Artificial rain and local fog dispersal

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Artificial rain and local fog dispersal

Schwob, M.

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P a g e s F i g u r e Ottawa, Canada TECHNICAL TRANSLATION D i v i s i o n of Mechanical E n g i n e e r i n g

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P r e f a c e

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4 Text

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30 s

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4 1 T e c h , T r a n s .

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TT-87 Date

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20 A p r i l 1949 F i l e No,

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12-R4-22 Lab, Order

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55258 T i t l e : La P l u i e A r t i f i c i e l l e e t l a D i s s i p a t i o n L o c a l e d e s B r o u i l l a r d s

By: Marcel Schwob

Referenca : 0 ,

N,

E , R , A , , P u b l i c a t i o n No, 1 0 , 1 9 4 8 , P a r i s , P a p e r r e a d 4 June 1948 a t La

Maison de l a Chimie

S u b j e c t : ARTIFICIAL R A I N AND LOCAL FOG DISPERSAL

SubmLtted by:

J ,

L o O r r , H e a d , Low Temperature L a b o r a t o r y , T r a n s l a t e d by: D , A , S i n c l a i r . Approved by :

J ,

H,

P a r k i n , D i r e c t o r ,

Page

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(ii)

Tech. Trans. TT-87

TABLE OF CONTENTS

List of Illustrations

Text

List of References

Bibliography

Photographs

Page

(111

1

1

28

Tech, T r a n s , TT-87

LIST OF ILLUSTHAT IONS

F i g u r e Curves of e q u i l i b r i u m of w a t e r / i c e and w a t e r v a p o u r _{1, 2} S a t u r a t i o n v a p o u r p r e s s u r e a t d i f f e r e n t t e m p e r a t u r e s 3 Water d r o p l e t s i n s u s p e n s i o n i n a c l o u d

-

by Houghton ( m a g n i f i e d 70 t i m e s ) 4 S t a t i s t i c a l c u r v e of w a t e r d r o p l e t d i a m e t e r s i n a f u l l y d e v e l o p e d c l o u d 5 S t a t i s t i c a l c u r v e of w a t e r d r o p l e t d i a m e t e r s i n a

newly formed c1ou.d 6

Cumulus and f r a c t o c u r n u l u s c l o u d s 7 Curr,ul-us c o n g e s t u s 1 0 "Sea" of s t r a t u s c l o u d s on t h e Limagne 11 S t r a t o c u m u l u s 1 2 V a r i o u s t y p e s o f n a t u r a l s o l i d p r e c i p i t a t i o n s 13 Ex.amples of s o l i d p r s c i p i t a t i o n s produced a r t i f i c i a l l y i n t h e l a b o r a t o r y 1 4 M ~ n t L a c h a t S t a t i o n 15-17 Clouds s e e d e d w i t h d r y i c e

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s u n r e f l e c t i n g from t h e i c e c r y s t a l s 18 Cl.ouds s e e d e d w i t h d r y i c e

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s o u t h e r l y d i r e c t i o n w i t h s u n t o s o u t h w e s t 1 9 Clouds s e e d e d w i t h d r y i c e

-

n o r t h w e s t e r l y d i r e c t i o n w i t h s u n t o s o u t h w e s t 20 S i l - v e r i o d i d e g e n e r a t o r

-

l a b o r a t o r y i n s t r u m e n t 2 0 ( a ) Appearance of s e e d e d c l o u d f o r t y - f o u r m i n u t e s a f t e r s e e d i n g 2 1

Page

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( i v )

Tech,

T r a n s .

TT-87

F i g u r e Development of seeded c l o u d shown i n F i g u r e s 20

and 2 1 22

Development of seeded c l o u d a s determined from

t h e photographs and a c c o r d i n g t o a l t i t u d e 23 Dry i c e d i s p e n s e r f o r f l i g h t o p e r a t i o n s 2 4 , 25 E l e c t r i c a l l y h e a t e d s i l v e r i o d i d e g e n e r a t o r f o r f l i g h t t e s t s 26 D i s s i p a t i o n of c l o u d s by shock waves ( t u n n e l e n t r a n c e and e x p l o s i v e s cage ) 27 C o n t a c t o r f o r c o n t r o l l i n g e x p l o s i o n s 2 8 S t r e a m l i n e s h i e l d 29 T e s t c y l i n d e r 30

T e s t w i t h c a r b o n i c snow i n a newly formed c l o u d 3 1 T e s t w i t h carbon d i o x i d e g a s r e l e a s e d from a c a r t r i d g e T e s t w i t h d r y i c e 33 T e s t w i t h . f l a s k f u l l of s o l i d C02 T e s t w i t h c o l d ZnO p a r t i c l e s T e s t w i t h l i q u i d a i r 36 S a t u r a t e d s o l u t i o n of calcium c h l o r i d e 37 S a t u r a t e d s o l u t i o m o f c a l c i u m c h l o r i d e and sodium c h l o r i d e E f f e c t of s e e d i n g w i t h d r o p s of w a t e r ( 5 0 t o 100p m )

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e l e c t r i c a l l y charged and uncharged E f f e c t of s e e d i n g w i t h c o l d w a t e r ( O ° C . ) E f f e c t of s e e d i n g w i t h warm water (55Oc.)

Tech, T r a n s .. T T - 8 7

AZTIFICIAL R A I N AND LOCAL FOG DISPEHSsL --- -- -".----A

---

The w e a t h e r we a r e p r e s e n t l y e x p e r i e n ~ i n g i n F r a n c e , p a r t i c u l a r l y in t h e P a r i s d i s t r i c t , and a l s o t h e f a c t t h a t v a c a t i o n t i m e i s a p o r o a c h i n g , m l g h t s u g g e s t t h a t a p a p e r on s u i t a b l e methods of p r e v e n t i n g n a t u r a l r a i n f a l l would be more t o t h e p o i n t t h a n a d i s c u s s i o n of artificial r a i n . However, i t i s t h e l a t t e r s u b j e c t w i t h which I am g o i n g t o a t t e m p t t o d e a l on t h i s o c c a s i o n -

S i n c e t h e end of 1946 t h e g e n e r a l p u b l i c h a s b e e n r e a d i n g i n t h e p r e s s a b o u t t h e s u c c e s s of American scientists i n p r o d u c i n g r a i n a r t i f i c i a l l y . J o u r n a l i s t i c i m a g i n a t i o n s have been g i v e n f r e e r e i n and t h e most f a n t a s t i c r e p o r t s have been c i r c u l a t e d a l o n g w i t h a l l s o r t s of schemes f o r u t i l i z a -

t i o n , many of which a r e a s f a r from p r e s e r l t olD f u t u ~ e r e a l i z a t i o n a s t h e y p o s s i b l y c o u l d b e - I n t h e meantjrfAe, however, p r a c t ! c a l e x p e r i m e n t s have b e e n c a r r i e d o u t on e v e r y c o n t i n e n t . A t t h e o u t s e t I t s n o u l d be rnade c l e a r t h a t , w h i l e g e n e r a l l y s p e a k i n g I n f o r m a t i o n a b o u t t h e c o n d i t i o n s and r e s u l t s of t h e s e experbimerlts i s r a t h e r meagre, i t i s v e r y p r o b a b l e t h a t t h e number of f a i l u r e s g r e a t i , y e x c e e d s t h e number of' s u c c e s s e s , A p e r s o n a l e x p e r i e n c e

may

p e r h a p s b e c i t e d " One day l a s t September, w h i l e I was a t te r ~ d i - n g t h e A e r o n a u t i c a l E x h i b i t i o n a t R a d l e t t , England, a d v e r t i s e m e n t s a p p e a r e d on

t h e f ' r o n t p a g e s of .the p o p u l a r S r f t i s l l n e w s p z c ~ ~ . j - s anno-:ulci.ng t h a t s y s t e m a t i c a t t e m p t s t o p r o d u c e ar t i f i c i a ? , r a i n would b e made t h e f o l l o w f n g d a y , Subsequenbly t h e t u n e i2ad t o be

changed. R a i n d i d n c x o c c u r 3 anywliere, and a f e w t e r s e bj;-- l i n e s e x p l a i n e d s i m p l y t h a t t h e ~ ~ e u u l t s had been n e g a t i v e b e c a u s e 'the e x p e r i m e n t s had n o t b e e n c a r r i e d o u t on t h e p r o p e r t y p e of c l o u d " ,

The i n c r e a s i n g i m p o r t a n c e a t t a c h e d t o t h e s e

expei-iments i n a l l c s o u n t ~ - i e s l n d i c a t e s t h a t t h e y arae some-

t h i n g more t h a n a game o r

a

symptom ~ f t h e w i l l t o power of mere human b e i n g s ("To make r a l n o r f i n e w e a t h e r ' i s t h i s n o t , s o t o s p e a k , t o u s u r p t h e suprame p o w e r ? ) , and s o m e t h i n g more t h a n a comnendable d e s i r e n o t t o a l l o w o n e ' s own n a t i o n t o be o u t s t r i p p e d by America, where d u r i n g t h e l a s t e i g h t y e a r s f o r m i d a b l e s t r i d e s have b e e n made i n a l l b r a n c h e s ~ f s c i e n c e and t e c h n o l o g y , i n advance of Europe p a r t i c u l a r l y .

