THE LIGHTNING DISCHARGE 
nism is not known, it cannot be determined whether 
all of the charges were conducted to ground or to what 
extent they occurred within the cloud only. 
Polarity 
The polarity of lightning strokes, in the great ma- 
jority of cases measured, is negative; that is, negative 
charges are lowered from the cloud. The relation for 
erounded structures is approximately 15:1 in favor of 
negative charges. Since most measurements have been 
made on transmission line towers and other rather 
well-grounded structures, it has been pointed out that 
this would be conducive to a higher number of negative 
strokes on account of the possibility of guiding a nega- 
tive leader to the structure by means of streamers 
produced at the positive grounded object. In the case 
of a positive stroke, a negative streamer would not be as 
likely to occur. From this reasoning it is expected that 
strokes to open ground may include a lower percentage 
with negative polarity. Measurements of field changes 
seem to favor this view because in many of these cases 
the ratio of negative to positive polarity cloud-to- 
ground strokes is of the order of 6:1 or less. 
In cases where direct strokes to ground have been 
measured by means of oscillographs or other devices 
which permit detailed examination of the stroke cur- 
rent throughout its duration, it was found that the 
majority of the strokes were entirely of negative po- 
larity. In some cases, however, the polarity of the 
continuing current flow changed, usually toward the 
end of the stroke. In other cases one of the current 
peaks was of positive polarity, while the remaining 
current peaks resulted from a negative cloud charge. 
The polarity relations throughout the length of the 
stroke are probably governed by the mechanism of 
multiple discharges which was discussed previously. 
The process must be governed by the means of charge 
exchange within the cloud during and following the 
first leader stroke to ground, as well as by the rate of 
production of charges within the original stroke center 
in the cloud. 
The fact that charges of negative polarity are lowered 
to the ground in the majority of the cases seems to 
indicate that negative charges predominate in the bot- 
tom of the cloud. The fact that some of the highest 
current peaks measured were of positive polarity might 
be accepted as proof that centers of positive polarity 
can exist in the lower portions of some storm clouds. 
Stroke Density 
It is extremely difficult to obtain information on 
stroke density. Analysis has shown that the density 
per square mile is approximately one-half of the number 
of storm days from the isokeraunic map. This was 
partially confirmed by a two-year count in a region 
with 27 storm days per year where the average stroke 
density was approximately 15 strokes per square mile 
per year. Observed figures depend on the size of the 
area under observation and rapidly decrease with in- 
creased area. Accurate data would be of value to de- 
termine lightning risks. 
141 
Forms of Lightning 
Names like streak, bead, ribbon, fork, heat, sheet, and 
ball lightning have been used to describe various ob- 
served forms of lightning discharges. Streak hghtning 
is the normally observed type as described in the fore- 
going discussion. Bead, ribbon, fork, and heat lightning 
probably have the same characteristics as streak light- 
ning. Bead lightning is assumed to be a form of streak 
lightning. The appearance of the beads may be caused 
by variations in the luminosity along the channel, 
perhaps caused by brush discharges. Ribbon lightning 
is probably a streak lightning stroke with multiple 
discharges where the channel is blown along by the 
wind. Fork lightning is the term used for strokes with 
several apparently simultaneous paths to ground. As 
explained, these ground termini may be formed simul- 
taneously or during successive discharges of multiple 
strokes. Heat lightning is a form of streak lightning 
at a distance sufficiently far away so that thunder is 
not heard. Sheet lightning usually occurs in clouds 
between the lower and upper atmosphere over a con- 
siderable area. This areal distribution of sheet lightning, 
and its long persistence, constitute the principal differ- 
ence between it and streak lightning. 
Ball lightning is described as a luminous ball of 
reddish color, with an average diameter of 20 cm. When 
seen emerging from the cloud, its velocity has been 
estimated as high as 100 m sec~!; while on the ground 
it travels at 1 to 2 m sec. Usually the ball explodes 
after an average life of 3 to 5 sec. There is much con- 
troversy regarding this form of lightning. It has been 
described as an optical illusion caused by retention of 
vision of a heavy lightning discharge in the retina of 
the eye. However, the testimony of a few apparently 
well-qualified observers seems to indicate that such 
phenomena may exist. No reliable explanation for the 
existence of ball lightning has been offered. However, 
several theories exist explaining the phenomenon on the 
basis of chemical and physical reactions. Some photo- 
graphie evidence submitted as possibly caused by ball 
lightning has been proved to have had an erroneous 
interpretation. 
Measurements of Lightning Characteristics 
The most exact measurements can be made at the 
ground end of a stroke. For this purpose a variety of 
instruments have been developed. 
The Lichtenberg figure on photographic film [7, 13] 
placed between two electrodes is a corona discharge. 
The size of the figure is a measure of the voltage applied 
and the polarity of the discharge. By suitable shunts, 
currents can be measured. The accuracy of such devices 
has been estimated at 25 to 50 per cent. 
A very simple method of measuring the peak value 
and polarity of surge or lightning currents is the mag- 
netic link [4]. It consists of a number of steel strips of 
high retentivity. The magnetic flux associated with a 
lightning stroke magnetizes the link. The magnetization 
measured after exposure gives a very accurate indication 
of current crest. Two links at different spacing from 
