
t r a c e w idth and th e d ro p le t charge was determ ined from th e d ro p le t
t r a j e c t o r y .
P e tro v c laim ed a s iz e range from 2 .0 to 28 ym in d iam eter fo r
h is method. Because th e a irs tre a m was slowed from 70 to 0 .2 m /sec, how
e v e r , th e re i s reaso n to b e lie v e th a t th e d ro p le t p o p u la tio n had changed
(befo re b e in g photographed .
F raunhofer D if f r a c t io n Methods
The "L aser Fog D isdrom eter"
A m ajor drawback o f th e co n v en tio n a l p h o to g rap h ic tech n iq u es
i s th a t o f th e l im ite d sample volume reco rd ed p e r p h o to g rap h . To o v e r
come t h i s , S ilverm an, Thompson, and ;Ward (1964) in tro d u ce d a new dev ice
c a l le d a " l a s e r fog d isd ro m e te r ." T his d e v ic e ,re c o rd e d on f ilm th e f a r
f i e l d d i f f r a c t io n p a t te r n s o f in d iv id u a l d ro p le ts w ith in a co llim a te d
beam o f l ig h t from a Q -sw itched ruby l a s e r . The o r ig in a l f ilm read o u t
Required th e measurement o f th e c h a r a c t e r i s t i c dim ensions o f th e d i f
f r a c t io n p a t t e r n s , These dim ensions were then r e la te d to th e d ro p le t
d iam eters by means o f e s ta b l is h e d d i f f r a c t i o n r e la t io n s h ip s . The au
th o rs claim ed a c c u ra te sam pling o f d ro p le ts 4 ym in d iam e te r and la rg e r
w ith in in d iv id u a l sample volumes o f up to 5 ml. Each sample volume was
r e g is te r e d on a s in g le 35-mm fram e, w ith a maximum sam pling r a te o f 10
fram es/m in. I t was no ted th a t th e f ilm p la n e was a t such a d is ta n c e
from th e sample volume as to approxim ate c o n d itio n s fo r F raunhofer d i f
f r a c t io n . The f ilm p lan e was n o t so f a r away as to be in th e f a r f i e l d
o f th e sample volume d iam e te r . A fte r t ry in g a s p a t i a l f i l t e r i n g te c h
nique and a co h eren t background te c h n iq u e , i t was found th a t th e co h er
en t background tech n iq u e was s u p e r io r in accuracy and in d is c r im in a tio n
a g a in s t p a r t i c l e s n o t in th e sample volume, in a d d itio n to be in g s im p le r
in c o n s tru c tio n and o p e ra tio n (F ig . 3 ) . A lthough th e d isd ro m ete r was
s p e c i f i c a l ly designed fo r ground u s e , p u ls e s s h o r te r th an th e 1.0 ysec
used in t h i s experim en t—say 10 n se c —would enab le t h i s tech n iq u e to be
employed fo r measurements from a i r c r a f t . :
Method o f Oura and Hori
Mason (1957, p . 88) b r i e f l y review ed a s im i la r d i f f r a c t io n te c h
n ique re p o r te d by Oura and H ori in 1953. Oura and Hori examined the
F raunhofer d i f f r a c t io n p a t te r n s a r i s in g from fog d ro p le ts i l lu m in a te d
w ith red l i g h t o f 650 nm w avelength . The au th o rs claim ed t h e i r te c h
n iq u e , which was used on th e ground, a p p lie d to d ro p le ts o f ra d iu s le s s
th an 10 ym. This method d i f f e r s from t h a t o f Silverm an and o th e rs (1964)
26
Film p laneStop
Image
Lens
> Lens
Sample volume
Lens
P inholeLens
A pertu re
A pertu re
Pulsed la s e r
F ig . 3. Schem atic Diagram o f Coherent Background A pparatus o f Silverm an and o th e rs (1964)
27
in th a t Oura and H o ri, working w ith a 50r-cm p a th in fo g , a ttem pted to
determ ine th e d ro p le t d i s t r ib u t io n from th e com posite p a t t e r n . S i lv e r
man and o th e rs used a 1.0-cm p a th and were ab le to work w ith th e d i f
f r a c t io n p a t te r n s from each d ro p le t w ith in th e sample volume.
Method o f Thompson and O thers
The read tiu t tech n iq u e p re s c r ib e d fo r th e method o f Silverm an and
o th e rs (1964), th a t o f m easuring th e d i f f r a c t io n p a t te r n s d i r e c t l y , p r e
sen ted d i f f i c u l t i e s th a t were removed by a new read o u t tech n iq u e d e v e l
oped by Thompson and o th e rs (1966). The new tech n iq u e was based on th e
r e a l i z a t io n th a t th e d i f f r a c t i o n p a t te r n reco rd ed on f ilm was in a c tu a l
i t y a new k ind o f hologram . Thus, Thompson and o th e rs were ab le to r e
c o n s tru c t a .th re e -d im e n s io n a l image o f th e sample volume. S iz in g and
coun ting o f th e d r o p le ts , ran g in g from 4 .0 to 200 pm in d ia m e te r , was
accom plished from th e images o f th e d ro p le ts r a th e r th an from com plicated
and o f te n o v e rlap p in g component d i f f r a c t io n p a t te r n s . T his new read o u t
techn ique enab led th e au th o rs to make lo c a l d e te rm in a tio n s o f th e s i z e ,
shape , and d i s t r i b u t io n o f p a r t i c l e s th roughou t an en la rg ed sample v o l
ume (F ig . 4 ) .