Page

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2

Tech. T r a n s , T T - 8 7

I n t h e c o u r s e of t h i s d i s c u s s i o n we s h a l l t r y t o a r r i v e a t t h e c r u x of t h e problem. The phenomena i n q u e s t i o n , w h i l e b a s e d on s i m p l e p h y s i c a l l a w s , a r e ol'ten complex. F o r some of them t h e r e i s s t i l l no s a t i s f a c t o r y e x p l a n a t i o n , f o r o t h e r s t h e r e a r e t o o many and a c h o i c e must be made between t h e d i f f e r e n t o n e s . I n a d d i t i o n I s h a l l t r y t o p l a c e t h e American r e s e a r c h i n i t s p r o p e r p e r s p e c t i v e . The magnitude of t h e means employed t h e r e i n d e a l i n g w i t h t h e i n d i v i d u a l problems i s c o n s i d e r a b l e , N e v e r t h e l e s s I hope t o show t h a t F r a n c e , i n p a r t i c u l a r , w i t h much more modest means, h a s made, and w i l l c o n t i n u e t o make v a l u a b l e c o n t r i b u t i o n s t o t h i s r e s e a r c h . F o l l o w i n g t h i s e x p o s i t i o n we s h a l l a t t e m p t t o o u t - l i n e

a

c o n c r e t e programme, i n d i c a t i n g b o t h t h e d i r e c t i o n s which p r e s e n t r e s e a r c h s h o u l d f o l l o w and t h e f u t u r e l i n e s a l o n g which i t s h o u l d S e d e v e l o p e d . F i n a l l y , coming t o t h e q u e s t i o n of a p p l i c a t i o n we s h a l l t r y t o p l a y t h e p r o p h e t , a l w a y s a d a n g e r o u s r o l e , a n d p r e d i c t what may r e a s o n a b l y be e x p e c t e d f r o m t h e work now i n p r o g r e s s . F o r t h e s a k e o f t h e u n i n i t i a t e d i t w i l l be n e c e s s a r y , of c o u r s e , t o r e v i e w b r i e f l y t h e u n d e r l y i n g p h y s i c a l phenomena and t h e m e t e o r o l o g i c a l b a s i s of t h e s u b j e c t , namely, c l o u d s and r a i n . I t i s hoped t h a t t h e s p e c i a l i s t s w i l l p a r d o n t h e i n t e n t i o n a l l y s k e t c h y c h a r a c t e r of t h e p r e s e n t a t i o n . I t i s n o t p o s s i b l e t o c o v e r e v e r y t h i n g i n an h o u r and a h a l f .

Everyone knows t h a t t h e a t m o s p h e r e , e v e n when i t i s c l e a r and w i t h o u t n o t i c e a b l e h a z e , c o n t a i n s a c e r t a i n amount of w a t e r i n t h e g a s e o u s s t a t e , and t h a t c l o u d s , f o g s and most

mists

c o n t a i n w a t e r i n t h e vapour s t a t e , and i n t h e l i q u i d s t a t e i n t h e form of t i n y d r o p l e t s ( t h e d i m e n s i o n s of t h e d r o p l e t s w i l l b e d i s c u s s e d l a t e r ) . What i s more i m p o r t a n t , and p e r h a p s n o t s o w e l l known t o some, i s t h a t w a t e r i n t h e s o l i d s t a t e , i n t h e form of srnall i c e c r y s t a l s , i s a l s o f r e - q u e n t l y p r e s e n t . Thus a l l t h e phenomena t o be s t u d i e d a r e governed by t h e l a w s ' o f e q u i l i b r i u m o r u n s t a b l e e q u i l i b r i u m of w a t e r i n t h e t h r e e s t a t e s , s o l i d , l i q u i d and g a s e o u s . .The vapour p r e s s u r e of t h e w a t e r c o n t a i n e d i n a g i v e n volume of a i r i s d e f i n e d a s t h e p a r t i a l p r e s s u r e of t h e w a t e r vapour c o n t a i n e d i n t h a t volume. A t t e m p e r a t u r e s above z e r o d e g r e e s c e p t i g r a d e a mass of a i r c a n o n l y be a t e q u i l i b r i u m w i t h t h e l i q u i d w a t e r i f t h e vapour p r e s s u r e i n t h e a i r h a s a v e r y d e f i n i t e v a l u e f o r e a c h t e m p e r a t u r e . I f t h e t e m p e r a t u r e f a l l s w h i l e t h e p r e s s u r e r e m a i n s c o n s t a n t , some w a t e r must condense. T h i s

Tech, Trans. TT-87 is ths familiar process of dew formation. The same effect would be rioted if there were a tendency for the vapour pressure to increase. This definite pressure for each

temperature is called maximum pressure or saturation vapour pressure. As soon as it is attained the air is said to be

saturated. The amount of water vapour contained in saturated air can be calculated. Some results of such a calculation are shown in Ffgure 3 ,

To conclude the definitions, the relative humidity of a given volume of air is the ratio of the vapour pressure of the water contained in it to the saturation pressure for the same temperature. It is expressed as a percentage. By this definition saturated air has a relative humidity of 100 per cent. It is very important to note that under certain conditions the air may have a relative humtdity greater than 100 per cent. It is then in a state of disequilibrium and is said to be "supersaturated",

At temperatures below zero the processes are similar. Here the phenomenon of sublimation occurs, and equilibrium between the ice and the water vapour can exist only for a definite value of the vapour pressure at each negative temperature.

At constant temperature the vapour transforms into ice when there is a tendency for the pressure to increase, If the pressure falls the ice must evaporate,

The curves of Figure 1 show the values of these pressures at different temperatures and the quantity+of water contained in each cubic metre of saturated air for each positive temperatureo

Finally, there is theoretically only one point of equilibrium at which the three phases can co-exist, namely, zero degrees and 4.6

mm.

of mercury.

The above applies to the equilibrium of a large plane surface of water in contact with aira The processes are different if the water is in the form of drops (effect of the radius of curvature), or if it contains salts in solution, etc. Thus curves of equilibrium might be drawn using the radius of curvature or the concentration of salts

in solution as abscissa and the corresponding saturation vapour pressure as ordinate.

All this appears very simple. Unfortunately, things do not normally happen fn this manner, Everyone knows that it is very easy to maintain water in the super-

cooled state. In meteorology, Indeed, it is the rule rather -.-.---.---.--.-.--

-

-

-.---- ---- .. . ,-.-- - .-.. . - .- . .- -.-.

-

.. C r

d s T r a n z l ~ ~ r ~ ~ ~ s TBote: There is a d . l s c r e ~ ~ ~ n c y b 1 3 ~ c

7 ;.ci -. t l i e i~.c.-isi~al document, Figure 1 s h o v s

no

Page

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4

Tech, Trans.

TT-87

than the exception. Liquid water may remain in unstable

equilibrium with the water vapour well below zero degrees,

and exceptional conditions are requjred to terminate this

unstable equilibrium.

A very important point, and it is the

accepted theory, is that the unstable curve lies above the

sublimation curve. The former, of course, is a continuation

of the curve of equilibrium for positive temperatures (cf.

Fig. 1).

To conclude this brief discussion of the physical

aspects, it is recalled that a change of state of the water

at constant temperature, e.g., change from the gaseous to

the liquid state, is accompanied by the liberation of a

definite amount of heat per gram of liquid water, called

heat of condensation.

During the inverse process the same amount of heat

will be absorbed. Indeed, cooling by evaporation is a

familiar phenomenon.

In

order to keep a bottle of wine cold

during a picnic everyone knows that it should be wrapped in

a damp cloth.

An analogous liberation of heat, called heat of

fusion, occurs

during

solidification.

After these few physical generalities it is

necessary to look at the meteorological aspects of the

question.

Everyone knows that it cannot rain if there are

no clouds and that there are often clouds without rain.

These aphorisms probably need to be stated.

You

will

.

soon understand why.

What is a cloud? It is an aerosol, i.e,, a collec-

tion of water droplets of various diameter (from a few

microns to some sixty microns) suspended in the air (Fig. 4).

The proportion of drops of each diameter in the cloud is

determined by a definlte statistical law. In the majority

of well-developed clouds the curve of distribution is a

bell-curve analogous to the one discovered by Houghton and

Radford (Fig. 5). In this case the most numerous drops

correspond approximately to the

mean

value of the diameter.

In the case of fogs, or clouds which are just

beginning to form, the curve may be L-shaped (Fig.

6);

the

smallest drops are then the most numerous. This will be

explained shortly.

