+800 
DISCUSSION 303 
= 
ee 
ae 
a= 
ie 
az 
een rx fa’ 
POTENTIAL GRADIENT (Yn) 
-800 
o --> POSITIVE CHARGE 
@--+NEGATIVE 4 
Fig. 8—Atmospheric field change, charge on raindrops and rain fall intensity during rain fall, 
Nov. 10, 1958 
tive charges below the line. The rate of rain is 
shown by the dashed line. 
Dr. Vonnegut—I think this is a very interest- 
ing observation. I am curious to know the magni- 
tude of the electric fields involved. Were these 
high enough so that point discharge was oc- 
curing or not? 
Dr. Magono—Observations were made on or- 
dinary rainfall not thunderstorms, so that the 
field was about a thousand volt per meter. 
Dr. Vonnegut—This value of the field is on 
the borderline so it is difficult to know whether 
or not point discharge was taking place. 
Dr. H. Kasemir (communicated)—I think I 
can give an explanation for the mirror effect and 
all the questions connected with it, which are 
brought up in the paper and discussion. Ob- 
servations such as reported here where the mirror 
effect has been observed in the presence of low 
fields apparently require that we abandon the old 
theory, as outlined by Dr. Mason, that the 
original precipitation charge is identical with 
the negative charge in the base of the cloud. We 
postulate a charging mechanism whereby the 
precipitation fallmg through the lower part of 
the cloud assumes a positive charge leaving nega- 
tive ions behind. These negative ions attach 
themselves to the cloud elements and thus they 
lose their ability to wander away. They are re- 
sponsible for the negative charge in the base of 
the cloud but not the precipitation. As long as 
the precipitation does not reach the ground, the 
net charge of the cloud elements and of the pre- 
cipitation are the same amount but of opposite 
sign. The electric field generated by the positive 
precipitation charge will almost be cancelled by 
the reverse electric field of the negative cloud 
charge. But as soon as the raindrops reach the 
ground, they get discharged and their charge 
is withdrawn from the picture. From there on 
the net charge of the precipitation remains at a 
constant value while the cloud charge continues 
to accumulate. A few minutes after the first rain- 
drop hits the ground the cloud charge becomes 
so predominant that the field reverses its sign 
and becomes negative while the rain charge re- 
mains positive. The negative field increases rap- 
idly until the discharging induction current bal- 
ances the charging effect of the precipitation. If 
for some reason the precipitation charge re- 
verses its sign and becomes negative then posi- 
tive charge is left behind in the cloud and after 
a short transient period the field also changes to 
positive values and we observe the mirror effect, 
that is, that field and precipitation charge usu- 
ally have the opposite sign. This picture explains 
all of the observed facts: (1) that we have the 
mirror effect with and without point-discharge ; 
(2) that the mirror effect exists even in the 
cloudbase itself, as the measurement of Dr. 
Vonnegut and others (Kuettner on Zugspitze, 
Israel and Kasemir on Jungfraujoch) have 
shown; (3) that we observe the mirror effect 
also, if the cloudbase is several kilometers above 
the ground. 
There are cases, where precipitation charge 
and field have the same sign. Also the precipita- 
tion may lose some of the charge on its way to 
the ground passing through ion clouds of oppo- 
site sign. But we have to consider these as sec- 
ondary effects while the basic mechanism works 
as outlined above. 
Dr. Vonnegut (communicated)—While Dr. 
Kasemir’s suggestion may explain the mirror ef- 
