138 
Stroke Mechanism for High Buildings 
Observations at the Empire State Building in New 
York [11] have shown that the starting mechanism of 
the stepped leader is quite different (bottom part of 
Fig. 1). In most cases the stroke starts as a stepped 
leader at the building rather than at the cloud. The 
length of the steps, the time interval between steps, 
and the velocity of propagation of the steps fall within 
the range of leaders from cloud to ground. Another 
significant difference is the absence of a return stroke 
after the leader has reached the cloud. Instead, a 
continuous flow of current of the order of magnitude of 
a few hundred amperes is observed. Frequently the 
stroke current stops without any further manifesta- 
tion. In many cases, however, the initial discharge is 
followed by subsequent continuous downward leaders 
from the cloud to the building, followed by a return 
stroke upward from the building, as in the case of 
strokes reaching the ground in open country. 
This sequence leads to the conclusion that charges 
in the cloud are not sufficiently concentrated to provide 
a return stroke. In spite of the large drainage of charges 
(as much as 80 coulombs) in the preliminary continu- 
ous-current period and a rather well-ionized channel, 
the cloud can precipitate a continuous leader to the 
building. This may be due to a more rapid increase of 
potential gradients within the cloud or to rapid inter- 
change of charges from other charge centers within the 
cloud. The reason for establishing a continuous leader 
in a channel, quite well ionized by the preceding con- 
tinuing current, is not clear. Photographs and oscillo- 
grams show that the return stroke, coupled with a 
heavy current discharge or peak current, is governed 
principally by conditions in the ground. 
Continuous Current 
Oscillographic and photographic evidence from the 
Empire State Building investigation seems to indicate 
that successive discharges are always connected by 
continuing current flow. Other investigators have made 
measurements which indicate that the current between 
successive discharges may drop to zero. The fact that 
successive lightning discharges in a stroke have the 
same shape is an indication that sufficient ionization 
remains in the channel for subsequent leaders to choose 
the same path. In some cases the time interval between 
such successive discharges is 0.5 sec. In the laboratory 
it was found that on establishing a long, sixty-cycle 
arc, a series of multiple discharges take place within a 
few hundreds of microseconds. The shape of these 
discharges differs greatly, indicating that ionization 
has ceased. The currents available in this case are of 
the order of a few amperes. 
To solve this question it is necessary to greatly en- 
large our knowledge on deionization time of the air and 
to determine more accurately whether current flow 
exists during deionization. 
The Channel of the Lightning Stroke 
The channel of the lightning stroke almost invariably 
follows an irregular path. This path and its branches 
ATMOSPHERIC ELECTRICITY 
are probably determined by the conditions in the atmos- 
phere surrounding the tips of the stepped leaders as 
they progress downward. Space charges produced by 
the leader mechanism, and perhaps charge distributions 
involved in the thunderstorm process itself, may be 
largely responsible for the field distortion resulting in 
the irregular pattern of the lightning channel. The 
diameter of the channel is apparently a function of the 
rate of rise of current flowing in the channel and the 
amplitude of current flow. Experiments have shown 
that the channel diameter experiences equilibrium when 
the density of the current flowing reaches 1000 amp 
em-*. Much greater current densities, as high as 30,000 
amp cm’, are reached when current flowing in the 
channel has a rate of rise of a few thousand amperes 
per psec. As the result of such measurements, the 
probable maximum diameter of the channel was 
deduced as of the order of 5 em. Other computations 
and experiments indicate diameters as high as 23 cm. 
The change in current density and consequent en- 
largement of the channel as a result of ionization, heat- 
ing, and disassociation along the path of the lightning 
discharge, results m pressure effects. For continuous 
discharges involving only low currents, the pressure 
effects are so negligible that thunder cannot be heard. 
Such observations permit the deduction that pressure 
effects will be greater the higher the rate of rise of cur- 
rent flowing in the channel and the greater the ampli- 
tude of the current. This is confirmed by laboratory 
experiments where the pressure effects are associated 
with high surge currents, while low-current discharges 
produce negligible pressure. 
At times the path of the lightning channel is affected 
by wind. In such cases a stationary camera film and 
lens will record the multiplicity of the discharge. In 
some such cases the stroke path changes its shape 
considerably, indicating perhaps the disruption of cur- 
rent flow. However, it is quite possible that differences 
in wind velocity at various heights are responsible for 
distortion of the pre-established channel. In other cases 
the channel retains its precise form to the end of the 
stroke. 
The Multiple Stroke 
The formation of multiple strokes [6, 11] has been 
explained as the extension of the stroke channel to 
new charge centers in other portions of the cloud. 
Alternatively, it has been suggested that new charge 
centers develop toward the original channel by means 
of the leader process. In some strokes the regularity 
of the time interval between successive discharges has 
been suggested to be due to repeated charging of the 
original stroke center. Statistical data on the number 
of multiple discharges in a stroke are given in Table II. 
Potential Involved in the Stroke Formation 
Based on potential gradients measured at the earth 
and within clouds, it now appears that a lightning dis- 
charge can be initiated and completed with average 
gradients of the order of 100 v em. On this basis the 
total voltage required to initiate a stroke of 10,000-ft 
