THE LIGHTNING DISCHARGE 
By J. H. HAGENGUTH 
General Electric Company, Pittsfield, Massachusetts 
The thunderstorm process and the theories relating 
to the formation of charges in the clouds are explained 
in another article.! This paper will contain information 
on the lightning discharge only. 
The mechanism of the lightning discharge can be 
divided into three regions of interest: (1) cloud-to-cloud 
discharges, (2) cloud-to-ground discharges, and (3) phe- 
nomena on the ground end of cloud-to-ground dis- 
charges. From the practical point of view, the greatest 
effort has been devoted to understanding and interpret- 
ing the phenomena associated with the lightning stroke 
when it contacts man-made installations [1, 2]. Quali- 
tative data have been obtained to give confidence in 
the principles of protection used to guard buildings and 
electrical transmission systems against the effects of 
lightning. It becomes progressively more difficult to 
determine the mechanism and physics of the lightning 
stroke as it occurs away from the earth. 
Leaders 
The study of the mechanism of strokes to ground has 
been accomplished primarily by means of photography 
[15, 16]. The lightning stroke starts at the cloud in the 
form of a stepped leader, as illustrated in the upper 
part of Fig. 1. The steps have an average length of 50 
m. The time interval between successive steps is of the 
Tasie I. CHARACTERISTIC DATA FOR LEADERS 
AND RETURN STROKES 
Minimum Average Maximum 
Stepped leaders 
Length of steps, m.... 10 50 200 
Time interval between 
SWOOSs (NEC. oocacsece 15 50 100 
Velocity of propaga- 
tion of step, em sec = 5 xX 10° = 
Velocity of propaga- 
tion of pilot streamer, 
CHUISECRE MER ree Il S< WO! |) to SK HO? 5 X 107 
Continuous leaders 
Velocity of propaga- 
tLOD, CM SeGal. 0... = 2X 108 = 
Return stroke 
Velocity of propaga- 
MOM, Gi SCO. oa dece 2102 5 xX 10° 18) 3< IG 
order of 50 usec. The average velocity of the individual 
steps is of the order of 5 X 10° cm sec, while the 
velocity of the total step mechanism is of the order of 
1.5 X 107 em sec“!. Thus, the total time required for 
the stepped leader to reach the earth may be greater 
than 0.01 sec. 
1. Consult ‘‘Precipitation Electricity’? by R. Gunn, pp. 
128-135 in this Compendium. 
After the leader reaches the earth, the photographic 
film shows a much brighter illumination traveling up- 
wards from the earth toward the cloud through the 
channel established by the stepped leader. This is 
called the return stroke. The average velocity of propa- 
gation is 5 X 10° em sec}. 
Subsequent to this first discharge there may be other 
discharges. These are also initiated by a leader, but of a 
different type, called dart leader or continuous leader, 
with a velocity of 2 X 10° cm sec”!. As in the case of 
the first discharge, return strokes result on contact of 
the continuous leader with the earth. Occasionally 
leaders on discharges subsequent to the first have been 
observed to have a few steps. Data on leader and re- 
turn-stroke characteristics are given in Table I. 
Branching 
Frequently the first discharge shows branching pro- 
duced by a stepped-leader process. Branching is always 
in the direction of propagation of the leader. In general, 
subsequent leaders do not show branching, choosing 
the path where ground previously has been reached, 
resulting in a return stroke. In many cases more than 
one branch may reach ground simultaneously. In other 
cases ground is reached at a different pomt on subse- 
quent discharges, followmg and completing a branch 
established on the first discharge. Upward branching 
has also been observed at the cloud end of the stroke. 
Photographs show that branching is the result either of 
discharges between portions of the cloud or of discharges 
from a different charge center in the cloud through 
part of the previously established channel. There are 
indications that branching is more profuse in hilly, 
wooded country than in flat, bare country. 
Theories have been developed to explain the leader 
mechanism [8, 14]. These theories consider the presence 
of a pilot streamer which progresses more or less con- 
tinuously, connecting the individual bright tips of the 
original leader. As the pilot streamer progresses, the 
ionization in the upper part of the streamer slows up 
and recombination of ions occurs. The impedance of the 
path therefore increases, and hence the potential dif- 
ference across the established channel is also increased. 
As the potential is raised, reionization occurs down the 
channel. The speed and intensity of ionization increase 
as ionization reaches the tip of the pilot streamer. This 
increases the energy at the tip and results in greater 
illumination. The pilot streamer then proceeds for an- 
other 50 m and the process is repeated. 
The breakdown or ionization process begins in a part 
of the cloud characterized by a high field gradient. At 
atmospheric pressure, 30,000 v em“ are required to 
begin ionization. At the reduced pressure in the clouds 
136 
