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.

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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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Borovikov, A. M ., I . I . G a iv o ro n sk ii, E. G. Zak, V. V. K ostarev , I . P.M azin, V. E. M inerv in , A. Kh. K hrgian, and S. M. Shm eter, inCloud P h y s ic s , Ed. by A. Kh. K hrgian. L en ingrad , G idro- m eteoro log ischeskoe I z d a te ls tv o , 1961. E n g lish t r a n s la t io n : Je ru sa lem , I s r a e l Program fo r S c ie n t i f i c T r a n s la t io n s , 1963, . Chap. XI, pp . 1 , 70, 71, 75, 76.

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, Q uenzel, H. , " In f lu e n c e o f r e f r a c t iv e index on th e accuracy o f s iz e , de­te rm in a tio n o f a e ro so l p a r t i c l e s w ith l i g h t - s c a t t e r in g a e ro so l c o u n te r s ," Appl. O pt. ; to be p u b lish e d .

49

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