112 
of Western Australia, is agitated enough by the wind 
to introduce puffs of electric charge into the air near 
the surface. There is some evidence that small varia- 
tions of conductivity are associated with the field 
changes and the former are doubtless dependent on the 
latter. The drifting of snow, like the drifting of sand or 
dust, also modifies the electric field, but during the 
former, positive charge is introduced into the air. 
A description of field changes of types (a), (b), and 
(c) is relevant here chiefly because these phenomena 
must be recognized for what they are and must be dis- 
tinguished from the universal aspects of the electric 
field in order that the latter may be identified. 
Changes of type (a), which are prominent during 
storms, have importance for the clues they provide 
MEAN 132 
FEB -MAR -APR. = En | 
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| il DAYS 7 = 
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[S MAY-JUN.-JUL. F as 
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Ww 
: 23 DAYS 
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— MEAN 
«!20 AUG.-SEP -OCT. 
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100 
(a) | 
: MEAN 140) & 
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120% = 
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JAN TO DEC. . a 
120. eres 
ifeye) | 
GMT 
Fie. 7—Diurnal variation of potential gradient on the 
oceans, Carnegie Cruises IV, V, and VI, 1915-21 and Cruise 
VII, 1928-29. 
concerning the electrical constitution of storms, but 
changes of types (6) and (c) are of minor interest as 
geophysical phenomena, especially when they depend 
upon the activities of man. 
The discovery of the universal diurnal variation of 
potential gradient was delayed because most of the at- 
mospheric electric observations, prior to about the year 
1915, were made near cities or at other places where 
field changes of types (a), (6), and (c) are prevalent. 
This feature, first noted by Mauchly in 1921 [6], has 
great importance for the subject. His analysis of the 
data for potential gradient over the oceans, obtained 
on Cruises IV, V, and VI of the magnetic survey yacht, 
Carnegie, showed that the diurnal variation of the gra- 
dient at sea proceeds according to universal time, not 
according to local time. 
This is illustrated by the results which are exhibited 
ATMOSPHERIC ELECTRICITY 
in Fig. 7. Average values of potential gradient, in volts 
per meter, are there plotted as ordinates against the 
hours of the Greenwich day, counting from midnight 
to midnight. The combined results for Cruises IV, V, 
and VI are shown separately from those for Cruise VIL. 
The latter, although generally of greater amplitude, 
vary in nearly the same manner as the former. If at- 
tention is fixed on either of the two lowest graphs, which 
represent hourly averages for the year, it is to be seen 
that the gradient is smallest about four hours after 
Greenwich midnight and greatest at about 19. That 
this feature varies somewhat throughout the year is 
shown by the other graphs which represent different 
quarters of the year. When these same data are aver- 
aged for hours of the local day, no significant diurnal 
variation is disclosed. Thus the change in gradient dur- 
ing the day does not depend upon daylight and dark- 
ness or other factors which vary in a manner directly 
related to the elevation of the sun at a given place. 
Another way of viewing this universal aspect is as 
follows. The negative charge of the earth as a whole in 
fair weather is greatest at about 19 GMT and least at 
about 4 GMT, and the total air-earth current J varies 
in a similar manner. The latter depends partly upon the 
fact that over the oceans the diurnal variation of 
d is negligible. 
This universal variation in potential gradient is also 
manifested in the diurnal variation of potential gradient 
at most places on land. But factors of local origin there 
often mtroduce component variations which may pre- 
ponderate, especially in or near centers of population 
where the conductivity of the air is affected by atmos- 
pheric pollution. The concentration of these impurities 
is, of course, dependent not only on the rate at which 
they are supplied to the atmosphere but on the strength 
and direction of the wind, on rain, or on other processes 
which scatter these substances or carry them to or from 
the place at which the electrical features are considered. 
Variations of the content of radioactive matter in the 
air over land also result in variations of the concentra- 
tion of the small ions, upon which the conductivity of 
the air chiefly depends. Over land the air conductivity 
and the gradient may, therefore, be expected to vary 
during the day im a manner and to an extent which is 
often dependent upon a complex and variable set of 
local circumstances. 
The average potential gradient of fair weather, the 
universal diurnal variation of gradient, and doubtless 
some other temporal variations are of type (d), that 1s, 
they depend directly upon V, which, in turn, is pre- 
sumably dependent upon the supply current. 
The universal aspects of the electric field of the atmos- 
phere which are of chief importance may be sum- 
marized briefly as follows: (1) the electric potential 
gradient in the atmosphere during fair weather is posi- 
tive everywhere on the earth and the average value is 
about 130 v m~; (2) the value near the equator is about 
80 per cent of the value at higher latitudes; (8) the 
gradient decreases with altitude in free air, rapidly in 
the first kilometer, then more slowly, till at an altitude 
of 10 km it is about 5 per cent of its value at the sur- 