S p e c tra l A tte n u a tio n Method
In a com plete ly d i f f e r e n t approach , E ld rid g e (1957a) in fe r r e d
th e d ro p le t s iz e d i s t r ib u t io n from an a n a ly s is o f th e in f r a r e d s p e c t r a l
tra n sm itta n c e o f n a tu ra l c lo u d s . R e fe rr in g to th e th e o r ie s o f Mie and
o th e rs on th e s c a t te r in g p ro p e r t ie s o f nonabsorb ing s p h e re s , E ld rid g e
based h is com putations on th e c o n d itio n th a t th e tra n sm iss io n c ro ss
s e c tio n was id e n t ic a l to th e s c a t te r in g c ro ss s e c t io n . I t was thus
28
TV camera
Lens
D rop le timages
Hologram
C o llim a to r
,— P inhole
Gas la s e r
TV m onitor
F ig . 4 . Schem atic Diagram o f Readout Device o f Thompson and O thers (1966)
Shadow
D rop le t moving out o f paper
L inearf ib e ro p tic s
Source
©
A perture
F ig . 5. Schem atic Diagram o f Shadow A pparatus o f N a tio n a l C en ter fo r A tm ospheric Research (1968)
29
p o s s ib le to compute s p e c t r a l tra n s m itta n c e from s c a t te r in g th e o ry fo r
any p a r t i c l e s iz e o r d i s t r ib u t io n o f s iz e s . By com parison o f s y n th e t ic
and ex perim en ta l t ra n s m is s io n s , th e s iz e d i s t r ib u t io n cau sin g th e ex p er
im en ta l tra n sm iss io n was in f e r r e d w ith an e r r o r o f 10 to 20%. Although
th e d i s t r ib u t io n s th u s in f e r r e d were n o t co n sid ered unique s o lu t io n s ,
E ld rid g e co n sid e red them s u f f i c i e n t ly unique fo r th e pu rposes o f th e
m e te o ro lo g is t. In an a c tu a l t e s t o f t h i s method on Mt. W ashington,
E ld rid g e in f e r r e d d i s t r ib u t io n s th a t were m arkedly s h i f te d tow ard th e
s m a ll- s iz e end o f th e spectrum . The r e l a t i v e number d e n s i ty o f th e
sm a lle r d ro p le ts was h ig h e r than had ev e r been measured b e fo re . ■'
Penndorf (1957) su b seq u en tly c r i t i c i z e d E ld r id g e 's te ch n iq u e ,
c i t in g p u rp o rted d e f ic ie n c ie s in th e th e o r e t ic a l assum ptions and in th e
ex p erim en ta l v a l id a t io n o f th e th e o ry . Penndorf s p e c i f i c a l ly q u estio n ed
th e un iqueness o f th e in f e r r e d s o lu t io n s in view o f th e experim en ta l ac
curacy o f th e te ch n iq u e . E ld r id g e 's (1957b) e lo q u en t re p ly to Penndorf
acknowledged in ad eq u ac ies in th e pap er r a th e r than in th e method and
f o r c ib ly contended th a t P enndorf, as a t h e o r i s t , was n o t a p p re c ia tiv e o f
an e x p e r im e n ta l is t 's compromise o f accuracy f o r th e sake o f o b ta in in g
u se fu l d a ta . E ld rid g e (1957a) d id , a f t e r a l l , c laim u t i l i t y r a th e r th an
a h igh degree o f accuracy fo r h is method.
E ld rid g e (1966) l a t e r re v is e d h is techn ique as a consequence o f
th e in c re a se d amount o f in fo rm a tio n a v a i la b le on s p e c t r a l a t te n u a tio n by
fog and haze d ro p le t d i s t r i b u t io n s . I n te r e s t in g ly , he l im ite d h is 1966
a r t i c l e to a few s p e c i f ic d i s t r i b u t io n s , th e re b y em phasizing th e l im i t a
t io n s o f th e method a t th e c u rre n t s ta g e o f developm ent. A r e p o r t by
H a rr is (1969) in d ic a te d th a t t h i s approach i s s t i l l b e in g t r i e d . H a rris
d e sc rib e d th e a c tu a l changes in th e p o la r iz a t io n and an g u la r d i s t r i b u
t io n o f s c a t te r e d r a d ia t io n as a fu n c tio n o f d ro p le t d i s t r i b u t io n . Sev
e r a l d i s t r ib u t io n s were d iscu ssed from a th e o r e t ic a l , r a th e r than e x p e r i
m en ta l, v iew p o in t.
Shadow Method
A re c e n t developm ent, th e O p tic a l E le c t r ic a l P a r t i c l e S ize D is
c r im in a to r designed by K nollenberg (N a tio n a l C enter f o r A tm ospheric Re
sea rch 1968), in c o rp o ra te s some unique te c h n iq u e s . The D isc r im in a to r
s iz e s d ro p le ts by m easuring th e s iz e o f th e shadows produced by d ro p le ts
t r a v e r s in g a c o llim a te d l ig h t beam. The c o llim a te d l i g h t t h a t bypasses
a d ro p le t i s focused in th e p lan e o f a sm all c i r c u la r s to p , which red u
ces s t r a y l ig h t and r e f r a c t iv e e f f e c t s due to th e tra n sp a ren c y o f th e
d ro p le ts . The d ro p le t,sh ad o w i s imaged on a p lan e c o n ta in in g 'a l in e a r
a r ra y o f end-on l ig h t p ip e s o f sm all d iam eter (F ig . 5 ) . . Each l ig h t p ip e
i s connected to an in d iv id u a l p h o to d e te c to r w hich, in tu r n , i s connected
to a lo g ic c i r c u i t .
