114 
wich day in the same manner as does the air-earth cur- 
rent of fair weather, This evidence is exhibited in Fig. 8, 
where the ordinates represent the area over which land 
thunderstorms are in progress at the time of the Green- 
wich day denoted by the abscissae. By comparing this 
graph with either of the lowest graphs for potential 
gradient in Fig. 7, a remarkable similarity will be noted. 
Whipple also concludes that the character of the diurnal 
variation changes during the year in about the same 
manner for the two phenomena. This result indicates 
that if the net current generated in the representative - 
thunderstorm is directed upward from the earth to the 
high atmosphere, and if the total current from all 
storms is great enough (average 1800 amp), the supply 
current would satisfy the requirements listed above. It 
is surprising, in view of the character of the data and 
their scantiness for large areas of the earth, that this 
result could be obtained. Whipple apparently used 
satisfactory methods in his analysis. Maybe this result 
indicates that thunderstorms over the oceans and in 
sparsely settled regions of the earth do not contribute 
much to the supply current. 
ie T ] 
100 SS] 
THUNDERSTORM AREA IN 104 KM 
(0) 2 4 6 8 lO 2 ! 6 I 
GREENWICH MERIDIAN TIME 
Fic. 8 —Diurnal variation in prevalence of thunderstorms. 
The ordinate is the estimate of the average area of land- 
thunderstorms in progress on the whole earth. (After F.J.W. 
Whipple and F. J. Scrase.) 
20) 22 24 
Other methods should be used to check Whipple’s 
results. It seems that it may be practicable now by the 
use of suitably designed atmospheric recorders at a com- 
paratively small number of well-distributed stations to 
make a complete “sweep” of the earth and with satis- 
factory registration for one year to obtain the required 
data. From such data one would hope to derive a 
record of lightning frequency as a function of Green- 
wich time which may be more suitable information for 
this purpose than records of thunderstorm occurrence. 
Although Whipple’s results seem to show that af 
thunderstorms are the seat of the supply current, the 
diurnal and annual variation of that current would each 
be of the right type, it is yet to be ascertained whether 
the thunderstorms do contribute a current of the right 
sign and magnitude to the circuit previously described. 
In principle this may be ascertained either by making 
measurements of the required elements at the earth’s 
surface beneath storms or by making the proper survey 
over the top. 
No adequate survey beneath a thunderstorm has ever 
been made. However, T. W. Wormel made part of the 
ATMOSPHERIC ELECTRICITY 
required measurements at Cambridge, England, which 
he used together with data obtamed elsewhere by other 
observers to draw up a balance sheet of the quantity 
of electricity lost and gamed by 1 km? in a year. 
Wormel’s balance sheet is as follows: 
Negative charge gained 
1. By natural point discharge 
2. By lightning discharge 20 
Coulombs km™ yr 
100 
Total negative charge gained 120 
Positive charge gained 
3. By atmospheric conduction 60 
4. By precipitation 20 
Total positive charge gained 80 
Net gain, negative charge 40 
Although it is interesting to see what comes from an 
attempt to strike such a balance it may not have much 
significance for the problem at hand. Item 3 is the best- 
determined one, but Wormel uses the small value ob- 
tained at a few places in England whereas the electric 
surveys of the Department of Terrestrial Magnetism, 
Carnegie Institution of Washmgton, indicate a value 
almost twice this for representative areas of the earth. 
The interpretation of the data from which Item 1 was 
obtained may be questioned and, furthermore, it cer- 
tainly is much too large for vast areas of the earth— 
especially the oceans and the polar regions. In addition 
to questioning whether Items 2 and 4 are representative’ 
it should be noted that uncertain elements enter ito 
their estimation. This approach to the problem seemed 
so difficult that another way was sought. 
Surveys of the electric current density over the tops 
of thunderheads seemed to the author to be feasible 
when in 1946 pressurized aircraft became available for 
use in scientific projects. Owing to the absence of pre- 
cipitation and the rarity of lightning in the clear air 
above a thunderhead, electric circumstances there are | 
simpler than beneath the storm. This was the basis for 
expecting that the transfer of electricity in the air above 
the storm occurs chiefly by electric conduction and that, 
as a consequence, measurements of the vertical com- 
ponent of the electric current density made at short 
intervals (2 sec) on a number of traverses over a 
storm would constitute an adequate basis for estimating 
the magnitude and direction of the total current from 
a storm. Surveys of this sort were made in 1947 and 
1948, as a jomt project of the Department of Terrestrial 
Magnetism, Carnegie Institution, and the U. 8. Air 
Force. A technical report is being prepared by O. H. 
Gish and G. R. Wait.® 
Successful surveys of twenty-four storms were made. 
In these storms the current was directed upward, that 
is, positive charge was transferred upward. This is 
favorable to Wilson’s suggestion. The magnitude of the 
current ranged from zero to 6.5 amp. The average cur- 
rent for all storms was 0.6 amp, and that for all except 
the one unusually large value, was 0.3 amp. 
Is an average current per thunderstorm cell of 0.3 to 
0.6 amp adequate to satisfy the requirement that the 
total supply current be 1800 amp? It would be, if the 
5. See “Thunderstorms and the Harth’s General Electrifica- 
tion,” by O. H. Gish and G. R. Wait, J. geophys. Res. 55: 473- 
484 (1950). 
