302 
DISCUSSION 
Discussion* 
(Discussion of the two immediately 
preceding papers.) 
Dr. B. J. Mason—The first thing that im- 
pressed me when Dr. Vonnegut showed his field 
records, was that his field variations fit very 
well the thunderstorm model of C. T. R. Wilson. 
If the thunderstorm is some distance away, then 
the electrical field at the point of observation is 
certainly controlled by the positive cloud dipole 
and one gets a positive field. Lightning dis- 
charges may cause excursions of negative sign 
for short periods. But if the thunderstorm ap- 
proaches the point of observation, the influence 
of the negative charge in the base of the cloud 
becomes predominant, and the field changes to 
the negative sign. The small positive charge 
pocket in the base of the cloud, in passing di- 
rectly over the observation poimt, might again 
cause a short positive excursion. I find nothing 
really very unusual about these field records 
since they are exactly what one would expect. 
Now the other important pomt is whether or 
not one can deduce the charge of precipitation 
in the cloud from precipitation charge measure- 
ments at the ground. A large concentration of 
negative drops in the cloud will cause a strong 
negative field which in turn produces point dis- 
charge at the ground. The point discharge will 
send a stream of positive ions up to the cloud, 
which the negative raindrops, falling to the 
ground, have to pass. By this passage the rain- 
drops may pick up enough positive ions to re- 
verse their charge. This is the commonly ac- 
cepted explanation for the ‘mirror image effect,’ 
which recognizes that the sign of the electric 
field and the sign of the precipitation current 
are inverted. So I would be very very reluctant 
to make any firm deductions about the pre- 
cipitation charge in the cloud from measure- 
ments on the ground. 
Dr. B. Vonnegut—I agree with Dr. Mason 
that because of point discharge it is difficult to 
ascertain from ground measurements what 
charge is carried by the rain within the cloud. 
However, I pointed out that occasionally we had 
* At the conference Dr. Vonnegut presented a 
condensed and slightly different version of the two 
immediately preceding papers. It is because of that 
reason that some of the discussion-remarks do not 
seem to have an immediate bearing to the papers 
as published. As the discussions are, however, of 
general interest and importance, they have not 
been omitted. Ed. 
rain arriving on the summit while the field was 
too low for point discharge. Under these condi- 
tions the rain was observed to transport only a 
very small current. It is quite possible that al- 
though the rain appears to carry little charge 
during these intervals it may become highly 
charged later on. If one is free to make whatever 
assumptions one chooses concerning the amount 
and polarity of charge carried by the rain in 
the cloud and to vary these assumptions at will, 
it is quite possible to account for the observed 
data. If rain is mdeed responsible for the pri- 
mary electrification process it is surprising, how- 
ever, to find that it generally carries such small 
currents. If we assume that the excursion of the 
electric field to negative values associated with 
the rain gush is caused by the falling of charged 
precipitation, this still leaves us with the prob- 
lem of explaining the very similar excursion also 
accompanied by a wind that we observed near 
the end of the storm, without any precipitation. 
Gunn’s observations, as Dr. Mason points out, 
indicate that the raindrops in thunderstorms are 
very highly charged. However, I think we should 
be somewhat cautious in accepting his findings 
for he himself acknowledges the difficulty of 
making these measurements and the possibility 
that some of his points may be appreciably in 
error. On the occasions when the cloud base was 
below the mountain top and we were in the 
cloud we found the currents carried by the rain 
were quite small. Even when the fields were large 
enough to give point discharge, the rain current 
was perhaps an order of magnitude smaller than 
that we measured with the cloud base some dis- 
tance above us. 
Dr. C. Magono—lI have been very happy to 
know your opinion concerning these observations 
because I am interested in field changes during 
rain and snowfall, and we made some observa- 
tions at Sapporo. We were wondering which is 
the origin: is the change in charge on raindrops 
the origin of field change, or the field change the 
origin of the change in raindrop’s charge. We 
carried out simultaneously the observation of 
field change, charge on raindrops and rainfall 
intensity. The result is given in Figure 8. The 
abscissa shows time, the ordinate shows field. 
The solid line indicates the field change. The 
charge on raindrops is given by the circles and 
dots along the field line. We find negative 
charges above the zero potential line and posi- 