New c a p a b i l i t i e s , n o t found in th e l i g h t - s c a t t e r in g methods d e s
c r ib e d l a t e r , a re a ffo rd e d by th e d esig n o f th e D isc r im in a to r . For one,
th e l in e a r l ig h t p ip e a rra y i s a rran g ed a t 90° to th e d i r e c t io n o f th e
a ir f lo w , and because th e u sab le dep th o f f i e ld , o f th e c o l le c t in g len s i s
a t l e a s t an o rd e r o f m agnitude g re a te r th an in th e s c a t te r in g m ethods,
th e sample volume i s v is u a l iz e d as a v e ry th in , r e c ta n g u la r sh ee t p e rp en
d ic u la r to th e d i r e c t io n o f th e a ir f lo w . This arrangem ent p e rm its a
h igh sam pling r a te w hile red u c in g co in c id en ce e r ro r s in co u n tin g . By
vary in g th e fo c a l le n g th o f th e c o l le c t in g le n s , th e r e s o lu t io n o f th e
dev ice may be changed s in c e th e shadow fo r a g iven s iz e d ro p le t w il l
cover more o r le s s o f th e l ig h t p ip e a r ra y . R eso lu tio n and accuracy o f
s iz e d e te rm in a tio n may a lso be in c re a se d by e le c t r o n i c a l ly s e t t in g the
lo g ic c i r c u i t to r e g i s t e r a count on ly fo r shadows hav ing a c o n tra s t
g re a te r than th e minimum c o n tra s t a c c e p ta b le . Such an ad ju stm en t, in .
e f f e c t , l im i t s th e dep th o f f i e l d o f th e c o l le c t in g le n s , th e reb y re d u c
ing th e sample volume. D ro p le ts to e i t h e r s id e o f th e p lan e con jugate
to th e d e te c to r a rra y produce shadows th a t have been degraded by d i f
f r a c t io n . The f u r th e r th e d ro p le t i s from th e co n ju g ate p la n e , th e
g re a te r i s th e d e g ra d a tio n o f th e shadow.
An a d d i t io n a l fe a tu re o f K n o llen b erg 's dev ice i s an e r r o r d e te c
t io n scheme th a t a llow s th e lo g ic c i r c u i t to r e j e c t shadows n o t ly in g
f u l ly w ith in th e l in e a r e x te n t o f th e l i g h t p ip e a r ra y . This i s accom
p lis h e d by re q u ir in g th a t th e two end f ib e r s o f th e a r ra y be i l lu m in a te d
d u rin g a shadow s iz e d e te rm in a tio n . K n o llenberg ’s tech n iq u e i s be ing
developed f u r th e r and i s expected to be a p p lie d to th e f u l l range o f
cloud d ro p le t s iz e s as w e ll as to la r g e r p a r t i c l e s such as snow and ic e
c r y s ta l s and ra in d ro p s (K nollenberg 1968, p r iv a te com m unication). P a r
t i c l e co u n ting r a t e s in excess o f 1 ,0 0 0 ,0 00 /sec were p re d ic te d , w ith
r e a l tim e d is p la y o f p a r t i c l e s iz e d i s t r i b u t io n s . ; •
S in g le S c a tte r in g Methods
Some o f th e more im p o rtan t o p t ic a l methods in c o rp o ra te p h o to
e l e c t r i c measurement o f l i g h t s c a t te r e d from s in g le d r o p le ts . Because
th e th e o ry behind th e se methods has been developed more f u l l y in re c e n t
y e a r s , g e n e ra l iz a t io n s about, th e se methods w i l l be d isc u sse d l a t e r .
32
These methods evolved from th e p r a c t i c a l n e c e s s i ty fo r a method le s s
cumbersome than th e im paction methods a lre a d y d e sc rib e d .
The P a r t i c le C ounter
E ld rid g e (1961) d iscu ssed an independent c a l ib r a t io n o f the
Armour R esearch Foundation P a r t i c le C oun ter, which was b u i l t in 1956 and
which had n o t p re v io u s ly been re p o r te d in th e jo u rn a l l i t e r a t u r e . The
P a r t i c le C ounter i s a dev ice designed to s iz e and count a irb o rn e , p a r t i
c le s from 1 .0 to 64 ym in d iam eter by means o f th e i n t e n s i t y o f s c a t
t e r e d , po lych rom atic v i s ib le and n e a r u l t r a v i o l e t l i g h t . L igh t s c a t
te re d in a re g io n c en te re d a t 90° from th e a x is o f th e i l lu m in a tin g beam
was measured by a p h o to m u lt ip l ie r tu b e . A m u ltich an n el p u lse h e ig h t an
a ly z e r q u an tiz ed th e v o lta g e p u lse s g en era ted by th e p re sen ce o f a p a r
t i c l e in th e sam pling volume.