Tech, T r a n s o T T - 9 7 The t e c n n i q u e of c o u n t i n g and m e a s u r i n g t h e s i z e of t h e d r o p s i s v e r y h i g h l y d e v e l o p e d i n F r a n c e , The

e n g i n e e r s of t h e Low Temperature L a b o r a t o r y a t ' B e l l e v u s , and t h o s e workirig a t t h e R e s e a r c h S t a t i o n on Mont L a c h a t , a

b r a n c h of t h e O o N e E . R . A , , h a v e , w i t h t h e h e l p 01' P r o f , Brun and T C r . L ~ z c i e n Demon, d i s c o v e r e d sorne very i r l g e n i o u s ways of c o l l e c t i n g t h e d r o p s i n a f o g o r c l o u d s o t h a t t h e y can

a c t u a l l y b e c o u n t e d and measured u n d e r t h e m i c r o s c o p e ,

O t h e r F r e n c h r e s e a r c h w o r k e r s , s u c h a s Mr, B r l c a r d and Mr, D e s s e n s , have a l s o done a g r e a t d e a l of work on t i i i s problem. F o r m e r l y t h e d r o p s were p r e c i p i t a t e d e l e c t r o -

s t a t i c a l l y i n o i l o Today

the

t e c h n i q u e p r e f e r r e d i s t o u s e v e r y d e n s e n e t w o r k s of e x t r e m e l y f i n e t h r e a d s on which t h e d r o p l e t s a r e c a u g h t . i?r0 :)emon makes t h e t n r e a d s s y n t h e t i - c a l l y from a s o l u t i o n 01' p l e x i g l a s , w h i l e Nr, Dessens p r > e f e r s t o c u l t i v a t e s m a l l s p i d e r s which do t h i s work r e m a r k a b l y

w e l l .

The mean s i z e of t h e d r o p s , of c o u r s e , depends on t h e t y p e of c l o u d and tiie s t a g e of i t s devel.opment. Iiowever, a g e n e r a l v a l u e of a b o u t twenty m i c r o n s can be g i v e n f o r i t "

\Vhen t h e Grops become v e r y 1art;e t h e y may be e x p e c t e d t o f a l l due t o t h e i r w e i g h t , and t h i s i s hvkat

a c t u a l l y o c c u r s , f o r example, i n c e r t a i n wet f o g s wilich a r e o f t e n s e e n i n mountafncus c o u n t r y . F o r d r o p s of t ~ v a r l t y m i c r o n s t h e r a t e of i s r e l a t i v e l y slow

-

1 , 3 cm, p e r second a c c o r d i n g t o s t o k e ' s l a w , Thus t h e s l P g h t e s c up- c u r r e n t and t h e l i t t l e t x r b u l e n t v o r t i c e s i n t h e c l o u d a r e s u f f i c i e n t t o keep them a l o f t , l i k e an egg on t h e wat,er* j e i

a t a f a i r . F u r t h e r m o r e , i f ~ i l e c l o u d i s a t a n a l t i t u d e of 3 , 0 0 0 i n e t r e s i t f'ollows t h a t t h e d r o p s would t a k e 390,000 s e c o n d s , o r 110 h o u r s , t o r e a c h t h e ground.

A

g r e a t many t h i n g s c a n happen t o a s m a l l d r o p of 2 0 m i c r o n s d u r i n g t h i s t i m e

.

The number of d r o p s p e r c u b i c c e n t t m e t r e of c l o u d d o e s n o t v a r y g r e a t l y , I t i s of t h e o r d e r of 500 t o 1 , 0 0 0 , The d i i r e n s i o n s of c l o u d s , ilowever, do d i f f e r g r e a t l y o Not i n f r e q u e n t l y a c l o u d may be s e v e r a l k i l o m e t r e s t h i c k and some t e n s of k i l o m e t r e s l o n g .

Another v e r y i m p o r t a n t c h a r a c t e r i s t i c of a c l o u d i s i t s w a t e r c o n t e n t , which i s of t h e o r d e r of s e v e r a l grams p e r c u b i c m e t r e . Under t h e a u s p i c e s of t h e O . N . E . R . A , a n a p p a r a t u s f o r s a m p l i n g c l o u d s h a s r e c e n t l y b e e n p e r f e c t e d at Nont L a c h a t . T h i s i s a s m a l l wind t u n n e l tllrougn which tile

T e c h , T r a n s . TT-87 a m e t a l c y l i n d e r w i t h a w i r e s t r e t c h e d a l o n g i t s a x i s , The wire c a r r i e s a v e r y h i g h v o l t a g e compared w i t h t h a t of t h e c y l i n d e r and a l l t h e w a t e r c o n t a i n e d i n t h e c l o u d i s d e p o s i t e d on t h e w a l l s of t h e c y l i n d e r by e l e c t r o s t a t i c a c t i o n . T h i s i s e x a c t l y t h e same t e c h n i q u e a s t h e f a m i l i a r one f o r d u s t e x t r a c t i o n . The c y l i n d e r i s weighed on a s m a l l p r e c i s i o n b a l a n c e b e f o r e and a f t e r d e p o s i t i o n of t h e w a t e r . Knowing t h e f l o w t h r o u g h t h e t u n n e l t h e mass of w a t e r c o n t a i n e d i n a g i v e n volume of c l o u d can e a s i l y be c a l c u l a t e d . M r , , Caron 1 ) h a s i n v e n t e d a n o t h e r a p p a r a t u s , c a l l e d an aquanebulometer, f o r measuring t h e r e l a t i v e v a l u e of t h e w a t e r c o n t e n t of a c l o u d . Two v e r s i o n s of t h i s i n s t r u m e n t a r e now b e i n g b u i l t a t t h e O , N . E . R . A o , one f o r ground

measurements a t t h e Mont Lachat L a b o r a t o r i e s , and t h e o t h e r f o r u s e aboard a n a i r c r a f t .

To conclude t h e d i s c u s s i o n of c l o u d s a few words a r e n e c e s s a r y c o n c e r n i n g t h e i r e l e c t r i c a l c h a r a c t e r i s t i c s n

I t i s w e l l known t h a t c l o u d s , p a r t i c u l a r l y s t o r m c l o u d s , may become e l e c t r i c a l l y c h a r g e d . The maximum p o s i t i v e o r nega-

t i v e c h a r g e which a s i n g l e d r o p i n a cloud may r e c e i v e i s of t h e o r d e r of some 10-7 c o g . s o e l e c t r o s t a t i c u n i t s . The e l e c - t r i c a l f i e l d of a cloud may a t t a i n s e v e r a l t e n s of k i l o v o l t s p e r c e n t i m e t r e . I t f o l l o w s t h a t t h e e l e c t r o s t a t i c f o r c e s of a t t r a c t i o n o r r e p u l s i o n between d r o p s a r e weako

A few remarks,, now, on t h e s u b j e c t of r a i n ,

Rain i s made up of much l a r g e r d r o p s , of t h e o r d e r of a m i l l i m e t r e i n d i a m e t e r . These have a volume e a s i l y 100,000 o r 1,000,000 t i m e s t h a t of t h e d r o p l e t s of a cloud o r f o g .

It w i l l p r e s e n t l y be shown how c l o u d s and r a i n form i n n a t u r e , b u t f f r s t i t i s n e c e s s a r y t o make a v e r y f u n d a m e n t a l o b s e r v a t i o n which I have borrowed from M r . R o u l l e a u , I n p o p u l a r p a r l a n c e i t i s s a i d t h a t a c l o u d f l b u r s t s " when i t r a i n s . , T h i s i s o b v f o u s l y a n e x a g g e r a t e d f i g u r e of s p e e c h , b u t i t i s one which might l e a d u s t o suppose t h a t a c l o u d c o n t a i n s a c e r t a i n q u a n t i t y of w a t e r which can be t r a n s f o r m e d i n t o r a i n , and t h a t when t h e c l o u d i s "empty" t h e r a i n must c e a s e . T h i s i s p r e c i s e l y t h e image of B i b l i c a l t i m e s , f o r , r e f e r r i n g t o t h e f l o o d t h e B i b l e s a y s t h a t " t h e same day were a l l t h e f o u n t a i n s of t h e g r e a t deep broken up, and t h e windows of Heaven were opened and t h e r a i n was upon t h e e a r t h f o r t y days and f o r t y n i g h t s " .

Many s c i e n t i s t s , w h i l e knowing v e r y w e l l t h a t a cloud i s n o t a v e s s e l f i l l e d w i t h w a t e r , have n e v e r t h e l e s s r e a s o n e d t h u s .

Tech, Trans.

TT-87

Now it can easily be shown by calculation that a cloud is able to provide an amount of rain greatly exceeding the amount of water contained in it in the form of droplets. It might be expected that a cloud three kilometres thick, containing three grams of water per cubic metre, would

provide only 9 rnm. rainfall. But often a cloud of this size yields five times this amount. This statement can easily be

verifSed with the aid of a rain gauge, a very simple instru- ment, for which, moreover, a jam ,jar may be substituted.

