UNIVERSAL ASPECTS OF ATMOSPHERIC ELECTRICITY 
face; (4) the gradient at the surface varies during the 
day according to a universal routine with a mimimum 
at 4" GMT and a maximum at 19" GMT and a diurnal 
range equal to 35 per cent of the daily mean; (5) the 
character and amplitude of this universal diurnal varia- 
tion and also the daily mean apparently vary during the 
year. Exceptions to the last two statements are con- 
spicuous at some times and places, but on a world- 
wide view the exceptions are presumably insignificant. 
THE AIR-EARTH CURRENT 
The electric current which flows from air to earth in 
fair-weather areas is predominantly an electric con- 
duction current. Although it is conceivable that electric 
convection, as in the case of charged rain fallmg to 
earth, or electric displacement which depends upon the 
rate of change of electric field strength, may sometimes 
play a part in determining the electric current density 
between air and earth, yet estimates of the magnitude 
of the effects of these factors mdicate that im fair 
weather they are usually small and of such character 
that they need not be considered in this brief review of 
the phenomena of atmospheric electricity. This is cor- 
roborated by several tests in which values of 7 deter- 
mined by the direct method were found to be about the 
‘same as those determined at the same place and time by 
the indirect method. The air-earth current density 7 is, 
therefore, essentially equal to X\#, and the description 
of it is implied in the foregoing discussions of \ and L. 
The average values of 7 derived from measurements 
over the oceans on cruises of the Carnegie are, in the 
opinion of the author, fairly representative for the earth 
as a whole. The bases for this opinion are largely as 
follows::(1) the geographical distribution of these meas- 
urements is much more extensive than that for all other 
data, and (2) the data are much freer from large and 
persistent local effects than those for most land sta- 
tions. It should be noted, however, that for some land 
stations far from centers of human activity, and notably 
for those in the polar regions, the data are relatively 
free from such local effects and have about the same 
characteristics as those for the oceans. It is on this 
basis that the following statements rest. The average 
value of 7 derived from measurements of \ and # at sea 
is about 10~° stat amp em. There is no clear evidence 
of any trend toward either a higher or a lower value in 
the 15-yr period, 1915-29, to which these data apply. 
But there is an indication that near the equator 7 is 
somewhat less than at higher latitudes [20]. 
The annual and the diurnal variation of 7 at sea have 
about the same character and relative magnitude as the 
potential gradient because only a small diurnal varia- 
tion of d is indicated by the observations and the latter 
is probably a local time effect. Also there is no definite 
evidence of an annual variation of ) at sea. 
The total electric current J from air to earth for all 
fair-weather areas is 7 multiplied by the area of the 
earth. That the error incurred by neglecting here the 
area of thunderstorms is less than one per cent may be 
inferred from the data assembled by Brooks [8]. 
The mean value of J obtained in this way is about 
113 
1800 amp. The error in this estimate is probably not 
greater than 10 per cent. The diurnal and possible an- 
nual variation of J are of the same character and rela- 
tive magnitude as the corresponding characteristics of 
the average for 7. This total current from air to earth 
must have a counterpart which ‘‘completes the cir- 
cuit.’”’ The term ‘“‘supply current” is used here to denote 
this counterpart, an elemental universal feature of at- 
mospheric electricity. Where and how is the supply 
current generated? The answer to that question has 
been sought during the last fifty years. The status of 
that search at the present time is discussed in the fol- 
lowing section. 
THE SUPPLY CURRENT AND THUNDERSTORMS 
Many proposals have been made to account for the 
fair-weather aspects of atmospheric electricity. Most 
of these were soon found to be untenable. The one sur- 
viving proposal which may give the answer as to where 
the supply current is generated is credited to C. T. R. 
Wilson. This is the suggestion that the supply current 
is generated in thunderstorms. If this is the case, the 
answer as to how it is generated doubtless must await 
the development of an acceptable theory for the genera- 
tion of electric charge in thunderstorms. This section is 
devoted to an appraisal of Wilson’s suggestion. 
The universal aspects of atmospheric electricity which 
should be recalled in this connection are, expressed in 
terms of the more comprehensive element J, as follows: 
(1) the yearly average of the total current / from air 
to earth in fair-weather areas seems to be nearly con- 
stant at a value of about 1800 amp; (2) the daily mean 
of I probably varies during the year, being greater in 
the six months from November to April than in the rest 
of the year; (3) the diurnal variation of / is such that on 
the average during the year the maximum occurs at 
about 19" GMT and the minimum at about 4" GMT; 
(4) the character and range of this diurnal variation 
vary during the year. 
The magnitude of the supply current and the charac- 
ter of its annual and diurnal variations must be the 
same as those listed here for 7. Near the earth the 
supply current must flow upward, from earth to air, 
(opposite to that for 7) but at some undetermined alti- 
tude, probably in the high stratosphere and below the 
ionosphere, the vertical component vanishes and the 
current is dispersed laterally. Then it returns to earth 
as an air-earth conduction current which is nearly uni- 
formly distributed over the earth. The circuit is com- 
pleted through the earth to the source, or sources, of 
the supply current, which may be located in thunder- 
storms. The fact that the air conductivity increases 
with altitude and has relatively large values at high 
altitudes seems to be of vital importance for the exist- 
ence of such an electric circuit. 
The first evidence in favor of Wilson’s suggestion that 
the supply current is generated in thunderstorms was 
obtained in an analysis, made by Whipple [22], of the 
thunderstorm data for the world, assembled by Brooks 
[8]. That investigation indicates that thunderstorm ac- 
tivity, for the earth as a whole, varies during the Green- 