E ld rid g e sp e c u la te d on some o f th e sam pling e r ro r s he co n sid ered
. to be in h e re n t in th e d e v ic e . The aerodynam ic desig n o f th e P a r t i c le
Counter p ro b e , fo r exam ple, was reg ard ed to be a l ik e ly source o f con
s id e ra b le sam pling e r r o r . E ld rid g e d id n o te , however, t h a t t h i s dev ice
p o ssessed th e c a p a b i l i ty o f d e te c t in g d ro p le ts sm a lle r than a few m icro
m eters in d ia m e te r - -a ’ c a p a b i l i ty n o t found in most o f th e d i r e c t ( i . e . ,
im paction) methods in u se^ a t th a t tim e . Although th e P a r t i c l e C ounter,
as th e name im p lie s , was designed to s iz e and count p a r t i c u la te s as w ell
as d r o p le ts , E ld rid g e was in te r e s te d in fog and cloud d r o p le ts , and he
d esigned h is c a l i b r a t io n s p e c i f i c a l ly fo r w ater d ro p le ts .
Lieberman and Katz (1962), in a c r i t i c a l d is c u s s io n o f E ld r id g e 's
(1961) c a l i b r a t io n , r e la te d some o f th e perform ance c h a r a c te r i s t i c s o f
33
th e P a r t i c l e C ounter. The coun ting e r r o r , fo r exam ple, due to th e p r e s
ence o f more than one p a r t i c l e w ith in th e sample volume in some in s ta n
c e s , was 3.5% when th e num erical p a r t i c l e d e n s ity was 125/ml and 25%
when th e d e n s ity was 1000/m l. A lso , a maximum coun ting r a t e o f 200/sec
was recommended to p rev en t o v e rlo ad in g the coun ting mechanism. Lieberman
and K atz, th e d e s ig n e rs o f th e P a r t i c l e C ounter, c la im an o v e r - a l l accu
racy in coun ting o r s iz in g o f 5%.
In a p r iv a te com m unication, Katz (1969) d iscu ssed th e e v o lu tio n
o f th e Armour R esearch Foundation (now IIT R esearch I n s t i t u t e ) A eroso lo -
sco p e , th e outcome o f a 1951 c o n tra c t awarded by th e Army Chemical C orps.
A lthough th e A erosoloscope and th e 11 models th a t fo llo w ed , in c lu d in g
th e co u n te r d e sc rib e d by E ld rid g e (1961), were n o t designed fo r sam pling
d ro p le ts from an a i r c r a f t , th e d esign p r in c ip le o f th e o p t ic a l s e c tio n
o f th e se co u n te rs i s id e n t ic a l to th a t o f th e a irc ra f t-m o u n te d dev ice o f
Konyshev and Laktionov (1966), which i s d e sc rib e d l a t e r in t h i s t h e s i s .
In f a c t , th e o p t ic a l schem atic show n.in F ig . 8 could j u s t as w e ll apply
to some o f th e co u n te rs in th e above-m entioned fam ily .
One o f th e fam ily o f co u n ters was su p p lie d to Dugway Proving
Ground fo r purposes o f d e te c tin g windborne p a r t i c l e s ran g in g in s iz e
from 0 .5 to 128 pm in d iam e te r . This d e v ic e , having an o p t ic a l ly sep a
ra te d sample volume sm a lle r than 0 .2 by 0 .2 by 1.0 mm, was capable o f
i s o k in e t ic sam pling a t wind speeds ran g in g from 0 .9 to 9 .0 m /sec. Of
a l l th e co u n te rs in th e ARF-IITRI fam ily o f c o u n te rs , perhaps t h i s coun
t e r could have been most e a s i ly m odified to sample c loud d ro p le ts from
a i r c r a f t . Most o f th e o th e r 11 co u n te rs were n o t designed s p e c i f i c a l ly
to sample w a ter d r o p le ts , b u t r a th e r p a r t i c u la t e s and o i l m is ts .
34
M ik iro v 's Method
One o f th e f i r s t p apers to appear in th e jo u rn a l l i t e r a t u r e ,
d e a lin g s p e c i f i c a l ly w ith a s in g le s c a t te r in g method, was th a t o f
M ikirov (1957). In t h i s method a narrow , continuous s tream o f d ro p le ts
i s t r a v e rs e d by a narrow , co llim a te d beam o f w hite l i g h t . The sample
volume, d e fin ed by th e volume common to th e a irs tre a m and th e l ig h t beam,
was sm all enough t h a t , acco rd in g to M ik iro v 's a p p l ic a t io n o f Poisson
s t a t i s t i c s , th e p r o b a b i l i ty o f th e re b e in g more than one d ro p le t s c a t
te r in g l ig h t a t a g iven in s t a n t was 0 .0 5 . L igh t s c a t te r e d in th e f o r
ward d i r e c t io n a t an g les g re a te r th an 1 .5° from th e a x is o f th e l ig h t
beam and le s s th an 20° from th e ax is o f th e l i g h t beam p assed through an
an n u la r a p e r tu re and impinged on th e cathode o f a p h o to m u lt ip l ie r tube
(F ig . 6 ) .
M ikirov c a lc u la te d a s c a t te r in g in d ic a to r , based on a summation
o f g eo m etrica l and d i f f r a c t io n e f f e c t s , and th e reb y r e l a t e d th e f lu x i n
te rc e p te d by th e an n u la r diaphragm to th e s iz e o f th e s c a t te r in g d ro p le t .