The very important conclusion to be drawn from this is that such clouds are continually rebuilding them- selves at the expense of the water vapour in the surrounding atnosphere. Over a certain period they may be compared to the storage batteries in automobiles, which are continually being discharged while a generator continually recharges

them. Or again, they are analogous to the famous bath tub which caused us so much trouble during our final examina- tions, the one which we vainly tried to fill while the drain faucet remained open.

Thus a cLoud now appears to be an indispensable medium for the occurrence of rain, but its actual capacity

greatly exceeds that which might be expected from a merely superficial examination of the problem. We are now faced with two fundamental questions:

1.

In what manner, and from what causes, does a cloud form in clear atmosphere? This is the study of the so-called condensation process.

2.

In what manner, and for what reasons, is rain generated in a cloud? The transformation of a cloud into rain, hail or snow is called precipitationo

These are two very different problems and many difficulties are due to the fact that they have frequently been confused.

How do clouds form?

(a) By the intermlngling of masses of homogeneous air, without change of altitude. This first and very old theory is that of Hutton, who used it to explain the

occurrence of rain as a result of the confusion just men- tioned. kTr. Marcel Brillouin proposed it as an explanation of the formation of clouds. When two masses of clear air at different temperatures and humidities mix, it is possible, under certain conditions, to get a vapour pressure above

the saturation point. -Condensationlmay then occur, For

Tech, T r a n s , TT-€37

r e l a t i v e h u m i d i t y and 2 , 3 mrn, vapour p r e s s u r e i s mixed w i t h two k i l o g r a m s of a i r a t 3 0 ° , 90 p e r c e n t r e l a t i v e h u m i a i t y and 2 8 , 6 mm, vapour p r e s s u r e , t h e r e s u l t s h o u l d be t h r e e k i l o g r m s of a i r a t a t e m p e r a t u r e and vapour p r e s s u r e d e t e r - mined b y t h e law of m i x t u r e s , namely, 20° and 1 9 , 8 mrn, of m e r c u r y n The s a t u r a t i o n v a p o u r p r e s s u r e of t h e m i x t u r e i s

o n l y 1 7 , 5 rnm, There may be c o n d e n s a t i o n , b u t when t h e h e a t of c o n d s n s a t i o n i s t a k e n i n t o a c c o u n t t h e amount o f w a t e r which c a n b e produced by t h i s p r o c e s s r e m a i n s v e r y s m a l l , Hence, t h i s would o n l y e x p l a i n t h e f o r m a t i o n of v e r y l i g h t c l o u d s , which a r e of l i t t l e i n t e r e s k irom t h e p o i n t of view oS p a i n ,

( b ) A d i a b a t i c e x p a n s i o n .

I f an a d i a b a t i c e x p a n s i o n , i , e , , a n e x p a n s i o n . u i t h o u t exchange of h e a t w i t h t h e e x t e r i o r , t a k e s p l a c e i n

a mass of humid a i r t h e t e m p e r a t u r e o f t h i s a i r w i l l d e c r e a s e , Now, from what was s a l d a t t h e begfnrling of t h i s p a p e r

i t

f o l l o w s t h a t , p r o v i d e d t h e c o o l i n g is n o t t o o i n t e n s e , l i q u i d w a t e r may condense i n t h e form o f d r o p l e t s , Note t h a t we s a y t h e d r o p l e t s "may c o n d e n s e r t ; t h e y do n o t do s o n e c e s s a r i l y , The r e a s o n f o r t h i s w i l l s o o n be a p p a r e n t , The h e a t of

f u s i o n p o f c o u r s e , p a r t i a l l y o f f s e t s t h e p r o c e s s . The e x p e r i - ment i s w e l l known and i s p r e s e n t l y b e i n g performed i n t h e l i i l s o n c o l d chamber, It i s v a l u a b l e i n c o n n e c t i o n w i t h n u c l e a r r e s e a r c h , b e c a u s e t h e d r o p l e t s a r e d e p o s i t e d on i o n s o r s m a l l p a r t i c l e s whose p a t h s o r t r a j e c t o r i e s may t h u s be eve a l e d , Such e x p a n s i o n s c a n o c c u r i n t h e atmosphere b u t t h e y may a r i s e from d i f f e r e n t c a u s e s : 1, The g e n e r a l f l o w a b o u t low p r e s s u r e a r e a s p e r - m i t s t h e a s s u m p t i o n t h a t c l o u d s c o u l d form a t t h e c e n t r e , The e f f e c t , i f i t e x i s t e d , c o u l d o n l y be a v e r y weak o n e , T h i s t h e o r y was once a d v o c a t e d by H e r s c h e l , b u t h a s s i n c e b e e n abandoned.

2 , The e x p a n s i o n may be t h e r e s u l t o f upward a i r d i s p l a c e m e n t s due t o c o n v e c t i o n , These may be of e f t h e r t , h e m a l o r dynamic o r i g i n , There may be mass d i s p l a c e m e n t s o r t u r b u l e n t d i s p l a c e m e n t s , The l a t t e r would be s o m e t h i n g between m o l e c u l a r d i f f u s i o n due t o t h e r m a l a c t i o n and l a r g e mass d i s p l a c e r n e n t s , ~

These phenomena a r e v e r y complex and have n o t y e t been c o m p l e t e l y a n a l y s e d , However, an a t t e m p t w i l l be made t o g i v e a q u a l i t a t i v e i d e a of t h e r m a l d i s p l a c e m e n t a s

d e s c r i b e d i n t h a t r e m a r k a b l e work "Physique d e s ~ u a g e s " ( P h y s i c s of C l o u d s ) by P ~ o f e s s o r s Coulomb aqd L o i s e l ,

- ~ ~ . . . - - . .. .

-

.-. .- . .-...--. .- .-.- --

I

Ji :e -1:; :: a k i:!.. i ,. : 1 :,re :

''

e b i . - ; : , : , The

Tech, Trans,

TT-87

It is common knowledge that at low altitudes, at least, there is a negative temperature gradient; in other words, the temperature decreases with altitude at the rate of approximately 0.6' per hundred metres, This is simply because the lower layers of the atmosphere are warmed by

contact with the earth, and especially by the earth's radiation, whereas the upper layers themselves radiate more energy than they receive from the sun.

I

hasten to add that this is true only of the first few kilometres, Also, the density of the air decreases with altitude- Hence a mass of air which tends to rise will cool, and its rise will continue as long as its density is less than that of the surrounding air at the same altitude; thereafter it will tend to descend again, and tkis depends generally on the gradient, The neutral equilibrium corresponds, more or less, to a variation in temperature of one degree per hundred metres; this is the so-called Mdry adiabatic gradient". Actually, even for larger gradients

there is still equilibri-an, for the expansion is not adiabatic, the air. being cooled by thermal exchanges at its boundaries. Thls brings us to the fundamental process.

If

the mass of air can rise to the point where condensation occurs, then the heat of condensation will enable it to rise further. This condition exists even in gradients of less than one degree per klundred metres

-

to be precise, in those which are of the ordexa of 0 e 6 0 , i.eo, the gradients most flqequently found in uur own regions" This is called "the wet adiabatic gradjentu0

Kith the aid of these very vague data we are now in a position to describe the formation of a cumulus cloud.

Everyone is familiar with these clouds, whose dense forms give such fine arti-stic effects (Fig" 7, 8, 9 ) 0

At sunrise the ground becomes heated by solar

radiation

and

in turn warms the air in its immediate vicinity. The gradient near the ground is now greater than the dry

adiabatic gradient, and the air begins to risee This air is replaced by air coming from cooler regions nearby, and these regions, in turn,are fed by still cooler regions above them. Thus

a

thermal convection is commenced,. As it develops its ceiling is progressively raised and the entire bubble of air beneath the ceiling is warmed. At a certain altitude the dew point is reached and condensation may occur. If so, a cumulus cloud begins to form. The heat of condensation now enables the cloud to ascend still further, so that more condensation can occur, even when the gradient of the cloud is no longer adiabatic. The summit of the cloud can then begin to flatten out, This is the process observed, for example, in the cumulus congestus (Fig* 10).

