THE LIGHTNING DISCHARGE 
length has been estimated at twenty to thirty million 
volts. Considering the formation of the channel by 
means of the stepped-leader process, it is reasonable to 
assume that the average gradient for the lightning 
stroke may be considerably less than that required to 
break down a gap in the laboratory (80,000 v cm“, 
uniform field; 5000 v em~, nonuniform field—large 
gaps). 
It is not known what gradients exist at the point 
where the leader is initiated in the cloud. The lower 
density of the air, and the presence of waterdrops 
with their associated charges, may greatly alter the 
gradient requirements initiating a discharge. 
Cloud-to-Cloud Discharges 
While the occurrence of cloud-to-cloud discharges 
is relatively much greater than that of cloud-to-ground 
discharges—estimates range between 50:1 and 0.7:1— 
the mechanism involved is not as well known because 
these strokes are frequently obscured by the cloud 
masses. However, the available photographs seem to 
indicate that cloud-to-cloud discharges are initiated 
by stepped leaders in the same manner as that of the 
first discharge in a cloud-to-ground stroke. Return 
strokes are not known to occur. 
Measurements of field changes associated with cloud- 
to-cloud strokes also indicate the absence of return 
strokes. The measurement of the wave form of atmos- 
pherics, however, has disclosed that some types of 
cloud-to-cloud strokes result in wave forms similar to 
cloud-to-ground strokes, but of smaller amplitude and 
separated by short, quiet intervals. These probably 
arise from multiple discharges within the clouds. Photo- 
graphic evidence indicates that discharges within clouds 
can take place from the tip to the bottom of the cloud, 
as well as in a horizontal direction within clouds. 
The importance of cloud strokes lies in their effects 
on radio reception and on the safety of airplanes. It 
has been suggested that airplanes may trigger off cloud- 
to-cloud discharges, and evidence is accumulating that 
the susceptibility of planes to lightning strokes increases 
as the size and speed of the planes become greater. 
Phenomena on Ground End of Cloud-to-Ground Dis- 
charges 
To safeguard electrical installations against damage 
from lightning strokes, a large number of measurements 
have been made to determine the characteristics of 
lightning strokes at the ground end. Such measurements 
have involved the determination of voltages on trans- 
mission lines (maximum measured—5,000,000 v), but 
have dealt principally with the statistical evaluation 
of the current in the stroke, the charges involved in 
the stroke, the wave shapes of currents, and other data 
needed to provide reliable protective systems. 
Figure 2 shows a composite oscillogram of a lightning 
stroke current to the Empire State Building. In this 
case an upward leader, followed by a long period of 
continuing current flow, initiated the stroke and ter- 
minated at 0.25 sec. At this moment a continuous 
downward leader to the building resulted in a return 
139 
stroke or current peak of approximately 15,000 amp 
in which the time to half value of the crest current 
was roughly 40 usec. A short period of continuing cur- 
rent was followed by a second current peak of 4000 amp. 
A total of four current peaks were measured in this 
stroke, the third one having the highest amplitude, 
23,000 amp. The total duration of the stroke was ap- 
proximately 0.46 sec. 
TIME IN SECONDS 
(INSERT SCALE — MICROSECONDS) 
() 0.02 0.04 0.06 0.08 
0 i 1 n : a Seite an 
fe ‘ay EO ahs 
=2 
0.08 0.10 0.12 0.14 0.16 
ip) 0+ 1 in 1 1 1 1 1 4 
tJ 
Ge =| | 
ive) 
= =2 
q 0.16 0.18 0.20 0.22 0.24 
a of L L ! f L \ i 
a 
= = 
-2 0.2515 0.2515+ 0.2618 0.2618 + 
2 ( psec. ( i sec. ( 
0.24 © 20 40 60 0.2670 20. 40 0.28 
= (0) ——— Fy 
Zz —>S 
me = &) -4- | 
«© -2 =U 0.320. ~8- 0.320+ 
> -15- No Hsec. ie 
S 9 20 40 60 0.34 
og See eee 
(S) 
z a -10- 
2 - 0.3418 py sec, Ce 3418+ _90_ 
= ose fs 20 do ea 0.36 0.38 0.40 
2 Saree 
erly | ee di aa 
-1 
0.40 0.42 0.44 0.46 0.48 
——L a 
oe = = 
TOTAL DURATION: 0.47 SECONDS 
TOTAL GHARGE: -—84 COULOMBS 
Fie 2.—Replot of low-speed cathode-ray oscillogram with 
inserts of high-speed cathode-ray oscillograms showing cur- 
rent peaks, Empire State Building, New York City, 1940. 
In strokes to open country, the stroke starts at the 
cloud rather than at the ground, and consequently a 
current peak would be the first measurement made. 
The value of continuing current between current peaks 
is expected to vary greatly and may even be zero. Never- 
theless, this oscillogram shows all the principal com- 
ponents of a cloud-to-ground stroke and the relative 
relations between amplitudes and wave shapes of the 
two principal components—current peaks and continu- 
ing currents—as well as the multiple character of the 
stroke. 
From the available measurements, it has been pos- 
sible to derive statistical data which are shown in 
Table II. These statistics in all cases give the minimum 
and maximum curves obtained from published data 
for 90, 50, and 10 per cent of the strokes, as well as 
the maximum available values. The great spread in 
some of the data is due to the various methods of 
observations used. 
Stroke Current 
Item 1 of Table II shows the amplitudes of current 
peaks measured in the path of the lightning stroke, 
with an average peak current of 7000 amp. A much 
greater number of records, which show a surprising 
agreement, have been obtained on transmission line 
towers (Item 2). Fifty per cent of the lower currents 
are in excess of 10,000 amp, while the maximum meas- 
