THE LIGHTNING DISCHARGE 
and due to the presence of water droplets, it is esti- 
mated that a gradient of 10,000 v cm! may be sufficient. 
The process is started by the acceleration of one or more 
free electrons in the air. By collision with air molecules, 
these electrons liberate more and more electrons as they 
advance in the field, the number increasing very rapidly 
in the form of an avalanche. Large numbers of positive 
ions are left behind in the field. The resulting positive 
space charge mcreases the field gradient sufficiently 
for the attraction of photoelectrons. These in turn 
produce greater ionization and rapidly complete the 
breakdown process. 
137 
ten to several hundred feet. However, the evidence is 
insufficient to determine whether ground streamers oc- 
cur on every lightning discharge. 
If hght intensity is considered a measure of the 
current amplitude in the return stroke, then in the 
majority of the cases photographed the first in a series 
of multiple strokes appears to carry current of the 
highest amplitude. This is not always the case. Oscillo- 
graphic evidence also indicates that the first current 
peak does not always have the highest amplitude. 
In many photographs taken in South Africa [9], 
the light imtensity of the channel decreases as the 
NEGATIVE 
_CLOUD >] 
UK 
a\\ 
i) 
BRIGHT 
TIPS OF 
LEADER RETURN 
STROKE STROKE 
INITIAL DOWNWARD 
STILL CAMERA 
tM Ee 
| 
LEADER 
{ 
RETURN 
STROKE 
SUBSEQUENT DISCHARGES WITH DOWNWARD 
CONTINUOUS LEADERS 
PHOTOGRAPH STEPPED LEADER 
F STROK 
Caps Sake HIGH SPEED BOYS CAMERA PHOTOGRAPH OF THE SAME STROKE 
CLOUD TO EARTH 
NEGATIVE 
BRIGHT TIPS OF TIME ————— 
CLOUD 
LEADER STROKE 
7) } } 
INITIAL UPWARD 
STEPPED LEADER 
STILL CAMERA 
PHOTOGRAPH 
OF STROKE 
= i 
LEADER_|/ | 
t RETURN 
STROKE 
SUBSEQUENT DISCHARGES WITH DOWNWARD 
CONTINUOUS LEADERS 
(_ HIGH SPEED BOYS CAMERA PHOTOGRAPH OF THE SAME STROKE 
CLOUD TO TALL CONDUCTING STRUCTURE 
Fie 1—Schematic diagram showing the mechanism of the lightning discharge from cloud to ground. 
Laboratory experiments on long sparks have con- 
firmed the stepped-leader process from negative point 
to plane, but not from positive pomt to plane. More 
sensitive measuring methods are needed to record the 
complete development of the streamers or leaders. There 
is considerable doubt as to the existence of the pilot 
streamer. The theory of the long discharge is not yet 
sufficiently developed, however, to provide a better 
answer to the formation of the lightning stroke by 
means of the stepped leader. 
The Return Stroke 
As the leader approaches the ground, the charges 
in the ground begin to move in the direction of the 
approaching leader. As the leader touches the ground, 
the charges in the channel and the ground charges 
can neutralize each other, resulting in the return stroke. 
Since the channel is partly ionized, further ionization 
can result at a more rapid rate. The current in the chan- 
nel increases rapidly and the velocity of propagation 
upward becomes of the order of ten times that of the 
continuous downward leader. 
There is some evidence that streamers from the 
ground start up toward the approaching leader, thus 
producing contact between leader and charges in the 
air [11]. The length of such streamers may vary from 
return stroke progresses toward the cloud. Poimts of 
discontinuity are observed, particularly at the junctions 
of the main channel and the branches. There is a ques- 
tion of whether current surges proceed from the branch 
to ground or from the ground to the branch. The veloc- 
ity im most cases exceeds 10!° em sec, and therefore 
cannot be accurately measured from the photographs. 
Measurements in other parts of the world do not seem 
to indicate such drastic changes in intensity in the 
channel as the return stroke progresses toward the 
cloud. This difference may be due to ground conditions. 
In those cases where charges are readily available, the 
current in the return stroke can be maintaimed up to 
higher altitudes at higher amplitudes than in ground 
with high resistivity. 
Data are not available on current amplitudes in the 
leader strokes, the relation between currents and charges 
in the leader stroke, the charges available in the ground, 
and the wave shape and amplitude of the return stroke. 
The mechanism involved in the wave shape of the 
current in the return stroke is not too clear. It is thought 
by some that the front is formed shortly before the 
leader touches the ground and completed on contact 
[12]. Other theories indicate that maximum peak cur- 
rent is reached at the ground end after the current 
wave has traveled a short distance upwards in the 
channel. 
