RAINDROP COLLECTION EFFICIENCIES IN ELECTRIFIED CLOUDS 
action of the electric fields we observed to appear 
shortly before the formation of rain [Moore and 
others, 1958]. 
From our observations, the appearance in a 
cloud of electrical activity above some threshold 
of perhaps 30V em seems to destroy the col- 
loidal stability of the cloud. Precipitation echoes 
appear very shortly after the electrification 
within the cloud; these echoes increase rapidly 
in intensity and extent despite the subsequent 
disappearance of the electrical activity. A gush 
of rain falls from the cloud for several minutes 
then ceases slowly. We then observe only drizzle 
type rain until after there is another episode of 
electrical activity whereupon the sequence re- 
peats itself. 
Effects of electric field on colliding water 
drops—Both laboratory and theoretical investi- 
gations indicate that electrical fields may have 
a very appreciable effect in speeding up the coa- 
lescence process in clouds. Laboratory experi- 
ments performed by Fuchs [1856], Plateau 
[1873], and Rayleigh [1879] have shown that 
even a rather weak field will cause colliding wa- 
ter drops to coalesce instead of bouncing off each 
other. 
Tn his well known book on surface tension phe- 
nomena, Boys [1911] discusses the action of 
electricity to cause coalescence of two impinging 
jets of water. He wrote, “A piece of sealing wax 
rubbed on (a) coat is electrified. ... The sealing 
wax acts electrically on the different water drops, 
causing them to attract one another, feebly, it 
is true but with sufficient power where they meet 
to make them break through the air film between 
them and join. To show that this is not imagi- 
nary, I have now in front of the lantern two 
fountains of clean water coming from separate 
bottles, and you can see that they bounce apart.” 
“To show that they really do bounce, I have 
colored the water in the two bottles differently. 
The sealing wax is now in my hand; I shall re- 
tire to the other side of the room, and the in- 
stant (the wax) appears the jets of water coa- 
lesce ... These two bouncing jets provide one of 
the most delicate tests for the presence of elec- 
tricity that exist. You are now able to under- 
stand the first experiment. The separate drops 
which bounced away from one another and scat- 
tered in all directions, are unable to bounce when 
the sealing wax is held up, because of its electrical 
action. They therefore unite, and the result is, 
that instead of a great number of little drops 
fallmg all over—great drops, such as you see in 
299 
a thunderstorm, fall on top of one another. There 
can be no doubt that for this reason the drops 
of rain in a thunderstorm are so large. This ex- 
periment and its explanation are due to Lord 
Rayleigh.” 
Electrical effects can be expected to increase 
not only coalescence (by preventing bounce-off) 
but also the frequency of collisions between drop- 
lets. From theoretical considerations, Sartor 
[1957] calculated that a field of 30v em™ might 
produce ‘collision efficiencies’ of 400% for neutral 
raindrops falling through a cloud of 10-micron 
droplets, and that larger fields in thunderstorms 
could produce ‘collision efficiencies’ of 10,000% 
or more. The electrostatically induced increase 
in collision efficiency arises from the force of 
attraction between the dipoles induced on the 
water drops in an electric field. This force varies 
with the square of the external field, the radius 
of the drops, and in a complex manner with the 
distance of drop separation. 
Sartor noted that this effect applies to all 
droplets, charged or uncharged, and that most 
of the droplets affected are those for which the 
computed efficiencies are very small. He con- 
cluded that “the electrostatic field, even when 
very small, plays an important role in the initia- 
tion and growth of precipitation, at least in 
warm or supercooled clouds and that the electri- 
cal manifestations of the thunderstorm are not 
merely its by-products but form an integral part 
of the precipitation mechanism.” 
Formation of rain by coalescence—The rapid 
growth in the intensity of initial radar echoes in 
convective clouds by coalescence has been noticed 
by Battan [1953] and other observers. The phe- 
nomenon is worthy of further study so that coa- 
lescence mechanisms in clouds may be evaluated 
more properly; from computations with ex- 
isting models, these mechanisms seem too slow to 
account for the observed production of precipi- 
tation. 
For example, Houghton [1951] (who early sug- 
gested that coalescence and accretion should be 
important precipitation forming mechanisms) 
calculated the time required for the growth of a 
raindrop by coalescence (using collection ef- 
ficiencies computed by Langmuir) and found 
that a 100-micron-diameter drop falling through 
a cloud of 20-micron drops (with a liquid water 
content of 1 gm m™“*) would require 24 min to 
grow to a diameter of 500 microns, and another 
7 min to grow to a diameter of 1 mm. (By 
Houghton’s caleulations, an initial period of 92 