Using o i l d r o p le ts , he was ab le to e m p ir ic a lly v a l id a te t h i s t h e o r e t i
c a l ly determ ined r e l a t i o n . O il d ro p le ts were used in p re fe re n c e to w a ter
d ro p le ts in th e c a l ib ra t io n .b e c a u s e o i l has th e same s c a t te r in g in d ic a
t o r as w a te r , f o r d ro p le ts la r g e r th an 4 .0 ym in d ia m e te r , b u t i s much
le s s v o l a t i l e . The c a l ib r a t io n re q u ire d th a t d ro p le ts p a ss through th e
sample volume and be c o l le c te d on a g la s s p l a t e . The s iz e o f th e c o l
le c te d d ro p le ts was determ ined under a m icroscope fo r com parison w ith
th e s ig n a l g en era ted by th e dev ice under c a l i b r a t io n .
Having been c a l ib r a t e d , th e o u tp u t o f th e d e v ic e , c o n s is t in g o f
v o lta g e p u lse s p ro p o r tio n a l in h e ig h t . to th e square o f th e in d iv id u a l
35
A irstreamC o llim a to r L igh t tra pCondenser
SourceP h o to m u ltip lie r
Stop
A perture
F ig . 6 . Schem atic Diagram o f M ik iro v 's Method (M ikirov 1957)
F i l t e r
Sampleda irs tre a m
In take
'A ir s leev e
F ig . 7. In tak e Arrangement o f Kazas (1963)
d ro p le t r a d i i , was q u an tized by a s ix -c h a n n e l p u lse h e ig h t a n a ly z e r .
M ikirov designed th e dev ice to respond to d ro p le ts ran g in g in s iz e from
2 .0 to 30 ym in ra d iu s and p o rtra y e d h is d a ta in h is to g ram form. The
dev ice was c a p a b le . o f h an d lin g d ro p le t number d e n s i t ie s o f up to
10 ,000/m l. In a f i e l d t r i a l i t a c tu a l ly measured 9950 d ro p le ts in 20
sec . • .
L ak tio n o v 's E x tension o f M ik iro v 's Method
Laktionov (1958) d e sc rib e d an a e ro so l sam pling dev ice s im i la r to
th a t o f M ikirov (1957). In L ak tio n o v 's d e v ic e , an e l l i p s o id a l m irro r
was used to c o l l e c t l ig h t s c a t te r e d in to a s o l id ang le o f 3ir s te r a d ia n s
by a d ro p le t p a ss in g th rough th e focus o f a converging l i g h t beam. This
dev ice was capab le o f a c c u ra te ly sam pling p a r t i c l e p o p u la tio n s w ith a
number d e n s ity o f up to 400 /m l. . T y p ic a lly , 4 to 5 l i t e r s were sampled
in 2 min by th e a irc ra f t-m o u n te d d e v ice . A fo u r-ch an n e l p u lse h e ig h t
an a ly ze r s o r te d s ig n a l p u lse s in to fo u r groups co v ering a d ro p le t diam
e t e r range o f 4 .0 to 8 .0 ym. Laktionov a ttem p ted to ensure is o k in e t ic
flow through h is p ro b e , b u t was n o t e n t i r e ly su c c e ss fu l in doing so .
K azas' Refinem ent o f L ak tio n o v 's Method
Kazas (1963) re p o r te d on an improved dev ice based on th a t o f
Laktionov (1958). O p tic a l ly , the dev ices were n e a r ly i d e n t i c a l , b u t th e
improved v e rs io n was h e rm e tic a lly s e a le d , in p a r t , in o rd e r to p rev en t
th e "sw eating" th a t had p re v io u s ly o ccu rred under c o n d itio n s o f low tem
p e ra tu re and h igh h u m id ity . An " a i r s le e v e " o f 100% h u m id ity , b u t con
ta in in g no d r o p le ts , surrounded th e 1 .0 mm d iam eter p a r t ic le - b e a r in g
a irs tre a m th a t p assed through th e sample volume o f th e d ev ice (F ig . 7 ) .
The humid a i r s le e v e was designed to p re v e n t ev ap o ra tio n o f th e d ro p le ts
in th e a irs tre a m b e in g m easured. R efinem ents in th e e le c t r o n ic s a ffo rd e d
an in c re a se in th e s e n s i t i v i t y and re s o lv in g power o f th e d ev ice . Five
s iz e g roups, co v ering d ro p le ts o f d iam eters rang ing from 4 .4 to 12.6 pm,
were measured fo r d ro p le t c o n c e n tra tio n s o f up to 450/ml a t an a irs tre a m
v e lo c i ty (through th e sample volume) o f 22 m /sec. The au th o r claim ed
th a t , d e sp ite a n is o k in e t ic flow and th e use o f a sm a ll-d ia m e te r m echani
c a l c o n s t r a in t on th e sampled a ir s tre a m , th e measured d ro p le t s p e c tra
were r e p re s e n ta t iv e o f d ro p le ts in an u n d is tu rb e d environm ent.
