118 
difficulty might be removed. But it may be that some 
force other than gravity effects the separation of the 
larger from the smaller charged particles. A combina- 
tion of centrifugal action and straight wind has been 
suggested, but it is not yet evident that the wind struc- 
ture of the thundercloud is suitable for this. 
This discussion of selected electric features of 
thunderstorms and of theories which have been pro- 
posed to account for those features was designed to 
show that at the present time there are only a few clues 
as to how the charge-clouds are developed. One of these 
is the recent discovery that the orderly freezing of very 
dilute solutions of certain salts is a very effective process 
for the imitial step m the generation of electric charge. 
The potency of this process is such that if conditions in 
the thunderstorm are favorable, the charge on the pre- 
cipitation particles may be maintained at a value so 
near the maximum, limited by electric breakdown, that 
the prospects of accounting for the large-scale separa- 
tion of charge will be improved. More observations of 
electrical and other conditions, especially within 
thunderstorms, and more carefully controlled specula- 
tion are required to find the answer to the question, How 
are the charge-clouds in thunderstorms developed? 
Until the foregomg question is answered, the added 
question, How is the supply current generated? will 
probably also remain unanswered. This statement de- 
pends upon the fact that with the evidence now at hand 
one may entertain the view that the supply current is 
generated in the thunderstorms of the earth. Before this 
view was advocated, the fair-weather aspects of atmos- 
pheric electricity and thunderstorm electricity were 
regarded as unrelated geophysical phenomena. Now, 
since it seems likely that the universal aspects of at- 
mospheric electricity derive from thunderstorms, a more 
unified exposition of the subject is feasible. 
From a remote position in space an ideal observer, 
whose acute vision could encompass the whole earth, 
might see in the broad prospect of atmospheric electric 
phenomena the following features: first, the numerous 
thunderstorms in progress on the earth—usually several 
thousand of them. He would notice that they are very 
scarce in the polar regions and especially abundant in 
the afternoon on land areas in middle and low lati- 
tudes. If each hour throughout a year he should count 
the total number of electric storm centers, he would 
probably note that, on the average, the count tends to 
be greatest for the hour when it is mid-afternoon at 
about longitude 75°W and least for the hour when it is 
mid-afternoon at about longitude 150°H. Other varia- 
tions in the count would doubtless also be found. 
If this observer had a special sense with which he 
could ‘“‘see” a flux of electricity, he would not only 
notice the lightnimg flashes in and about the electric 
storm centers, but would also see a complicated pattern 
of electric flux in, about, and beneath each electric 
storm center. But the most alluring feature would be a 
narrow stream of the electric effluvium which emerges 
from the top of the upper charge-cloud, flows upward 
and along its course, widens and becomes less dense 
until it merges at a high level with similar streams from 
ATMOSPHERIC ELECTRICITY 
the other storms to form a world-wide ocean of electric 
effluvium. From this ocean a much more tenuous but 
nearly uniform electric flux could be seen to proceed 
downward to the earth everywhere except in and about 
electric storms. The circuit continues through the earth 
and back to the storm centers. The density of this 
universal flux from air to earth would be seen to vary 
during the day, from day to day, and during the year, 
in about the same manner as does the corresponding 
count of storms. But apparently it varies little, if at all, 
from year to year. 
This 1s an impressionistic sketch of the broad prospect 
of atmospheric electricity as it is seen by the author in 
the light of evidence now available. 
OUTSTANDING PROBLEMS 
In the broadest sense there are two main problems in 
the field of atmospheric electricity: 
1. To locate the source of those universal aspects 
which are epitomized in the concept, supply current. 
2. To elucidate the mechanism which generates the 
supply current. If, as now seems likely, the supply cur- 
rent is generated in thunderstorms, the last-mentioned 
problem and that posed by the electric aspects of the 
thunderstorm have much in common. 
It is of course necessary to have an adequate quanti- 
tative description of the phenomena before the rationale 
of the subject is developed. In this respect there are 
many minor and some major inadequacies. Some of the 
more important observations required in order to fill 
this need are: 
1. Measurements of air conductivity in the high at- 
mosphere, especially in the altitude range 18 km to 60 
km. New techniques would be required to make the 
measurements in the upper part of this region. 
2. Measurements of air conductivity, and counts of 
Aitken nuclei m thunderstorms. These items are sug- 
gested, rather than measurements of small-ion and 
large-ion concentrations, because measurement of the 
latter elements is more difficult. The technical difficul- 
ties of making reliable measurements of air conductivity 
from aireraft, during flight through clouds, have not 
yet been overcome. 
3. More extensive, or more comprehensive, informa- 
tion about the population of the electric storm centers 
that are in progress on the earth. Until this information 
is available, no reliable estimate can be made of the 
average current which a typical electrical storm center 
must contribute to the make-up of the supply current. 
It is desirable that data be collected to verify, or to 
refute, the diurnal variation in storm population re- 
ported by Whipple. Perhaps this could be done by radio 
methods used for measuring atmospherics and “back- 
ground noise,” with some modification to adapt them 
for use in a world survey of electric storm activity. _ 
Apparently it would be necessary to make such meas-— 
urements at a relatively small number of well-distrib- 
uted stations. 
4. More data for the air-earth current density 7 or 
for the elements \ and Z, in order to ascertain whether, 
and in what manner, the supply current varies during 
