UNIVERSAL ASPECTS OF ATMOSPHERIC ELECTRICITY 
By O. H. GISH! 
Lincoln, Nebraska 
One becomes aware of weather through sensibility 
to heat or cold, calm or storm, brightness or gloom, but 
one’s unaided senses do not reveal the ever-present uni- 
versal aspects of atmospheric electricity. Of course one 
sees lightning and hears thunder, but these are heralds 
of an impetuous, stormy aspect of atmospheric elec- 
tricity which is not universal. Lightning is seldom seen 
in the polar regions and even in temperate latitudes it 
is seen only a small fraction of the time. The study by 
Brooks [8] indicates that on one-fourth of the earth’s 
surface thunder may be heard on less than one day in 
a hundred. 
But everywhere on the earth, electric forces in the 
open atmosphere are readily detectible with instru- 
ments. Many measurements made in most representa- 
tive regions of the earth show that (1) during fair 
weather the average electric field strength or potential 
gradient is usually more than 100 volts per meter, (2) 
the electric potential increases with elevation above the 
surface but the field strength, or the potential gradient, 
decreases, and (8) the direction of the electric field in 
fair weather is such that positive ions in the air drift 
towards the earth and negative ions drift away. One 
must infer from these observations of the electric field 
that the electric charge on the surface is always negative 
everywhere on the earth, except in the vicinity of 
thunderstorms or where drifting dust or some other 
local charge-generating process temporarily disturbs the 
normal aspect. 
Another fact of fundamental importance is that air, 
in the open, is not a perfect insulator: Although the 
electrical conductivity of air is so small that for most 
practical affairs it need not be considered, this property 
does play an important role in determining the electric 
state of the atmosphere. For example, because of this 
property and the existence of an electric field, an electric 
current flows from air to earth and the average magni- 
tude of this current, based on numerous measurements, 
is such that 90 per cent of the negative charge on the 
earth would be neutralized in thirty minutes. Despite 
this, the charge at the present time doubtless is about 
the same as it was one hundred years ago when Sir 
William Thomson (later Lord Kelvin) made the first 
reliable quantitative measurements of electric field 
strength. 
__ How is the negative charge of the earth or the corre- 
sponding electric field maintained? or, in other words, 
How and where is negative electricity supplied to the 
earth at the rate required to compensate, on the aver- 
age, for the loss by electric conduction? Many attempts 
1. Retired from Department of Terrestrial Magnetism, 
Carnegie Institution of Washington. 
to answer this question have been made since the prob- 
lem was first recognized, about the beginning of this 
century, but most of the answers have been definitely 
confuted. The challenge presented by this problem 
seemed to some to call for radical measures. Several 
eminent physicists thought that the answer might re- 
quire some modification of basic laws of physics or 
might depend on some physical entity or property that 
had not yet been discovered [1]. These nonclassical sug- 
gestions were made at a time when all other proposals 
seemed inadmissible, but since then some evidence has 
been found which lends support to the suggestion that 
thunderstorms supply negative electricity to the earth 
at a rate which is adequate to maintain the negative 
electric charge of the earth and the electric field of the 
atmosphere. 
According to this view, each thunderstorm which 
has developed sufficiently acts as a generator of elec- 
tricity and all the thunderstorms of the earth, acting as 
generators connected in parallel between the earth and 
the high atmosphere, provide the supply current which 
maintains the high atmosphere at a potential of several 
hundred kilovolts positive with respect to the earth. 
The requirements which this supply current must 
satisfy will be seen better after the review, which fol- 
lows, of the chief universal aspects of atmospheric 
electricity, but from what has already been said it will 
be evident that the more general requirements are (1) 
the supply current generated in the thunderstorms must 
flow upward into the high atmosphere and then spread 
out and return to earth as a more or less uniformly dis- 
tributed air-earth conduction current, (2) the magni- 
tude of the supply current must equal the total current 
from air to earth in all fair-weather areas of the earth, 
and (3) the variations of the total supply current should 
correspond to the variations in the total air-earth cur- 
rent. Whether or not the net electric current which flows 
between the earth and thunderstorms meets these re- 
quirements has not been definitely ascertained because 
the electrical circumstances under thunderstorms are 
so complex that it has not been feasible to make the 
measurements which are needed. 
The foregoing requirements are clearly indicated by 
extensive measurements of air-earth current density 
which haye been made in representative areas of the 
earth, and it is from these measurements that the 
magnitude and characteristics of the supply current 
may now be inferred. 
Values of the air-earth electric current density 7, al- 
though very small, have been obtained satisfactorily 
at several places for a number of years by automatic 
registration. The value of 7 may be obtained either (1) 
by a ‘direct’? method [19] in which the current is “‘col- 
lected” on an insulated plate set flush with the earth’s 
101 