F u r th e r Work by Kazas and O thers
Kazas and o th e rs (1965) d e sc r ib e a fo rw a rd -s c a t te r in g dev ice de
signed s p e c i f i c a l ly to measure th e s iz e d i s t r ib u t io n o f c loud d ro p le ts .
ran g in g in s iz e from 30 to 150 ym in d iam e te r. This d ev ice had a design
p r in c ip le g e n e r ic a l ly s im i la r to t h a t o f th e dev ice d e sc r ib e d by M ikirov
(1957), b u t n o t q u ite so s im p le . The au th o rs took care to m a in ta in i s o
k in e t ic in ta k e o f w a te r d r o p le ts , c i t i n g th a t th e most im p o rtan t source
o f e r r o r in d ro p le t sam pling from a i r c r a f t was a n is o k in e t ic flow o f th e
sampled a ir s tre a m . They determ ined th a t th e measurement e r r o r due to
d ro p le ts s t r ik in g th e s id e w a lls o f th e e n tran ce tube was a maximum o f
20%. The o v e r - a l l maximum e r r o r o f th e dev ice was c laim ed to be 20%
over a d ro p le t s iz e range o f 20 to 150 pm in d iam eter a t a maximum d rop
l e t number d e n s ity o f 15/m l. :
Method o f Konyshev and Laktionov' " : ' . c :
O p tic a l s e p a ra tio n o f th e sample volume m ight appear to have
d i s t i n c t advantages over m echanical s e p a ra t io n . With o p t ic a l s e p a ra tio n
38
th e re would be le s s d is tu rb a n c e o f th e d ro p le t env ironm ent, r e s u l t in g ,
p e rh a p s , in a probe d esig n th a t would a llow a c lo s e r approach to i s o
k in e t ic flow . Measurement e r r o r s p e c i f i c a l l y due to d ro p le ts s t r ik in g
th e i n t e r i o r o f th e probe would be re d u c ed .
I t was, however, th e ic in g problem s a s s o c ia te d w ith m echanical
s e p a ra tio n in sup erco o led clouds t h a t le d to th e method o f Konyshev and
Laktionov (1966), A K oh ler-type i l lu m in a tio n system p ro je c te d th e image
o f a square a p e r tu re ac ro ss to a focus in th e c e n te r o f a 25-mm-diameter
a ir s tre a m . A m icroscope o b je c t iv e , w ith i t s a x is o rth o g o n a l to th e a i r
flow and th e i l lu m in a to r a x i s , c o l le c te d l ig h t s c a t te r e d a t 90° from th e
i l lu m in a to r a x is . D ro p le ts in th e re g io n o f th e in -fo c u s image o f th e
i l lu m in a to r a p e r tu re were imaged on o r n ea r th e cathode o f a p h o to m u lti
p l i e r tu b e . Because a square a p e r tu re was p laced n e a r th e ph o to ca th o d e ,
th e en tran ce p u p il o f th e m icroscope was sq u a re , be in g lo c a te d a t , and
p e rp e n d ic u la r t o , th e in -fo c u s i l lu m in a to r a p e r tu re (F ig . 8 ) . The sam
p le volume, n e g le c tin g v ig n e t t in g e f f e c t s o f th e a p e r tu re s , was a cube
3 .0 mm on a s id e . The v ig n e t t in g caused in c o r re c t s iz in g o f n o t more
than 6% o f th e d ro p le ts sam pled. An a d d i t io n a l e r r o r , a fu n c tio n o f
d ro p le t s i z e , o ccu rred in th e s iz in g o f th o se d ro p le ts in te r c e p t in g
th o se boundaries o f th e sample volume p a r a l l e l to th e a ir f lo w .
The au th o rs claim ed th a t th e range o f d ro p le t d iam eters th a t
could be r e l i a b ly determ ined extended from 25 to 150 pm. T y p ic a lly , 500
ml o f cloud a i r was sampled p e r sec a t an a irsp e e d o f 70 m /sec. D ro p le ts
la r g e r th an 30 pm in d iam e te r were found ini number d e n s i t ie s ran g in g from
0 .1 to 10/m l. ; ' :
39
Prism ir l
Square a p e r tu re
P h o to m u ltip lie r
O b jec tiv elens
Sample volume
O b jec tiv elen sL ight t ra p s
1r qg> Source
,ens
Squarea p e r tu re
F ig . 8. Schem atic Diagram o f th e Method o f Konyshev and Laktionov (1966)
40
T h e o re tic a l Developments
Work o f S h if r in and Kolmakov
The th e o ry a p p lic a b le to l i g h t s c a t te r in g te ch n iq u es fo r s iz in g
■and coun ting cloud d ro p le ts has developed ra p id ly w ith in th e p a s t few
y ears to th e p o in t th a t th e perform ance o f proposed s c a t te r in g methods
can now be p re d ic te d . S h if r in and Kolmakov-(1966), f o r exam ple, were
ab le to d isc u ss th e e f f e c t s o f l im i t in g th e s o l id ang le w ith in which
s c a t te r e d l i g h t i s measured and o f l im i t in g th e range o f p a r t i c l e s iz e s
to be accounted f o r .
Work o f Hodkinson and G re en fie ld
Hodkinson and G re en fie ld (1965) d is c u s se d , a t le n g th , th e v a r i
ous in s tru m e n ta l pa ram ete rs p e r t in e n t to th e desig n o f a e ro so l c o u n te r s .
In p r in c ip le , th e a e ro so l co u n te rs d iscu ssed were s im i la r to th e l ig h t
s c a t te r in g d ro p le t co u n te rs a lre ad y d is c u s se d . The d is c u s s io n o f
Hodkinson and G re en fie ld was based on computed Mie th e o ry an g u la r s c a t
te r in g d i s t r ib u t io n s fo r s p h e r ic a l p a r t i c l e s having V arious in d ic e s o f
r e f r a c t io n . V arious g e n e ra liz e d in s tru m en t desig n s ad ap tab le to d ro p le t
coun ting and s iz in g were d isc u sse d . Because th e e f f e c t o f r e f r a c t iv e
index on th e an g u la r d i s t r ib u t io n was em phasized, much o f th e d is c u s s io n
was n o t p e r t in e n t to th e sam pling o f w a te r d ro p le ts .
