LIGHTNING DISCHARGE AND 
mately equal and opposite charge into the 
charged region. According to this view the light- 
ning discharge would produce a flow of current 
that would greatly reduce the potential differ- 
ences within the cloud but would not cause com- 
plete neutralization such as occurs when a spark 
jumps to a charged metal electrode. 
This view of what may happen in the cloud 
has been arrived at largely on an extrapolation 
of what happens in the pastic, but it seems rea- 
sonable from another point of view. Before a 
lightning stroke can oceur a large volume of 
charge, of the order of 10 or 20 coulombs must 
accumulate within the cloud. When the discharge 
begins, large voltages and high currents are avail- 
able to supply the large energies required to form 
and maintain the intensely ionized path. As the 
stroke progresses and the flow of current con- 
tinues, it is clear that the initial large reservoir 
of charge steadily decreases and along with it 
the potential gradients and electric currents. 
Finally a point is reached at which the energy 
required to form the ionized path is greater than 
the available energy. It seems likely that this 
happens long before the discharge proceeds to 
each electrified cloud particle. 
Effect of discharge on precipitation particle 
growth—The lightning discharge as envisioned 
above would suddenly introduce a large quantity 
of concentrated charge into the cloud that would 
cause neutralization on a gross scale. Following 
the stroke, neutralization on a much finer scale 
would occur by the migration of ions through 
the cloud. It appears that this final stage of 
neutralization following the stroke could result in 
a rapid and effective coalescence process. 
The largest part of the length of the disrup- 
tive discharge pattern in the plastic is in the 
form of the rather fine terminal branches of the 
discharge. Similarly the greatest length of the 
lightning structure in the cloud is probably also 
in the form of the branch sparks, which are far 
smaller in diameter than the main stroke. When 
a discharge into a cloud takes place, we can 
imagine that these smaller branches penetrate 
into the charged region and suddenly deposit 
along their path a localized intense electric 
charge of approximately equal magnitude and 
opposite sign to that in the charged region. This 
intense charge introduced by the lightning creates 
an intense local electric field. Initially this charge 
is doubtless in the form of fast ions, and under 
the influence of the electric field these ions move 
rapidly away from the stroke. 
PRECIPITATION FORMATION 289 
We have no knowledge of the intensity of the 
local electric field in the region of the stroke just 
after 1t occurs but it is unquestionably high, per- 
haps approaching the dielectric breakdown 
strength of air. If this is so we may expect that 
the fast ions move outward with initial velocities 
of the order of 20 to as high as 100 m/sec. 
The heat released in the immediate region of 
the stroke is probably sufficient to vaporize the 
cloud droplets so that initially the ions may 
move in clear air. After moving only a short 
distance away from the stroke the ions will en- 
counter cloud droplets to which they will become 
attached and impart their charge. The intensity 
of charge acquired by the cloud droplets near 
the lightning stroke depends on the ion density 
and electric field, both of which are probably 
quite high. It appears hkely that in the im- 
mediate vicinity of the stroke the droplets will 
acquire sufficient charge to raise the fields on 
their surface to values approaching dielectric 
breakdown. A rough estimate shows that if this 
occurs, these highly charged droplets will move 
outward in the local intense field with initial 
velocities that may be as high as 100 m/sec. 
The process visualized above would be exceed- 
ingly effective in causing coalescence. The highly 
charged droplets would move rapidly away from 
the stroke out into the unaffected region of the 
cloud where they would collide with the other 
cloud particles. If we assume that the droplets 
in the region of the stroke are charged to values 
approaching dielectric breakdown this is equiva- 
lent to a space charge density of about two or 
three orders of magnitude greater than the av- 
erage values one might expect in a thunder cloud. 
If this is true, the highly charged droplets pro- 
duced by the hghtning stroke will collide with 
hundreds or thousands of the oppositely and far 
less strongly charged cloud droplets in the sur- 
rounding cloud before their charge is neutralized. 
By the time the highly charged droplet has lost 
most of its charge by collision, its mass will 
probably be so large that it will have an ap- 
preciable rate of fall and gravitational forces 
will begin to play a part in its growth. 
Discussion—If lightning does indeed cause a 
rapid coalescence of the sort suggested here, one 
would expect to observe an increase in precipi- 
tation shortly after a lightning stroke. 
Tt has been observed that gushes of heavy rain 
frequently follow a lightning stroke. Weickmann 
[1953, p. 111] has stated, “The precipitation left 
the cloud base regularly at those places where 