The cloud, of course, may also be carried up by a mass movement of purely dynamic origin, in which case the

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1 0 T e c h , T r a n s . TT-87 e x t e r n a l g r o w t h may be t h e same a s t h a t j u s t d e s c r i b e d . Now a word on t h e f o r m a t i o n o f c l o u d s b y t u r b u l e n t c o n v e c t i o n , s i n c e t h f s i s p r o b a b l y t h e most i m p o r t a n t p r o c e s s f o r t h e u l t i m a t e d e v e l o p m e n t o f t h e s t u d i e s c o n c e r n i n g artificial r a i n , I n t h i s t y p e of c o n v e c t i o n we c a n i m a g i n e t h e upward movement of

a

g r e a t many s m a l i m a s s e s of a i r , t h e d i s p l a c e m e n t b e i n g p r o d u c e d , g e n e r a l l y s p e a k i n g , b y more o r l e s s r e g u l a r v o r t i c e s w i t h h o r i z o n t a l a x e s o The m a s s e s of a i r t h u s t r a n s p o r t e d h a v e d e f i n i t e t e m p e r a t u r e s and h u m i d i t i e s , v a r y i n g a c c o r d i n g t o t h e i r p l a c e of o r i g i n , a n d i t c a n t h e n b e shown t h a t on t h e a v e r a g e t h e r e l a t i v e h u m i d i t y t e n d s t o i n c r e a s e c o n s i d e r a b l y t o w a r d s t h e u p p e r p a r t s o f t h e atmos- p h e r e when t h e y have b e e n s t i r r e d by t h e s e v o r t i c e s . If t h i s h u m i d i t y r e a c h e s 1 0 0 p e r c e n t c o n d e n s a t i o n may o c c u r T h i s seems t o be t h e p r o c e s s of f o r m a t i o n of s t r a t u s and s t r a t o - cumulus c l o u d s , which a r e c l o u d s c o n t a i n i n g l e s s w a t e r t h a n t h o s e formed i n t h e l o w e r l a y e r s of t h e atmosphere ( F i g o 11 a n d 1 2 ) .

May I a g a i n draw y o u r a t t e n t i o n t o t h e manner i n which t h i s i s e x p r e s s e d , When s a t u r a t i o n i s a t t a i n e d

?r

e x c e e d e d c o n d e n s a t i o n may o c c u r . I h a v e b e e n c a r e f u l n o t t o s a y t h a t i t must o c c u r , I n d e e d , when d i s c u s s i n g t h e e q u i - l i b r i u m of w a t e r i n t h e t h r e e s t a t e s i t was s t a t e d v e r y s p e c i f i c a l l y t h a t s t a b l e e q u i l i b r i u m was t h e e x c e p t i o n and t h a t s p e c i a l p r e c a u t i o n s were n e c e s s a r y t o p r e v e n t s u p e r - c o o l i n g o r s u p e r s a t u r a t i o n , A s a m a t t e r of f a c t r e l a t i v e h u m i d i t i e s up t o 400 o r 500 p e r c e n t w i t h o u t c o n d e n s a t i o n a r e q u i t e p o s s i b l e o As f a r b a c k a s 1 8 8 0 A i t k e n o b s e r v e d i n h i s l a b o r a t o r y e x p e r i m e n t s t h a t s u c c e s s i v e a d i a b a t i c e x p a n s i o n s i n t h e e x p a n s i o n chamber p r o d u c e d f o g s of l o w e r and l o w e r d e n s i t y , and, i f t h e a i r i s f i l t e r e d t h r o u g h c o t t o n , n o c o n d e n s a t i o n w h a t e v e r t a k e s p l a c e , S i n c e t h a t t i n e t h e a c c e p t e d t h e o r y h a s b e e n t h a t no c o n d e n s a t i o q i s p o s s i b l e u n l e s s c e r t a i n s e e d s , a l s o c a l l e d ' g n u c l e i ' f o r " c o n d e n s a t i o n c e n t r e s i f , a r e p r e s e n t , Near t h e g r o u n d t h e s e s e e d s a r e e v e r y w h e r e , b u t t h e i r number d e c r e a s e s r a p i d l y w i t h a l t i t u d e , , A c c o r d i n g t o Wigand, a t 1 , 0 0 0 m e t r e s t h e r e a r e a p p r o x i m a t e l y 2 , 5 0 0 p e r c u b i c c e n t i m e t r e , b u t above 5 , 0 0 0 m e t r e s t h e r e a r e n o more t h a n 8 0 0 Nef t h e r t h e n a t u r e of t h e c o n d e n s a t f o n c e n t r e s n o r t h e manner i n which t h e y promote c o n d e n s a t i o n i s a s y e t w e l l u n d e r s t o o d , a l t h o u g h t h e r e have b e e n many i n v e s t i g a t i o n s of t h e p r o b l e m

-

C . T , A . W i l s o n ' s , Junge f s , and e s p e c i a l l y L a n g e v i n f s may be c i t e d . A t p r e s e n t F r o B r i c a r d and R ! r o Demon a r e s t i l l working on t h i s q u e s t i o n . A t d i f f e r e n t t i m e s i t h a s be'en t h o u g h t t h a t t h e s e n u c l e f m i g h t b e v e r y s m a l l m i n e r a l p a r t i c l e s , b i g o r s m a l l i o n s , o r h y g r o s c o p i c p a r t i c l e s , L a s t week M r * , Demon i n f o r m e d me t h a t h e h a d

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a

s e r i e s o f e x p e r i m e n t s a t Mont L a c h a t w h i c h seemed t o p r o v e t h a t i n t h e c a s e o f c l o u d s t h e s e c e n t r a s a r e o f t e n h y g r o s c o p i c p a r t i c l e s , M r , Dernon o b t a i n e d some n a t u r a l c l o u d d r o p l e t s w i t h . h i s p l e x i g l a s n e t s and t h e n p e r m i t t e d thern t o e v a p o r a t e i n s i d e t h e l a b o r u t o r y , 'iilatching t h e a v a p o r a t i o n u n d e r t h e m i c r o s c o p e he o b s e r v s d t h a t a n e x t r e m e l y l i g h t d e p o s i t , v i s i b l e o n l y now and t h e n b y a v a r i a t i o n i n c o l o u r i n g , r e m a i n e d on t h e t h r e a d s , T h i s d e p o s i t c e r t a i n l y c o ~ l s t i t u t e d t h e much d i s c u s s e d c o n d e n s a t i o n c e n t r e s , f o r , when h e b r e a t h e d l i g h t l y on t h e t h r e a d s d r o p l e t s a g a i n a p p e a r e d a t t h e p l a c e s where t h e p a r t i c l e s h a d b e e n s e e n , i , e , , a t t h e s a n e p l a c e s where t h e d r o p s h a d b a o n p r s v i o u s l y , However, i n my o p i n i o n t h i s s t i l l d o e s n o t e n a b l e u s t o s a y a n y t h i n g c o n c e r n i n g t h e n a t u r e o f t h e c e n t r e s ( w h e t h s r c r y s t a l l i n e o r a n ~ o r p h o u s p a r t i c l e s , w h e t h z r i o n i z e d cy n o t ) , Howev3r, t h e i m p o r t a n t f a c t t o remember i s t h a t t h e s e c o n d e n s a t i o n n u c l e i a r e i n d i s p e n s a b l e i n t h e f o r m a t i o n o f c l o u d s , and t h a t c l o u d s s u c h a s t h e c u m u l u s , which b e g i n t o f o r m a t t h e ground by t h e p r o c e s s of t h a r m a l c o n v e c t i o n , n e v e r l a c k t h e s e n u c l e i , f o r t h e r e i s a s u p e r a b u n d a n c e of

them n e a r t h e g r o u n d which a r e b o r n e upwards by t h e same p r o c e s s o f t h e r m a l c o n v s c t i o n , But t h e l a y e r s of a t m o s p h e r e above 3 , 0 0 0 m e t r e s c o n t a i n v e r y f e w , and i t i s p r o b a b l e t h a t i n many c a s e s t h e r e a r e enormous s u p e r s a t u r a t i o n s a t t h e s e a l t i t u d e s j u s t b e c a u s e o f t h i s s c a r c i t y , i3y s u p p l y i n g t h e n u c l e i i t m i g h t p e r h a p s be p o s s i b l e t o make c l o u d s , t h e u l t i m a t e s o u r c e o f r e i n ; f o r , a f t e r a l l ( p l e a s e p a r d o n t h i s r e p e t i t i o n o f a n o b v i o u s t r u t h ) , t h e r e c a n be n o r a i n w i t h o u t c l o u d s

.