F raunhofer A pproxim ation o f Hodkinson: \
In a very in s t r u c t iv e and re le v a n t p a p e r , Hodkinson (1966) d is -- ■;
cussed th e p o te n t ia l o f p a r t i c l e s iz in g by means o f th e fo rw ard s c a t t e r
in g lo b e . The a u th o r n o ted th a t fo r p a r t i c l e s .10 o r more tim es la rg e r
41
than th e i l lu m in a tin g w avelength , F raunhofer d i f f r a c t io n accounts f o r
th e forw ard s c a t te r in g lobe f a i r l y w e ll . Even f o r p a r t i c l e s iz e s w ith in
th e range w herein Mie th e o ry i s u s u a lly a p p lie d , th e r e l a t i v e shape o f
th e forw ard s c a t te r in g lobe may. be determ ined from F raunhofer d i f f r a c
t io n . Hodkinson th e o r iz e d th a t i t was p o s s ib le to s iz e p a r t i c l e s by
m easuring th e r e l a t i v e an g u la r d i s t r ib u t io n o f i n t e n s i t y in th e forw ard
lo b e , w ith o u t knowing th e r e f r a c t iv e index o f th e p a r t i c l e . This p o in t
was dem onstrated w ith graphs comparing an g u la r d i s t r ib u t io n s due to Mie
th eo ry w ith th o se due to F raunhofer d i f f r a c t i o n . These graphs may be
used to p r e d ic t th e th e o r e t i c a l re s o lv in g power o f in s tru m e n ts th a t s iz e
p a r t i c l e s by means o f th e l ig h t s c a t te r e d in th e forw ard lo b e . Hodkinson
in c lu d ed a c tu a l d a ta computed from d i f f r a c t i o n c o n s id e ra t io n s , as a p p lie d
to some measurement schem es.
Q uenzel’s Work w ith Mie Theory
In a p ap er to be p u b lish e d , Quenzel d iscu ssed th e s c a t te r in g
p ro p e r t ie s o f a e ro so l p a r t i c l e s w ith d i f f e r e n t in d ic e s o f r e f r a c t io n .
E x tensive c a lc u la t io n s o f s c a t te r e d l i g h t , in accordance w ith Mie th e o ry ,
were accom plished w ith th e a id o f a d i g i t a l com puter. These d a ta , i n
c o rp o ra tin g th e s p e c t r a l response o f th e a e ro so l c o u n te r , had n o t been
p re v io u s ly a v a i la b le . Q u enze l's c a lc u la t io n s were r e s t r i c t e d to p a r
t i c l e s ran g in g from 0 .2 to 6 .0 ym in d ia m e te r . A lso , a source having a
c o lo r tem p era tu re o f 2800° wa^ assumed. Hodkinson (1966) had r e s t r i c t e d
h is use o f Mie th e o ry to p a r t i c l e s in th e s iz e range o f 0 ,05 to 5 .0 ym
in d ia m e te r , f o r an i l lu m in a tin g w avelength o f 0 .5 ym. For such i l lu m i
n a tio n , th e n , Mie th e o ry i s o f r a th e r l im ite d use over th e nom inal 1 .0
to 100 ym s iz e range o f cloud d r o p le t s . Q u enze l's c a lc u la t io n s may be
u s e f u l , however, in th e a cc u ra te d e te rm in a tio n o f d ro p le ts ran g in g in
d iam eter from 1 .0 to 6 .0 ym--a range in which e m p iric a l c a l ib r a t io n o f
d ro p le t s iz in g d ev ices i s most d i f f i c u l t
CHAPTER IV
DISCUSSION :
Of a l l th e d ro p le t s iz in g methods d e sc r ib e d , th e most w idely
used have been th e v a rio u s r e p l ic a t io n methods u s in g s o o t , magnesium ox
id e , o r Formvar c o a tin g s ( in re c e n t y e a r s ) . The problem s a ss o c ia te d
w ith o b ta in in g a c c u ra te samples by th e se te ch n iq u es a re enormous, Drop
l e t im paction a t a i r c r a f t sp eed s—u s u a lly more than 50 m /s e c - - is n e ce s
s a r i l y accompanied by a d is tu rb a n c e o f th e d ro p le t environm ent. The
e f f e c t s o f d is tu rb in g th e d ro p le t environm ent were n o t f u l l y a p p re c ia te d
o r f u l l y known by th e e a r l i e r re s e a rc h e rs u s in g im paction te c h n iq u e s .
May's work (1950, 1961) w i l l se rv e to s u b s ta n t ia te t h i s s ta te m e n t. The
m ajor such e f f e c t i s th a t o f d ro p le t ev ap o ra tio n due to a change in
p re s su re o r tem p era tu re o f th e cloud a i r j u s t b e fo re im paction o c c u rs .
