UNIVERSAL ASPECTS OF ATMOSPHERIC ELECTRICITY 115 
average number of thunderstorm cells or centers of 
electric activity in progress on the earth were between 
3000 to 6000. This is several times Brooks’ estimate of 
1800. The latter estimate is, however, not applicable 
here for the following reasons. The data used by Brooks 
consisted of reports of thundery days—days on which 
thunder was heard—and there is no indication that he 
took into account the fact that at some places two, and 
occasionally more, visually distinct thunderstorms oc- 
cur within hearing distance of a station on the same 
day. Neither was it considered that in a given thunder- 
storm there are sometimes several well-separated cen- 
ters of electrical activity in progress simultaneously. 
These considerations indicate that a world population 
of between 3000 and 6000 centers of electrical activity 
may be admissible, and, that the answer to the fore- 
going question is “‘Probably, yes.” 
This provisional answer is, however, subject to an- 
other condition, namely, that the values for total cur- 
rent obtaimed from these surveys may be too large 
because part of the current, represented by the measure- 
ments, may flow in a local circuit either to the lower 
pole of the cloud or to bound charge on the earth in the 
vicinity of the storm. An adequate evaluation of this 
effect has not yet been completed. Until this is done 
and until a more reliable estimate can be made of the 
world population of centers of electric activity, one can 
say only that the available evidence is favorable for the 
view that the supply current is generated in the 
thunderstorms, but much more investigation is re- 
quired in order to reach a definite and reliable conclu- 
sion. This theory has at the present time no serious 
rival. No other theory known to the author has the 
potentiality of accounting for the universal, diurnal, 
and annual variation of J. 
An interesting corollary of this theory is that if the 
supply current originates in thunderstorms, the uni- 
versal, diurnal, and other variations may be regarded 
as a measure of the thunderstorm activity of the whole 
earth. It accordingly seems likely that the record of 7 
at a station where local disturbances are small would 
constitute an approximate record of the thunderstorm 
activity of the earth, showing how it varies from day to 
day and perhaps even from hour to hour. Small local 
effects are more likely to obscure the latter than the 
former. Such effects could, however, doubtless be largely 
eliminated from the data if 7 were registered at a 
moderate number of suitably selected stations. Such 
records might be valuable in the study of some prob- 
lems of world meteorology. For example, they might 
provide answers to questions such as, Does world 
thunderstorm activity vary from year to year and is it 
correlated with sunspot activity? The data now avail- 
able for 7 apparently point toward a negative answer 
to these questions. 
How is the supply current generated? How are the 
electric charge-clouds of thunderstorms developed? 
These are two questions which doubtless have a single 
answer, provided that the supply current is generated 
in thunderstorms. In the author’s opinion, no adequate 
answer to these questions has yet been published. Be- 
cause of this circumstance the discussion which follows 
is limited to an outline of important aspects of the 
theories which have been proposed, the chief electrical 
features which must be accounted for, and some other 
conditions which must be satisfied. 
The electrical cycle of a thunderstorm may be re- 
garded as consisting of the following parts: (1) an mitial 
separation of charge, a small-scale phenomenon in which 
some particles of precipitation become charged with 
electricity of one sign and the adjacent air, or some of 
the smaller particles, becomes charged with the other 
sign, (2) a large-scale separation of charge, possibly 
occurring in several steps, by which large charge-clouds 
of several cubic kilometers in volume are formed at 
more or less definite levels in the thundercloud, (3) the 
initiation of discharge from a cloud, namely, the mo- 
bilization of the charges residing on particles widely dis- 
tributed throughout the charge-cloud, thus preparing for 
the succeeding steps which constitute (4) the lightning 
discharge. 
The last of these, the lightning discharge, has been 
comparatively well elucidated in recent years. It is the 
subject of a separate article in this Compendium’ and 
will not be discussed here. 
Knowledge regarding the other steps, however, is in 
an unsatisfactory state. More factual evidence is re- 
quired before theories of these aspects of the electric 
cycle of the thunderstorm can be securely established. 
The initiation of a lightning discharge—the getting 
together of the charges which are widely distributed on 
ice particles or raindrops or both—although one of the 
unexplained aspects of the thunderstorm has little rele- 
vance to the development of the charge-cloud, and will 
be dismissed with the statement that the glow. dis- 
charge, which starts at the surface of charged particles, 
is presumably involved. When the glow discharge 
spreads throughout a sufficient volume of the charge- 
cloud, the discrete charges on the particles are mobil- 
ized for the lightning discharge. However, there.is little 
direct evidence to support this surmise. 
The electric structure of the thunderstorm is often so 
complex that exact descriptions, such as could be made 
with the aid of general harmonic analysis when ade- 
quate data are available, have not been undertaken. 
One might say, however, that in the classical work of 
C. T. R. Wilson a fair estimate of the principal moment 
was obtained. An advance beyond this was realized in 
the work of Workman, Holzer, and associates [23]. The 
results of these and other investigators are in fair agree- 
ment on the gross aspects of the electric charge-clouds. 
These and some other features which bear on the later 
discussion are listed here. 
1. There are two principal charge-clouds in the typi- 
cal thunderstorm. The positive cloud is usually located 
at an altitude greater than that of the negative cloud. 
An altitude of 6 to 7 km for the former and of 3 to 4 
km for the latter has been estimated. 
2. Charge-clouds tend to be cumuliform rather than 
stratiform. 
6. Consult ‘‘The Lightning Discharge” by J. H. Hagenguth, 
pp- 136-143. 