The s t u d y of t h e h u m i d i t y o f t h e a t m o s g h e r e above 3 , 0 0 0 m e t r e s , i n my o p i n i o n , i s a b s o l u t . e l y f u n d a m e n t a l . It n e e d s t o be i n v e s t i g a t e d s y s t o m a t i c a l l y , and t h e O , N , E , R , A , h a s i n c l u d e d t h i s i n i t s programme. A t t h e same t i m e s y s t e m a t i c a t t e m p t s a t " s e e d i n g " w i t h c o n d e n s a t i o n n u c l e i , e , g o , s u i t a b l e h y g r o s c o p i c n u c l e i , s h o u l d be made

*

It i s v e r y p r o b a b l e , I t h i n k , t h a t a r t i f i c i a l c l o u d s c a n be c r e a t e d i n t h i s way, a l t h o u g h t h e r e i s no a s s u r a n c e t h a t r a i n c o u l d be made t,a f a l l f r o m them, However, I must add t h a t t h i s s u p e r s a t u r a t i o n i s m y o n l y hope o f c r a a t i n g a r t i f i c i a l c l o u d s l a r g e e n o u g h t o y i e l d r a i n , +k Mors t h a n a y e a r ago M r , M a u r i n s u g g e s t e d u s l n g h y g r o s c o p i c n u c l e i i n u n s a t u r a t e d a t m o s p h e r e . I do n o t t h i n k t h e r e c o u l d be any fbrmati.on o f c l o u d s i n s u c h a c a s e ,

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12

Tech, T r a n s . T T - 8 7

I n d e e d , i f we were r e s t r i c t e d o n l y t o t h e s t a b l e e q u i l i b r i u m , w e would have t o be a b l e t o pxaoduce a r t i f i c i a l a d i a b a t i c e x p a n s i o n s , f o r example, by c o n v e c t i o n , But M r . R o u l l e a u 3 ) h a s c a l c u l a t e d t h a t i n o r d e r t o c a u s e

a

column of a i r f i v e k i l o m e t r e s t h i c k and one s q u a r e k i l o x o t r e I n a r e a t o r i s e f o r f i v e h o u r s a t t h e r a t e o f 3 cm, p e r second

(which would produce 5 mrn, of r a i n o v e r an a r e a o f 100

h e c t a r e s ) i t would be n e c e s s a r y t o expend

an

e n e r g y o f t h r e e m i l l i o n k i l o w a t t h o u r s , assuming one hundred p e r c e n t

e f f i c i e n c y , It

i s

o b v i o u s t h a t t h e s o l u t i o n t o t h e problem d o e s n o t l i e i n t h i s d i r e c t i o n .

We have now r s a c h s d t h e f o l l o w i n g p o i n t : We know approxima.tely how c l o u d s form

-

by t h e p r o c e s s of condensa- t i o n . We a l s o knox t h a t t h e r e c a n be no q u e s t i o n f o r u s of making c l o u d s a r t i f i c i a l l y by a d i a b a t i c e x p a n s i o n a l t h o u g h

t h e r e i s some hope i f , a s i s p r o b a b l e , s u p e r s a t u r a t e d zones e x i s t i n g a t h i g h a l t i t u d e s a r e p r o v i d s d with c o n d e r l s a t i o n n u c l e i .

Plce come now t o t h e p r o c e s s o f r a i n f o r m a t i o n , T o e . , t h e growth o f d r o p s from t w e n t y o r so m i c r o n s t o a p p r o x i - m a t e l y one m i l l i m e t r e i n d i a m e t e r . T h i s i s t h e phenomenon o f p r e c i p i t a t i o n . F o r t h e r a i n t o r e a c h t h e g r o u n d i t i s c l e a r l y n e c e s s a r y t h a t t h a d r o p s t h u s formed must n o t e v a p o r a t e d u r i n g t h e i r f a l l , I am s u r e t h a t some of you w i l l be s u r p r i s e d t o l e a r n t h a t t h e c a u s e s o f r a i n have been known f o r l e s s t h a n f i f t e e n y e a r s . The d i s c o v e r y came l a t e r t h a n t h o s e o f

r e l a t i v i t y , t h e wave mechanism, and t h e n e u t r o n , Only t h e f u t u r e c a n t e l l u s w h e t h e r i t i s more o r l e s s i m p o r t a n t t h a n t h e s e .

J u s t a s s m a l l s t r e a m s u n i t e t o make g r e a t r i v e r s , i t seems r e a s o n a b l e t o suppose t h a t l a r g s d r o p s a r e fomned by t h e u n i o n of a number o f s m a l l d r o p s , T h i s phenomenon h a s b e e n much d e s c r i b e d , I t i s t h e p r o c e s s known a s c o a l e s c e n c e . U n f o r t u n a t e l y , i t p r o b a b l y d o e s n o t e x i s t a t a l l , o r a t l e a s t i t i s v e r y r a r s a s a c a u s e o f n a t u r a l r a i n , Vu'hat, i n d e e d , a r e t h e p o s s i b l e c a u s e s o f c o a l e s c e n c e ? 1, Aerodynamic a t t r a c t i o n of d r o p l e t s f o l l o w i n g p a r a l l e l p a t h s and r e m a i n i n g c l o s e t o one a n o t h e r d u r i n g t h e f a l l . B j e r n k n e s s t u d i e d t h i s problem. The f o r c e s

i n v o l v e d a r e v e r y weak o n e s . The d r o p s would have t o r e m a i n n e a r one a n o t h e r a t l e a s t f o r a d i s t a n c e of a t e n t h of a m i l l i m e t r e and a t i m e o f r o u g h l y one m i n u t e . F o r t h i s t o happen t h e d r o p s would have t o be of e x a c t l y t h e same d i a - m e t e r s o t h a t t h e y would have t h e saxe r a t e o f f a l l , It i s u n l i k e l y t h a t s u c h c o n d i t i o n s e v e r o c c u r ,

Tech, T r a n s . TT-87 2 , Due t o i n e r t i a , m i c r o - t u r b u l e n c a could c a u s e collisions between m o l e c u l e s of d i f f e r e n t d i a m e t e r . A t

Trappes M r . Guy Dady 4 ) observed a n a r t i f i c i a l f o g w i t h mean drop d i a m e t e r of 4p and maximum drop d i a m e t e r of 15p f o r a p e r i o d of t e n h o u r s . D e s p i t e t h e f a c t t h a t t r a j e c t o r i e s a p p a r e n t l y c o i n c i d e d f r e q u e n t l y (once i n about e v e r y t e n s e c o n d s ) , he d i d n o t s e e a s i n g l e c a s e of c o a l e s c e n c e . According t o h i s t h e o r y he e x p e c t e d 9 0 0 ?

It

i s p r o b a b l e t h a t s u r f a c e e f f e c t s p r e v e n t drops of t h e s e dimensions from u n i t i n g . It s h o u l d b e o b s e r v e d , however, t h a t t h e average drops i n a c l o u d a r e l a r g e r t h a n t h o s e o f t h e f o g s t u d i e d by M r . Dady. 3. E l e c t r o - s t a t i c e f f e c t s . It was l o n g b e l i e v e d t h a t i n s t o r m c l o u d s e l e c t r i - c a l c h a r g e s kept t h e d r o p s a p a r t , b u t t h a t a f t e r d i s c h a r g e n o t h i n g more p r e v e n t e d t h e i r c o a l e s c e n c e . We have j u s t s e e n t h a t t h i s e x p l a n a t i o n 'is q u i t e improbable. Furthermore

i t

h a s been proved t , h a t r a i n b e g i n s t o f a l l b e f o r e l i g h t n i n g o c c u r s , E l e c t r i c a l a t t r a c t i o n s due t o c h a r g e s of o p p o s i t e s i g n a r e e q u a l l y improbable. I n f a c t , such c h a r g e s have no o p p o r t u n i t y of o c c u r r i n g t o g e t h e r , And f i n a l l y , t h e a t t r a c t i v e f o r c e s of c h a r g e s of t h e o r d e r of 10'8 c , g , s , u n i t s , such a s a r e found i n e l e c t r i c a l l y charged c l o u d s , a r e v e r y s m a l l .

So much f o r c o a l e s c e n c e . Let u s t u r n now t o some much more s a t i s f a c t o r y t h e o r i e s based on t h e i d e a of t h e growth

of a l i q u i d o r s o l i d phase a t t h e expense o f t h e vapour p h a s e . A s a m a t t e r of f a c t , i f o n l y t h e s a t u r a t i o n vapour p r e s s u r e of t h e two phases a r a d i f f s r e n t , w a t e r may be t r a n s f e r r e d fzborn t h e phase of h i g h e r p r e s s u r e t o t h a t of lower p r e s s u r e ; i n o t h e r words t h e r e i s e v a p o r a t i o n from t h e f i r s t and condensa- t i o n on t h e second. This happens i n s e v e r a l v e r y i m p o r t a n t c a s e s which w i l l be d i s c u s s e d a g a i n i n c o n n e c t i o n w i t h t h e c r e a t i o n of a r t i f i c i a l r a i n :

1, A l l o t h e r t h i n g s being e q u a l , t h e s a t u r a t i o n vapour p r e s s u r e i n c r e a s e s w i t h t h e c u r v a t u r e of t h e s u r f a c e of s a t u r a t i o n of t h e two media. Thus drops of l a r g e d i a m e t e r must i n c r e a s e i n volume a t t h e expense of s m a l l d r o p s , pro- v i d e d t h e d i f f e r e n c e of c u r v a t u r e i s g r e a t enough,

2 . Sold drops i n c r e a s e a t t h e expense of warm d r o p s , T h i s i s t h e p r i n c i p l e of t h e c o l d w a l l , Indeed, t h e s a t u r a t i o n vapour p r e s s u r e i n c r e a s e s w i t h t e m p e r a t u r e ( F i g , 1, 2 , 3 1 0 3 , I c e c r y s t a l s grow a t t h e expense of s u p e r c o o l e d w a t e r d r o p l e t s , s i n c e ( F i g . 1) a t c o n s t a n t n e g a t i v e tempera- t u r e t h e l i q u i d water-vapour s a t u r a t i o n vapour* p r e s s u r e

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1 4 Tech, T r a n s , TT-87 ( u n s t a b l e e q u i l i b r i u m ) i s g r e a t e r t h a n t h e i c s - v a p o u r p r e s s u r e ( s t a b l e e q u i l i b r i u m )

.