A nother e f f e c t , t h a t o f d ecreased c o l le c t io n e f f ic ie n c y f o r th e sm a lle r
d r o p le ts , re g a rd le s s o f e v a p o ra tio n , has been in v e s t ig a te d e x te n s iv e ly
(Langmuir and B lo d g e tt 1946). Perhaps th e need f o r f u r th e r work in t h i s
a re a was in d ic a te d by th e comments o f Lodge (Weickmann and Smith 1957,
pp. 182-183),
With th e im paction methods th e re rem ains some u n c e r ta in ty as to
th e sample volume re p re se n te d by a g iv e n , exposed t a r g e t and to th e de
gree to which th e r e p l ic a t io n s re p re s e n t f r e e -c lo u d d ro p le t d i s t r i b u
t io n s . A m ajor drawback to th e se methods i s th e te d io u s d a ta re d u c tio n
p ro cess w herein in d iv id u a l r e p l ic a t io n s a re examined, s iz e d , and counted
under a m icroscope. Such a p rocedure as t h i s i s a d e te r r e n t to o b ta in
in g sam ples la rg e enough to be s t a t i s t i c a l l y r e l i a b l e . In a word, th e
r e p l ic a t io n methods a re cumbersome.
The o p t ic a l sam pling te c h n iq u e s , n o ta b ly th o se in v o lv in g s in g le
p a r t i c l e s c a t te r in g and th a t o f K nollenberg (N ationa l C en ter f o r Atmos
p h e r ic R esearch 1968), show much prom ise in overcoming many o f the
problem s a s s o c ia te d w ith im paction sam pling . Less d is tu rb a n c e o f th e
d ro p le t environm ent occurs w ith some o f th e se methods th an w ith any o f
th e im paction m ethods. H igher a i r c r a f t speeds may be to le r a te d w ith o u t
lo s s o f accu racy . A lso , th e se te ch n iq u es may e a s i ly be read out in r e a l
tim e , p ro v id in g a f a s t measure o f d ro p le t d i s t r i b u t io n . This f e a tu re
should appeal to th o se m e te o ro lo g is ts who wish to fo llo w th e p ro g ress o f
a c lo u d -seed in g ex p erim en t, f o r exam ple, w ith o u t w a itin g hours fo r r e
duced d a ta on changes in d ro p le t d i s t r ib u t io n w ith tim e .
In t h i s th e s i s an a ttem p t has been made to rev iew some o f th e
S o v ie t s in g le s c a t te r in g methods fo r s iz in g and cou n tin g c loud d ro p le ts
from a i r c r a f t . W estern m e te o ro lo g is ts seldom re fe re n c e th e ex ten s iv e
S o v ie t l i t e r a t u r e on t h i s s u b je c t , perhaps due to th e d i f f i c u l t y t h i s
rev iew er ex p erienced in t ry in g to lo c a te th e re le v a n t S o v ie t jo u rn a ls .
I t i s n o t c le a r w hether W estern m e te ro lo g is ts p r e f e r th e im paction meth
ods because o f t h i s communication d i f f i c u l t y o r because th ey reg a rd th e
s in g le s c a t te r in g methods as le s s a c c u ra te o r le s s r e l i a b l e .
K nollenberg (1969 p r iv a te communication) b e lie v e s th e l a t t e r to
be th e c a se , c la im ing an in h e re n t s iz in g e r r o r o f 25% f o r th e s in g le
s c a t te r in g m ethods. This re v ie w er, how ever, b e lie v e s t h a t th e claim o f
a 5% e r r o r fo r th e IIT R esearch I n s t i t u t e co u n ter (Lieberman and Katz
45
1962) i s r e l i a b l e . A lthough t h i s c o u n te r d e s ig n , o r ig in a l ly developed
in 1951, has n o t been used on a i r c r a f t , i t does seem to form th e b a s is
fo r a S o v ie t p a te n t (Konyshev and Laktionov 1966) on a dev ice th a t has
been used on a i r c r a f t f o r sam pling cloud d ro p le ts .
The d i f f i c u l t y in comparing th e s iz in g a c c u ra c ie s o f th e v a rio u s
methods makes i t d i f f i c u l t , i f n o t im p o ss ib le , to draw any d e f in i te con
c lu s io n s re g a rd in g t h e i r pe rfo rm an ces . N e v e r th e le s s , i t seems to t h i s
rev iew er t h a t , w ith th e su p p o rt o f re c e n t th e o r e t i c a l s tu d ie s by
Hodkinson and G re en fie ld (1965) and Hodkinson (1966), f u r th e r a t te n t io n
should be p a id to th e l ig h t s c a t te r in g te c h n iq u e s , some o f which a re
used in p a r t i c l e co u n te rs com m ercially a v a i la b le in t h i s co u n try .
In t h i s re g a rd , i t w i l l be o f i n t e r e s t fo r th e re a d e r to n o te
th a t D avies (1969 p r iv a te communication) re fe re n c e d a re c e n t a r t i c l e ap
p e a r in g in C ontam ination C on tro l (1969), which surveys th e v a rio u s com
m erc ia l d e v ic e s . This a r t i c l e in c lu d e s a com prehensive l i s t i n g o f U. S.
m anufactu rers and th e ty p es o f co u n te rs th e y m an u fac tu re .
Lieberman (1967) d e sc rib e d some o f th e methods used w ith th e
commercial d e v ic e s . He n o t only d isc u sse d e s ta b l is h e d te c h n iq u e s , b u t
a lso d iscu ssed b r i e f l y some new te c h n iq u e s . , In g e n e ra l , how ever, th e
methods d iscu ssed a re n o t e a s i ly a d ap ta b le to c loud d ro p le t sam pling
from a i r c r a f t , p r im a r i ly due to th e r i g i d requ irem en t th a t th e cloud
d ro p le ts n o t be d is tu rb e d p r io r to and d u rin g sam pling.
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