A Possible Effect of Lightning Discharge on 
Precipitation Formation Process 
BERNARD VONNEGUT AND CHARLES B. Moore 
Arthur D. Little, Inc., Cambridge, Massachusetts 
Abstract—The electrical effects caused by lightning strokes within clouds may cause 
a rapid and effective drop coalescence process. It is suggested that the lightning stroke 
intensely electrifies cloud drops in its immediate vicinity. These drops acquire a charge 
opposite to that carried by the rest of the cloud and under the influence of electrical 
forces move rapidly away from the stroke, colliding and coalescing with the unaffected 
droplets. 
Introduction—Atmospherie electrical measure- 
ments show that the great majority of precipita- 
tion-producing storms are accompanied by rather 
intense electrification even though they may not 
produce lightning. It follows that the precipita- 
tion-forming processes often occur in the pres- 
ence of electrical activity. 
It is well known that electrical effects are of 
great importance in determining the stability of 
emulsions and suspensions and there appears to 
be good reason to believe that atmospheric elec- 
trical activity may play a significant role in de- 
termining the stability of clouds. Various work- 
ers have approached the problem of the effect of 
electric fields on precipitation growth both ex- 
perimentally and theoretically and in general 
they conclude that strong electric fields can ac- 
celerate coalescence phenomena. 
With few exceptions these investigations have 
been confined to considerations of the effect of 
steady electric fields and have not dealt with the 
transient effects that might occur when a light- 
ning stroke occurs. It is the purpose of this 
writing to offer some speculations on the nature 
of a lightning discharge in a cloud and its effects 
on the formation of precipitation. 
Nature of lightning discharge within cloud— 
An understanding of the effects that a hghtning 
stroke has on precipitation growth processes is 
obviously dependent on knowledge of the details 
of the discharge within the cloud. Unfortunately 
we presently know very little of these details, for 
this portion of the stroke is always screened from 
view. In this discussion it is therefore necessary 
to make some guesses concerning the nature of 
the stroke. 
It is generally agreed that a large part of the 
electric charge is carried on cloud particles. Since 
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a lightning stroke obviously serves as the mecha- 
nism for the release of the electrical energy ac- 
cumulated in a cloud, it is frequently assumed 
that the lightning discharge neutralizes electri- 
fied precipitation and cloud particles. 
Atkinson [1887] has considered the effect of 
lightning on the coalescence process in the light 
of this idea, that the lghtning removes the 
charge from the cloud droplets. He says, “The 
small drops which were before kept apart by 
mutual repulsion from being highly charged and 
of the same potential now coalesce and form the 
large drops, which being too heavy to be sus- 
tained in the atmosphere, fall.” 
While it is undoubtedly true that some of the 
electrified particles in a cloud are neutralized by 
a lightning stroke, we question that this occurs 
generally. We prefer instead a somewhat differ- 
ent picture based to a large extent on the ap- 
pearance of the lightning-like patterns that are 
results of the dielectric breakdown of electrified, 
methylmethacrylate plastic blocks. Gross [1958] 
has described an interesting phenomenon which 
he produced by irradiating plastie with high en- 
ergy electrons from an accelerator until the elec- 
trie field within the plastic became sufficiently 
large to cause a spark quite similar to hghtning. 
The pattern produced within the plastic by the 
electrie discharge is shown in Figure 1. This phe- 
nomenon is in many ways analogous to a light- 
ning stroke within the cloud, for here we have 
a charged region in a dielectric that produces 
sufficiently high electric fields to cause a spark 
discharge. Because the plastic is quite trans- 
parent, it is possible to see the fine details of 
how the discharge behaves within the highly 
charged region that gave rise to it. 
An examination of Figure 1 shows that the 