4 , Drops c o n t a i n i n g

a

s a l t i n s o l u t i o n grow a t t h e e x p e n s e of d r o p s of p u r e w a t e r , b e c a u s e t h e v a p o u r p r e s s u r e of t h e s o l u t i o n i s l e s s t h a n t h a t o f t h e p u r e w a t e r , T h i s e f f e c t , i s a l l t h e more i m p o r t a n t b e c a u s e t h e s o l u t i o n h a s a l o w e r s a t u r a t i o n v a p o u r p r s s s u r e . Which of t h e s e c a u s e s a c t u a l l y p r o d u c e n a t u r a l r a i n ? We have Sean t h a t d i r e c t c o a l e s c e n c e i s h i g h l y i m p r o b a b l e . It a p p e a r s t h a t i n t h e m a j o r i t y of c a s e s t h e g r o w t h o f i c e c r y s t a l s i s r e s p o n s i b l e . T h i s i s t h e f a m i l i a r t h e o r y of B e r g e r o n and F i n d e i s e n , f i r s t p r e s e n t e d i n 1 9 3 3 , It i s b a s e d o n a h y p o t h e s i s which h a s s i n c e b e e n v e r i f i e d many t i m e s , namely, t h a t t h e u p p e r p a r t s o f most l a r g e r a i n

c l o u d s ( g e n e r a l l y s p e a k i n g cwnulo-nimbus c l o u d s s e v e r a l k i l o - m e t r e s t h i c k ) w i l l have a n e g a t i v e t e m p e r a t u r e ,. Thus t h e w a t e r i n t h e c l o u d i s n o r m a l l y i n a s u p e r c o o l e d s t a t e , f o r we know t h a t s u p e r c o o l i n g i s t h e r u l e and s t a b l e e q u i l i b r i u m i s t h e e x c e p t i o n , But a s soon a s t h e d r o p l e t s f i n d t h e m s e l v e s i n t h e p r e s e n c e o f some agency which s t a r t s t h e p r o c e s s t h e i c e c r y s t a l s b e g i n t o a p p e a r , You a r e a l l f a m i l i a r w i t h t h e b e a u t i f u l g e o m e t r i c forms of snow c r y s t a l s , A few of them a r e shown i n F i g u r e 13, These a r e a b l e t o grow by t h e above d e s c r i b e d p r o c e s s

-

t h e t r a n s f e r of w a t e r from t h e s u p e r - c o o l e d d r o p s t o t h e c r y s t a l s . A s t h e y grow, of c o u r s e , t h e y b e g i n t o f a l l as snow f l a k e s of v a r i o u s s i z e , I f t h e y a r e s u f f i c i e n t l y warned d u r i n g t h e i r f a l l , so t h a t t h e y m e l t b u t do n o t e v a p o r a t e , i t r a i n s . I f e v a p o r a . t i o n o c c u r s b e c a u s e t h e a i r i s t o o d r y and t o o warm, t h e w a t e r d o e s n o t r e a c h t h e g r o u n d and we t h e n h a v e t h e phenomenon of " v i r g a " . I f t h e t e m p e r a t u r e r e m a i n s low enough a l l t h e way t o the ground I t snows. I f t h e r e a r e a s c e n d i n g c u r r e n t s i n t h e c l o u d which c a u s e t h e l a t t e r t o r e t a i n t h e c r y s t a l s f o r some t i m e t h e y t h e n have a n o p p o r t u n i t y t o i n c r e a s e i n s i z e and t h e r e s u l t i s h a i l , T h i s t h e o r y w a s c o m p l e t e l y c o n f i r m e d between 1932 and 1937 i n America by S t i c k l e y 5 ) , who made numerous f l i g h t

i n v e s t i g a t i o n s . I t a c c o u n t s f o r a l m o s t 97 p e r c e n t o f t h e o b s e r v e d p r e c i p i t a t i o n s . I t a p p e a r s , however, t h a t c e r t a i n t r o p i c a l r a i n s and summer r a i n s i n m o u n t a i n o u s c o u n t r y a r e b e s t e x p l a i n e d by t h e t r a n s f e r of w a t e r from warmer d r o p s t o c o l d e r o n e s i n c l o u d s i n which t h e r e a r e no n e g a t i v e t e m p e r a t u r e s a t a l l , Reynolds p r o v e d t h a t t h i s c o u l d h a p p e n , e x p l a i n i n g how n o c t u r n a l r a d i a t i o n from t h e t o p of t h e c l o u d , o r t h e c l o u d

f s

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1 5

Tech, T r a n s . TT-87 own shadow when t h e s u n i s low, c o u l d b r i n g a b o u t d i f f e r e n c e s i n t e m p e r a t u r e between n e i g h b o u r i n g d r o p s w i t h i n t h e same cl.oud. A l l t h e o t h e r e x p l a n a t i o n s which have been d i s c u s s e d above seem v e r y i m p r o b a b l e .

I n any c a s e t h e B e r g e r o n e f f e c t i s t h e c h i e f c a u s e of r a i n ,

There i s one f u r t h e r p o i n t t o be c l a r i f i e d . Uhat c a u s e s t h e s u p e r c o o l i n g i n t h e u p p e r p a r t of t h e c l o u d t o c e a s e , A l l t h a t i s n e c e s s a r y i s f o r tile t o p of t h e c l o u d t o r i s e u n t i l

i t

s n t e r s t h e v e r y c o l d r e g i o n , It i s known, of c o u r s e , t h a t s u p e r c o o l i n g g e n e r a l l y c e a s e s below - 3 5 O C , The d r o p s c a n t h e n b e g i n t o s o l i d i f y . It i s e q u a l l y p o s s i b l e t h a t t h e f r e e z i n g i s s t a r t a d by t h e a p p e a r a n c e of c e r t a i n c r y s t a l l i n e a g e n t s , c e r t a i n o x i d e s of n i t r o g e n , whose forma- t i o r l i n t h e atmos9hero may be due t o l i g l i t n i n g ; d i s c h a r g e s i n s t o r m c l o u d s w i t h i n t e r m e d i a t e f o r m a t i o n o f o z o n e , The p o w e r f u l s h o c k waves p o s s i b l e w i t h l i g h t n i n g d i s c h a r g e s c o u l d l i k e w i s e l e a d t o f r e e z i n g , T h i s h a s b e e n proved by i n v e s t i - g a t i o n s of M r , J a c q u e s K a u r i n which w i l l bz d i s c u s s e d a g a i n s h o r t l y . The c r y s t a l s c r e a t e d by i n o c u l a t i o n i n t h e l a b o r a t o r y a r e s i m i l a r i n e v a r y way t o t h e n a t u r a l snow c r y s t a l s of t h e p r e c e d i n g f i g u r e ( c f . F i g . 1 4 ) .

vie now know how c l o u d s a r e formed, and we a l s o know why i t r a i n s , Vie t h e r e f o r e have a t o u r command a l m o s t a l l t h e knowledge n e c e s s a r y i n o r d e r t o make a r t i f i c i a l r a i n ,

F i r s t l e t u s suppose t h a t we have c l o ~ d s a t o u r d i s p o s a l ,

3ut

a t tho, sams t i m e l e t me remind you t h a t e v e n i f we h a v a no c l o u d s we may s t i l l hope t o form them i f we c a n f i n d zones i n which t h e h u m i d i t y i s g r e a t e r t h a n 100 p e r c e n t ,

i

, e , , s u p e r 3 s a t u r a t o d zorlos, and i f w e c a n s u p p l y s u i t a b l e

n u c l o i f o r t e r m i n a t i n g t h e s u p e r c o o l i n g . 'This d o e s n o t ' m e a n t h a t c l o u d s c r e a t e d i n t h i s way w i l l n e c e s s a r i l y be c a p a b l e o f c a u s i n g p r e c i p i t a t i o n s which w i l l r e a c h t h e ground. The s i z e of t h e d r o p s , t h e t e m p e r a t u r e s between t h e c l o u d and t h e ground, and t h e h u m i d i t y must s t i l l be s u c h t h a t t h e r a i n w i l l n o t e v a p o r a t e d u r i n g i t s f a l l .

F o r t h e p r a s e n t , however, l e t u s assume t h a t we have a n a t u r a l c l o u d ,

Two s e p a r a t e c a s e s must be c o n s i d e r e d : t h a t of a c l o u d i n which t h e r e a r e r e g i o n s of n e g a t i v e terriperature, and t h a t o f a c l o u d which h a s n o s u c h r e g i o n s . The f i r s t i s t h e s i m p l e r c a s e , , 111 o r d e r t o make r a i n

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n o t h i n g c o u l d be

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Tech, T r a n s . TT-87

t h e s u p e r c o o l i n g . We might c o n s i d e r s e e d i n g t h e c l o u d w i t h o r d i n a r y snow, T h i s method i s u n s u i t a b l e because t h e snow w i l l f u s e i n t o lumps, owing t o t h e tendency of t h e c r y s t a l s t o conglomerate, and w i l l be d i f f i c u l t t o s c a t t e r . We s h a l l n o t p r o v i d e enough n u c l e i i n t h i s way f o r t h e s n o w f a l l t o s p r e a d . Carbonic snow, o r b e t t e r s t i l l , s m a l l p e l l e t s of d r y i c e h a v i n g a t e m p e r a t u r e lowsr t h a n -80°C. a s e much more

s a t i s f a c t o r y . By t h i s means s u p e r c o o l i n g can be brought t o an end even i n v a r y l a r g e c l o u d s . T h i s i s t h e method which h a s been employed s o f a r . We s h a l l r e t u r n t o t h e h i s t o r y of t h e problem and t h e b e s t ways of a p p l y i n g t h e p r i n c i p l e i n a l i t t l e w h i l e , It i s e a s y t o s e e t h a t any v e r y c o l d s u b s t a n c e w i l l g i v e t h e same r e s u l t , e , g . , l i q u i d a i r , o r any s o l i d p a r t i c l e s which have been c o o l e d t o v e r y low t e m p e r a t u r e s . I n a few moments I am going t o t e l l you about t h e r e s u l t s of M r . Demon's e x p e r i m e n t s w i t h t h e s e v a r i o u s materials

.

Another method might c o n s i s t i n i n o c u l a t i n g t h e c l o u d w i t h i n s o l u b l e c r y s t a l l i n e s e e d s which a r e isomorphous w i t h i c e and have t h e same c r y s t a l s t r u c t u r e . Th3se would a l s o b r i n g t h e s u p e r c o o l i n g t o an end. The procedure i s based on a w s l l known phenomenon, c a l l e d e p i t a x i s , which i s

i n v o l v e d i n t h e manufacture of mixed c r y s t a l s . U n t i l now o n l y one s u b s t a n c e , s i l v e r i o d i d e , h a s g i v e n r e s u l t s , and what r e s u l t s ! This w i l l be d i s c u s s e d a g a i n i n a few moments.

F i n a l l y , r e c a l l i n g t h a t i c i n g c l o u d s , which a r e s j m p l y c l o u d s a t t e m p e r a t u r e s below z e r o , c a n be d i s p e r s e d by powerful shock waves, we could perhaps employ e x p l o s i v e s o r u l t r a - s o n i c waves t o t e r m i n a t e t h e s u p e r c o o l i n g .

Any o f t h e above methods may b r i n g about r a i n by t h e a p p l i c a t i o n of t h e Bergeron t h e o r y . A l l of them have a l r e a d y been used t o produce a r t i f i c i a l r a i n , e i t h e r i n

n a t u r e o r i n t h e l a b o r a t o r y , w i t h v a r i o u s r e s u l t s . Can r a i n a l s o be made w i t h c l o u d s a t t e m p e r a t u r e s above z e r o ? I f s o , t h e methods d i s c o v e r e d s h o u l d a l s o succeed when a p p l i e d t o c l o u d s which a r e p a r t i a l l y o r wholly a t n e g a t i v e t e m p e r a t u r e s , Needless t o s a y , t h e i n v e r s e p r o p o s i t i o n does n o t h o l d .

F i r s t of a l l , by s p r i n k l i n g t h e c l o u d w i t h l a r g e drops we may e x p e c t t h a t t h e growth of t h e s e drops a t t h e expense of t h e s m a l l ones i n t h e c l o u d w i l l l e a d t o e v e n t u a l r u p t u r e a s soon a s t h e s p r i n k l e d d r o p s become v e r y l a r g e and t h e i r speed becomes v e r y g r e a t , and t h a t t h e newly formed drops i n t u r n w i l l draw w a t z r from new s m a l l d r o p s . Thus we can

hope t o d i s p e r s e c l o u d s by s p r a y i n g them w i t h w a t e r . The

e x p s r i m e n t has been t r i e d by P r o f e s s o r Langmuir i n America and was s u c c e s s f u l . M r , Demon h a s a l s o v e r i f i e d t h e r e s u l t s i n t h e l a b o r a t o r y . Of c o u r s s , i f t h e l a r g e drops a r e c o l d e r t h a n t h o s e of t h e c l o u d , t h e d i f f e r e n c e i n vapour p r e s s u r e due t o

Tech, T r a n s , TT-87 t e m p e r a t u r e w i l l be added t o t h a t due t o c u r v a t u r e , s o t h a t a more r a p i d a c t i o n c a n be e x p e c t e d , Moreover, i f t h e d r o p s a r e c o l d , s m a l l e r o n e s may be employed t o g a i n m e q u a l e f f e c t . T h i s i s an I m p o r t a n t c o n s i d e r a t i o n when t h e w a t e r s u p p l y i s t o be t r a n s p o r t e d t o t h e c l o u d by a i r c r a f t . S i m i l a r l y , by u s i n g a s a l t s o l u t i o n s m a l l e r d r o p s c a n be employed t o promote r a p i d growth and s u b s e q u e n t r u p t u r e t h a n if p u r e w a t g r i s u s e d , Some verJy e n c o u r a g i n g l a b o r a t o r y r e s u l t s have been o b t a i n e d on t h i s by h r . Demon and h i s co- w o r k e r s , You w i l l be shown a few c u r v e s i n a l f t t l e w h i l e . F i n a l l y , w i t h d r o p s of w a t e r c a ~ ~ r y i n g e l e c t r i c a l c h a r g e s v e r y much s t r o n g e r t h a n t h o s e met i n t h e s t o r m c l o u d s

( P r o f . Brun, P r o f , P a u t h e n i e r and M r . Demon 6 ) have r e p o r t e d methods whereby c h a r g a s may be a t t a i n e d which a r e o f t h e o r d e r

of 10-5 c o g o s , u n i t s p e r d r o p , i o e , , s e v e r a l h u n d r e d s o f t i m e s g r e a t e r t h a n t h e c h a r g e s on t h e n a t u r a l c l o u d d r o p l e t s ) e l e c - t r o s t a t i c f o r c e s of a t t r a c t i o n s u r f i c i e n t f o r a c t u a l c o a l e s c e n c e . v i l l be o b t a i n e d , The e x p e r i m e n t h a s a l r e a d y s u c c e e d e d i n t h e l a b o r a t o r y . M r , Demon's r e s u l t s w i l l be shown i n a l i t t l e w h i l e . The i n t e r e s t i n g p o i n t a b o u t e l e c t r i c a l l y c h a r g e d w a t e r i s t h a t i t w i l l p r o b a b l y e n a b l e u s t o i n i t i a t e p r e c i p i - L a t i o n s w i t h d r o p l e t d i a m e t e r s of t h e same o r d e r o f magnitude :IS t h o s e of t h e c l o u d i t s e l f , i , e o , 2 0 p i n s t e a d of 200, Thus t h e q u a n t i t i e s of w a t e r t o be t r a n s p o r t e d by a i r c r a f t w i l l be a t h o u s a n d t i m e s l e s s t h a n i f p u r e w a t e r were b e i n g u s e d t o a c h i e v e s i m i l a r r e s u l t s . A l l h o w n p o s s i b i l i t i e s o f c r e a t i n g a r t i f i c i a l r a i n have now b e e n r e v i e w e d , Not a l l , of c o u r s e , have b e e n g i v e n t ' u l l - s c a l e t r f a l s , b u t t h e r e s u l t s of t h e s y s t e m a t i c i n v e s t i - g a t i o n s a t t h e B e l l e v u e l a b o r a t o r y by M r , Brun, M r . Demon and

t h e i r c o l l a b o r a t o r s , e n a b l e u s t o f o r e s e e new p o s s i b i l i t i e s of s u p p l e m e n t i n g , and, abovs a l l , of s y s t e m a t i z i n g t h e somewhat h a p h a z a r d and p e r h a p s p r e m a t u r e a t t e m p t s which have b e e n made

f o l l o w i n g t h e f i r s t e x p e r i m e n t s by Langmuir.

We come now, a s a m a t t e r of c o u r s e , t o a n a c c o u n t of t h e e x p e r i m e n t s a l r e a d y completed t h r o u g h o u t t h e world i n . l a b o r a t o r y and f i e l d , and t o an a s s e s s m e n t of t h e i r v a l u e ,

I n F e b r u a r y 1942 a t t h e G e n e r a l E l e c t r i c Company, P r o f e s s o r I r v i n g Langmuir and M r . V i n c e n t

J,

S c h a e f e r were working on a smoke g e n e r a t o r f o r m i l i t a r y u s e , These two s c i e n t i s t s began by c a l c u l a t i n g t h e t h e o r e t i c a l r a t e o f